The figure shows a seven level hierarchy in which levels 1–3 are the

supervisory levels and levels 4–7 are the information levels. The top

management system devises plant optimising control strategies and level 2

deals with the optimum strategies for efficient production. Level 3 looks after

the overall supervision of a process. The field instruments, i.e., sensors and

actuators (at level 7) send their information through a process bus and/or

device bus (at level 6) and Local Area Network (at level 5) to the information

network (at level 4). This information is passed on to levels 3, 2 and 1 for final

control of the overall plant. In this chapter, levels 4-7 have been dealt with since

the supervisory levels are dependent on the information levels for the efficient

operation of the plant.

6.2 Evolution of Industrial Control Process

Industrial process control has evolved through hteh developments of

direct Digital Control (DDC), Distributed Control Systems (DCS) and Field

Control Systems (FCS). In all of them, PLC is the main component for control

system logic implementation. In every step of the evolution, the control of the

process has moved closer to the sensors and actuators. Fig. 6.2 shows DDC.

Fig. 6.2 Evolution of Control Process

DCS and FCS in which the following functions occur: (i) In a DDC system a

Proportional Integral Differential (PID) function is executed I the primary

computer, and analog signals are separately transmitted to the sensor and

actuator (S & A) in the field. (ii) In a DCS, a digital signal bus is used through

which digital signals are allowed to pass to a single PID controller in the field

and then analog signals are sent to individual S & A. (iii) In an FCS, digital

signals are passed to the field, where there is a digital signal bus, from where

individual S & A are provided with PID controller through which digital signals

can pass.

The control function executors are thus moved nearer the sensors and

actuators in the field. The control functions are in fact executed by PLCs. This

movement of control process, reduces wiring, aids in troubleshooting and

decreases maintenance costs of an industrial network, thus enhancing the

overall reliability of a process.

In this control and automation process, signal communication and

networking play the most vital role. This chapter deals with different generalized

industrial communication and networking systems. Since in general, in a plant

the host control station is at a long/medium distance from field devices, serial

communication is preferred to parallel communication. Keeping this in view,

serial communication techniques have been discussed rather more elaborately

than parallel communication.

6.3 Types of Communication Interface

Communication interfaces are categorized by their data handling

capacity and their ability to handle single or multiple devices. Different types of

communication interface are shown in Fig. 6.3

Fig. 6.3 Different Types of Communication Interface

6.4 Types of Networking Channels

There are two basic types of networking channels, namely, physical

channels and logical channels.

(a) Physical channels: In this case, two nodes are physically connected by

a channel. This channel may be a twisted pair of wires, coaxial cable,

optical fibre etc. The general characteristics of physical channels are low

speed and low cost hence, they are preferred for short distance

communication.

(b) Logical channels: If the nodes in a network are not connected

physically, the channel is called a logical channel. A microwave link and

a satellite link are examples of al logical channel. It is much faster and

costlier than a physical channel and hence is mainly used for long

distance communication.

6.5 Parallel Communication Interface

Parallel communication requires handshaking and transmits data one

byte (8 bits) at a time. When data are transferred from the host processor to a

peripheral device, the following steps take place:

The host sets a bit on the bus, signaling to the peripheral that a byte of

data has been sent.

The peripheral receives data and sets a bit on the bus, signaling to the

host that data have been received.

Parallel bus structures include IEEE-488, IBM PC, VME, MULTIBUS, Q

and STD. IEEE-488 is extensively used for instrument interfacing.

6.5.1 IEEE-488 Bus

The IEEE-488 bus was developed to connect and control programmable

instruments, and to provide a standard interface for communication between

instruments from different sources. Originally developed by Hewlett-Packard,

the interfacing technique gained popularity in industry. Afterwards the IEEE

committee renamed it General Purpose Interface Bus, (GPIB). The IEEE-488

interface system consists of 16 signal lines and 8 ground lines. The 16 signal

lines are divided into 3 groups, namely, 8 data lines, 3 handshake lines, and 5

interface management lines.

Any instrument can be used with the IEEE-488 specification, because it

says nothing about the function of the instrument itself, or about the form of the

instrument’s data. Rather, the specification suggests that the instrument should

be interfaced, to make it compatible with the bus. The structure of the IEEE-488

is shown in Fig. 6.4.

Fig. 6.4 Structure of IEEE-488 (GPIB)

6.5.2 Devices Useable with IEEE-488

There are basic three types of devices that can be sued with the IEEE-

488. These are shown in Fig. 6.5.

Fig. 6.5 Classification of Devices Useable with the IEEE-488

As shown in the figure, there are three types of devices that can be

connected to the IEEE-488 bus, namely, listeners, talkers and controllers.

Some devices include more than one of these functions. The standard allows a

maximum of 15 devices to be connected on the same bus. A minimum system

consists of one controller and one talker or listener device.

It is possible to have several controllers on the bus but only one may be

active at any given time. The active controller may pass control to another

controller, which in turn can pass it back to another controller. The system

controller may optionally pass control to another controller, which then

becomes the active controller. A talker transmits data on to the bus following an

instruction. The controller can set up a talker and a number of listeners so that

it is possible to send data between the devices.

The data lines DIO-1 through DIO-8 are used to transfer addresses,

control information and data, the formats for which are defined by the IEEE-488

standard.

6.5.3 Handshaking Process

There are three handshake lines, as shown in Table 6.1, which control

the transfer of message bytes among the devices and from the method for

acknowledging the transfer on data. This handshaking process guarantees that

the bytes on the data lines are sent and received without any transmission

errors.

Table 6.1 Details of Handshaking Lines

Name Description Function

NRFD Not Ready

For Data

The NRFD handshake line is asserted by a listener to

indicate that it is not yet ready for the next data or

control byte. It is to be noted that the controller will not

see NRFD released (I.e., ready for data) until all devies

have released it.

NDAC Not Data

Accepted

The NDAC handshake line is asserted by a listener to

indicate it has not yet accepted the data or control byte

on the data lines. It is to be noted that the controller will

not see NDAC released (i.e., data accepted) until all

devices have released it.

DAV Data Valid The DAV handshake line is asserted by the talker to

indicate that a data or control byte has been placed on

the data lines and has had the minimum specified

stabilizing time. The devices can now safely accept the

byte.

The handshake process is shown in fig.6.6 in the form of a flowchart.

6,5.4 Interface Management Lines

There are five-interface management lines to manage the flow of control

and data bytes across the interface. These are summarized in Table 6.2

Table 6.2 Details of Interface Management lines

Name Description Function

ATN Attention This signal is asserted by the controller to indicate that it

is placing an address or control by the on the data bus.

ATN is released to allow the assigned talker to place

status or data on the date a bus. The controller regains

control by reasserting ATN: this is normally done

synchronously with the handshake to avoid control

between control and data bytes.

EOI End of

Identify

This signal has two uses. A talker may assert EOI

simultaneously with the last byte of data to indicate end-

of-data. The controller may assert EOI along with ATN

to initiate a parallel poll. Although many devices do not

use parallel poll, devices should sue EOI to end

transfers.

IFC Interface

Clear

This signal is asserted only by the system controller in

order to initialize all device interfaces to a known state.

After releasing IFC, the system controller is the active

controller.

REN Remote

Enable

Only the system controller asserts this signal. Its

assertion does not place devices into remote control

mode: REN only enables a device to go into remote

mode when addressed to listen,

SRQ Service

Request

This is like an interrupt: it may be asserted by any

device to request the controller ot take some action. The

controller must determine which device is asserting

SRQ by conducting a serial poll. The requesting device

releases SRQ when it is polled.

Fig. 6.6 flowchart showing the Handshaking process

6.6 Serial Communication Interface

In serial transmission, one bit follows another and hence one

communication channel is required to establish communication between two

devices.

6.6.1 Balanced and Unbalanced Systems

A serial communication interface requires two conductors to transmit

each signal. If the return wire is grounded and the information is sent by putting

absolute voltage on the signal wire (+10V or –10V for instance) then the

transmission is said to be unbalanced. It is not the fastest way of using the two-

wire channel. If the voltage put on the signal wire is only positive (or only

negative) the signaling is implore. If both positive and negative are used it

bipolar.

The performance of a channel and the speed of communication can be

significantly increased by two methods. The first way is to terminate the two-

wire path correctly to avoid earth-return. In the second method, signals can be

driven on both wires, instead of using an earth-return path and driving only one

wire I an unbalanced fashion. For example, the true signal might be +5V on

one wire and –5V on the other. The false signal would be the reverse, -5V and

+5V respectively. (This allows a shock wave to be generated that can be

detected more rapidly and followed more closely by the next bit).

The choice between unbalanced and balanced transmission lines is an

important consideration when selecting a data communications system. The

balanced transmission line permits a higher rate of data transfer over longer

distances. The differential method of data transfer is preferable in industrial

applications where noise can be a major problem. The disadvantage is that a

balanced system requires two conductors for every signal.

The successful transfer of voltage signals across two conductors in the

presence of noise is based on the assumption that the conductors have similar

characteristics and will be affected equally by noise and voltage drops. It does

not mean that noise does not exist in the balanced differential system. The

voltages on both conductors should rise and fall together, and the differential

voltage should remain the same.

6.7 Communication Mode

In any communications link connecting two devices, data can either be

sent in one of three communications modes:

Simplex

Half duplex

Duplex

6.7.1 Simplex Mode

The simplest of serial links is called simplex or channel connection. It

provides a single path in one direction only and involves a driver circuit at one

end (Tx) and a receiver circuit at the other (Rx) as shown in Fig. 6.7. Simplex

mode is of limited interest in an industrial communications system since

feedback from the instrument is essential to confirm whether the action

requested has indeed occurred or not.

Fig. 6.7 Simplex Mode of Communication

6.7.2 Half Duplex Mode

Half duplex communication occurs when data flows in both direction; but

in only one direction at a time as shown in Fig. 6.8. Half duplex communication

is provided by the RS-485 physical standard where only one station can

transmit at a time.

Fig. 6.8 Half Duplex Mode of Communication

6.7.3 Full Duplex Mode

In a full duplex system, the data can flow in both directions

simultaneously. Examples of hardware standards supporting full duplex are the

physical standard EIA-232E, sometimes referred to as RS-232C.

In duplex transmission shown in Fig. 6.9, a copy of any byte received is

sent back to the sender to verify that it was sent and received correctly.

Fig. 6.9 Full Duplex Mode of Communication

6.8 Synchronisation and Timing in Communication

A number of techniques exist to ensure that a receiving station reads

data at the correct times. As far as binary signaling is concerned, there are four

basic methods to synchronise reception with transmission.

1. Synchronous transmission – with an explicit clock

2. Enchronous transmission – with an embedded clock

3. Isochronous transmission – with two similar clocks

4. Asynchronous transmission – synchronizing without a clock

6.8.1 Synchronous Transmission

This type of transmission uses a clock signal produced by the transmitter

and sent along a separate channel in parallel with the data channels. Fig. 6.10

shows a changing data stream with the clock (option A) being true if the data is

valid and false if it is changing.

Fig. 6.10 Synchronous Data Transfer

This is a simple system, but the clock signal has to change twice as fast

as the maximum rate of change of data. Option B uses a single edge to make a

point where data are known to be valid. This is called transition clocking.

Synchronous transmission is fast but channel efficiency is 50%. It is used in

high speed local area networks (10 Mbps or higher).

6.8.2 Enchronous Transmission

The same total efficiency can be achieved by simply changing the timing

and data information on a single channel. This embeds the clock into the data

giving enchronous transmission. This is done by methods called phase

encoding, modified frequency modulation etc. They employ clock edges at

regular times between each data time but differ in the method used to

determine whether the data is zero or one. The various methods used for this

purpose are the state, change or absence of change, or direction of change.

There is a maximum of two edges per data bit (minimum of one) and so the

efficacy on the single channel is only 50%. The data is transferred at half the

rate of that of synchronous transmission but only one channel is required and

there is no skew problem. These techniques are common for magnetic tape

and disc encoding as well as LAN.

6.8.3 Isochronous Transmission

In isochronous data transmission, the receiver is assumed to have a

local clock running with 10% accuracy of that of the transmitter. With this

addition, the amount of synchronization carried in a channel can usually be cut

down to 20%. Fig. 6.11 shows isochronous data transfer. Since both

synchronous and enchronous methods have given up 50% of their available

bandwidth in carrying synchronization, this method enhances its efficiency.

Fig. 6.11 Isochronous Data Transfer

6.8.4 Asynchronous Transmission

An asynchronous point-to-point transfer control needs two channels in

addition to the data channels as shown in Fig. 6.12.

Fig. 6.12 Asynchronous Data Transfer

The sequence of action starts with the transmitter waiting until the

receiver indicates on its Receiver Ready (RR) channel that it is ready to receive

as at (1) as in Fig. 6.12. The transmitter puts out new data and then asserts

that they are valid on its Transmitter Data Valid (TDV) channel as at (2), in Fig.

6.12. The receiver may take whatever time it needs to take the data, and then

indicates it has done so by lowering its RR channel signal as at (3). This also

indicates that the receiver is not ready to accept another data item as it still

handling the data it has just taken. The transmitter then removes its data,

asserting that it is not longer valid, by lowering the signals on its DV channel as

at (4). The whole cycle then repeats at whatever rate the receiver will accept

and the transmitter will run. The structure of valid data is shown in Fig. 6.13.

Fig. 6.13 Structure of Valid Data

The different parts of data carry the following indications:

Before >> this is a period when no bit is being sent and the line is true

Start >> a single bit

Data >> bit (01001000)

Parity >> even parity

Stop >> one or two stop bits can be used

Idle >> a period of time when limit is true before the next byte

The sending and receiving computers must know what these settings

are in order to properly receive and decode the data. The transmission speed is

the maximum number of bits that can be sent per second. The baud rate

includes the start, parity and stop bit.

For example, a 9600-baud transmission of data would transfer

9600/1+8+1+2 = 800 bytes per second. Lower baud rates are 120k, 300k, 1.2k,

2.4k and 9.6k. Higher speeds are 19.2k, 28.8k and 33.3k.

6.9 Comparison between Synchronous and Asynchronous Transmission

Since synchronous and asynchronous transmission methods are

commonly used, a comparative study of their performances is given in Table

6.3.

Table 6.3 Comparison between Synchronous and Asychronous

Transmission

Synchronous Transmission Asynchronous Transmission

Synchronization errors result in loss of

a complete block

Synchronization error results in loss of

only one character

High speed in communication Low speed in communication

Block length is fixed Block length is unlimited

Constant bit rate over a block Constant bit rate within a character

No idle time between characters Variable idle time between characters

Characters are buffered into blocks for

transmission

A character can be transmitted at

random

High transmission efficiency Reduced transmission efficiency

6.10 Standard Interface

In process control applications, communication between two or more

devices is always required for which interfacing is the bridge. A good interface

should have the following features:

Completeness in respect of electrical, physical and logical definition

Flexibility in respect of connecting a wide range of devices

Simplicity in respect of definition, structure and use

Symmetry in applications

Transparency in data pattern

Security of transmission mechanism

Some interface mechanisms have been devised to comply with these

features, and some additional ones also, which have been accepted

internationally. They are called standard interfaces. Some commonly used

standard serial interfaces applicable for industries are discussed in the

subsequent sections.

6.11 Serial Interface RS 232C

RS 232C stands for: RS for Recommended Standard and C for Third

Version

RS 232C is a standard interface approved by the Electronic Industries

Alliance (EIA) for connecting serial devices. In 1987, the EIA released a new

version of the standard and changed the name to EIA-232-D. In 1991, the EIA

teamed up with the Telecommunications Industry Association (TIA) and issued

a new version of the standard called EIA/TIA-232-E.

It should be emphasized that RS-232 and other related EIA standards,

define the electrical and mechanical details of the interface and do not define a

protocol. The RS-232 interface standard specifies the method of connecting

two devices, the Data Terminal Equipment (DTE) and the Data

Communications Equipment (DCE), also called Data Circuit-terminating

Equipment in EIA/TIA-232E.

The levels of voltage signals used in RS 232C are shown in Table 6.4.

Table 6.4 Voltage Levels in RS 232C

Transmitter Receiver

+5V to +15V

-5V to –15V

+3V to +25V

-3V to –25V

Logic ‘0’

Logic ‘1’

Equipment that uses the RS-232 standard has the following features:

Point-to-point communication

Suitable for serial, binary and digital data communication

Communication is generally asynchronous; meaning that there is fixed

timing between data bits, but variable time between character frames

Full duplex communications

Unbalanced transmission and, therefore, susceptible to noise

This type of interfacing is reliable up to a distance of about 16m (50ft),

depending on the type of cable used and is capable of a speed of data rates of

up to about 20 kbps according to the standard (but 115 kBaud in practice).

RS-232 is the serial connection found on IBM-compatible PCs. It is used

for many purposes, such as connecting a mouse, printer, or modern as well as

industrial instrumentation. Due to improvements in line drivers and cables,

application often increase the performance of RS-232 beyond the distance and

speed listed in the standard. RS-232 is limited to point-to-point connections

between pc SERIAL PORTS AND DEVICES.

The EIA-232 standard defines 25 electrical connections. The electrical

connections are divided into four groups, namely, (i) signal common, (ii) data

lines (transmitted data, received data), (iii) control lines (request to send, clear

to send, DCE ready, DTE ready) and (iv) timing signals.

Fig. 6.14 shows a female DB-25 connector of RS 232c along with detail

terminal identifications.

Receiver Signal Timing (DCE Source)

Fig. 6.14 Female DB-25 Connector of RS 232C

A male DB-25 connector of RS 232C is shown in Fig. 6.15 with all the

terminal identifications.

DTE Ready

Fig. 6.15 Male DB-25 Connector of RS 232C

Figs. 6.16 and 6.17 show female and male DB-9 connectors along with

their terminal identifications.

Fig. 6.16 Female DB-9 Connector of RS 232C

Fig. 6.17 Male DB-9 Connector of RS 232C

Fig. 6.18 shows the cable connection between DTE and DCE through

RS 232C DB9 connector.

Fig. 6.18 Cable Connection between DTE and DCE through RS 232C DB9

Connection

6.12 Serial Interface RS 422

RS 422 (EIA RS 422-A Standard) is the serial connection used on Apple

Macintosh computers. RS 422 uses a differential electrical signal, as opposed

to the unbalanced signals referenced to ground with RS 232. Differential

transmission, which uses two lines each for transmit and receive signals,

results in greater noise immunity and longer distances as compared to RS

232. The greater noise immunity and longer distances are big advantages in

industrial environments.

6.13 Serial Interface EIA 485

The EIA 485 standard is the most versatile of the EIA interface

standards and is a true balanced or differential standard. EIA 485 permits a

‘multidrop’ network connection on two wires and allows reliable serial data

communication for (i) distances of up to 1200m, (ii) data rates of up to 10 Mbps,

(iii) up to 32 line drivers on the same line and (iv) up to 32 line receivers on the

same line.

The major enhancement of EIA 485 is that a line driver can operate in

three states called tri-state operation, namely: (i) logic 1, (ii) logic 0 and (iii)

high-impendence. In the high impedance state the line driver draws virtually no

current and appears not to be present on the line. This is known as the

‘disabled’ state and can be initiated by a signal on a control pin on the line

driver integrated circuit. Tri-state operation allows a multidrop network

connection. Up to 32 transmitters can be connected on the same line, although

only one can be active at any one time. Each terminal in a multidrop system

must be allocated a unique address to avoid conflicting with other devices on

the system.

The EIA 485 interface standard is very useful for systems where several

instruments or controllers are connected on the same line. Special care should

be taken to choose coordinating software, which is used to decide which device

will be active at what time. In most cases, a master terminal, such as a PC or a

computer, controls which transmitter/receiver will be active at any one time.

The two-wire data transmission line does not normally require special

termination. On long lines, the leading and trailing edges of data pulses will be

much sharper if terminating resistors approximately equal to the characteristic

impendence (Zo) of the line are fitted at the extreme ends. For twisted pairs the

characteristic impedance is typically between 100 to 120 ohms.

An EIA 485 network can also be connected as a four-wire configuration.

In this type of connection, it is necessary that one node be a master node and

all others be slaves. The master node communicates to all the slaves, but a

slave node can communicate only to the master. Since the slave nodes are not

aware of another slave’s response to the master, a slave node cannot reply

incorrectly to another slave node. This is an advantage in a mixed protocol

environment.

6.14 Comparison of Different Recommended Standards

A comparative study between the different recommended standards is

presented in Table 6.5

Table 6.5 Comparisons of Different Standards

Specification Rs 232 Rs 423 Rs 422 Rs 485

Mode of operation Single

-ended

Unbalanced

Single

-ended

Balanced

Differential

Unbalanced

Differential

Balanced

Total number of drivers and

receivers on one line (one driver

active at a time for Rs 485

networks)

One driver

One receiver

One driver

Ten

receivers

One driver

Ten receivers

32 Drivers

32 receivers

Maximum cable length 16M

(50ft)

1300M

(4000ft.)

1300M

(4000ft.)

1300M

(4000ft.)

Maximum data rate

(13M-1300M for RS 422/RS 485)

20kbps 100kbps 10Mbps-

100kbps

10Mbps-

100kbps

Maximum driver output voltage +/25V +/-6V -0.25V to 6V -7V to+ 12V

Driver output

signal level

(unloaded max)

Loaded

+/5V to

+/-15V

+/-3.6V +/-2.0V +/-1.5V

Driver output

signal level

(Unloaded max)

Unloaded +/-25V +/-6V +/-6V +/-6V

Driver load Impedance 3k to 7k 450 100 54

Maximum driver

current in high Z

state

Power on

Power off

N/A

+/-6mA

and +/-2V

N/A

+/-100 A

N/A

+/-100 A

+/-100A

+/-100A

Slew rate (maximum) 30V/S Adjustable N/A N/A

Receiver input voltage range +/-15V +/-12V -10V to 10V -7V to +12V

Receiver input sensitivity +/-3V +/-200mV +/-200mV +/-200mV

Receiver input resistance

(one standard load for RS 485) 32 to 7k 4k min. 4k min. 12k

6.15 Software Protocol

Standard serial interfaces have their own specifications and characteristics

as discussed in the previous sections, but they do not mention any specific

software protocol to be used to communicate with devices. For use in an

industrial environment, serial instrument manufacturers use several protocols,

the most common of which are, (i) ASCII Protocol, (ii) Highway Addressable

Remote Transducer (HART) Protocol and (iii) Manufacturer Specific Protocol.

6.15.1 ASCII Protocol

ASCII protocol has a set of ASCII strings that constitute commands to the

device. This protocol is commonly implemented as asynchronous protocol,

because the transmitter and receiver do not have any tightly coupled

synchronisation mechanism. These networks are commonly master/slave,

where one computer is the master and the field devices are slaves. Normally,

all devices power up in receive mode, waiting to receive messages. When the

master transmits a message, all devices receive the message. Each of the

devices checks whether the message is addressed to it and then acts upon the

message.

6.15.2 HART Protocol

HART protocol finds wide usage across process control instrumentation.

This is commonly used with standard PC serial ports using an electrical adapter

for physical signal conversion. HART is a hybrid network that uses the 4-20rnA

analog signal commonly found on process control instrumentation and adds to

it a digital signal. The digital signal is added in such a way that it does not

interfere with the standard 4-20mA functionality. In a control system where the

HART protocol is understood, the digital signals can be read from the device to

read and write data.

6.15.3 Manufacturer Specific Protocol

There are many protocols that are used on standard serial interfaces, but

use much more sophisticated data packaging schemes. These protocols use

different mechanisms to ensure reliable, robust communication. Most

manufacturers of serial devices invent their own ASCII protocol for their

particular device. It is a relatively straightforward task to read and write these

ASCII strings using the standard serial functions built into a language such as

Visual Basic. OptoMux and ModBus ASCII are examples of serial protocols that

use standard serial ports and have achieved broad acceptance across many

devices and suppliers. Most such protocols are proprietary to a specific

supplier, requiring special software drivers and often - special interface

hardware. Examples of these protocols are those used for communicating with

PLCs, for example Data Highway+ from AIlen-Breadley or ModBus+ from

Modicon.

6.16 Industrial Network

A computer with a single network interface can communicate with many

other computers. This economy and flexibility has made networks the interface

of choice. Typical advantages of networks include resource sharing and ease

of communication.

Small networks are often called Local Area Networks (LANs). These may

connect a few hundred computers within a distance of hundreds of meters.

These networks are inexpensive. Data can be transmitted at rates of millions of

bits per second. Many control systems are using this kind of network to

communicate with other controllers and computers. Typical applications

include:

Taking quality readings with a PLC and sending the data to a database

computer.

Distributing recipes or special orders to batch processing equipment.

Monitoring remote equipment.

Wide Area Networks (WANs) are used by LANs to communicate overlong

distances. These are not common in controls applications, but might be needed

for a very large-scale process. An example might be an oil pipeline control

system that is spread over hundreds of kilometers.

6.17 Network Topology

Local Area Networks share information and resources. To enable all the

nodes on the network to share information effectively, they must be connected

through some transmission medium in some specific fashion. The method of

connection is known as the network topology. The nodes need to share this

transmission medium in such a way as to allow nodes access to the medium

and minimize disruption of an established sender.

A physical topology defines the wiring layout for a network. This specifies

how the elements in the network are connected to each other electrically. This

arrangement will determine what happens if a node on the network fails. There

are basic four types of network topology as shown in Fig. 6.19.

Fig. 6.19 Classification of Network Topology

Bus and Ring topologies both share the same network wire. In a star

configuration each computer has a single wire that connects it to a central hub.

Combinations of these can be used to form hybrid topologies, which are used

to overcome the drawbacks of each of the three component topologies.

6.17.1 Bus Topology

A bus topology describes a network in which each node is connected to

a common single communication channel or “bus”. This bus is sometimes

called a backbone, as it provides a spine for the network. Every node can hear

each message packet as it goes past. Each node checks the destination

address that is included in the message packet to determine whether that

packet is intended for the specific node. When the signal reaches the end of

the bus, an electrical terminator absorbs the packet energy to keep it from

reflecting back along the bus cable, possibly interfering with other messages

already on the bus. Each end of a bus cable must be terminated, so that

signals are removed from the bus when they reach the end. A network with a

bus topology has been shown in Fig. 6.20

Fig. 6.20 Bus Topology of a Network

In a bus topology, nodes should be far enough apart so that they do not

interfere with each other. However, if the backbone bus cable is too long, it may

be necessary to boost the signal strength using some form of amplification, or

repeater. The maximum length of the bus is limited by the time interval that

constitutes "simultaneous" packet reception.

The advantages and disadvantages of bus topology are tabulated in

Table 6.6.

Table 6.6 Advantages and Disadvantages of Bus Topology

Advantages

A bus uses relatively small cable length compared to other topologies

having the simplest wiring arrangement.

Bus topology is simple and flexible.

Since nodes are connected by high impedance tapings across a

backbone cable, it is easy to add or remove nodes from a bus. This

makes it easy to extend a bus topology.

The broadcasting of messages is advantageous for one-to-many data

transmissions.

Disadvantages

There can be a security problem, since every node may see every

message, even if the message is not meant for it.

Diagnosis and fault-isolation can be difficult, since the fault can be

anywhere along the bus.

The bus cable can be a bottleneck when network traffic gets heavy. This

is because a considerable amount of time is taken by the nodes in trying

to access the network.

There is no automatic acknowledgement of messages, since messages

get absorbed at the end of the bus and do not return to the sender.

6.17.2 Ring Topology

A ring topology is both a logical and a physical topology. As a logical

topology, a ring is distinguished by the fact that message packets are

transmitted sequentially from node to node, in a predefined order, and as

such, it is an example of a point-to-point system. Nodes are arranged in a

closed loop, so that the initiating node is the last one to receive a packet. As

a physical topology, a ring describes a network in which each node is

connected to exactly two other nodes. Information traverses a one-way path,

so that a node receives packets from only one node and transmits them only

to one other node. A message packet travels around the ring until it returns

to the node that originally sent it. In a ring topology, each node can act as a

repeater, boosting the signal before sending it on. Each node checks

whether the message packet's destination node matches its address. When

the packet reaches its destination, the destination node accepts the

message and then sends it back to the sender, to acknowledge receipt. Fig.

6.21 shows a network in Ring topology, and its advantages and

disadvantages are tabulated in Table 6.7.

Fig. 6.21 Ring Topology of a Network

Table 6.7 Advantages and Disadvantages of Ring Topology

Advantages

A physical ring topology has minimal cable requirements.

No wiring centre or closet is needed.

The message can be automatically acknowledged.

Each node can regenerate the signal.

Disadvantages

If any node goes down, the entire ring goes down.

Diagnosis/troubleshooting is difficult because communication is only

one-way.

Adding or removing nodes disrupts the network.

There will be a limit on the distance between nodes.

Since ring topologies use token passing to control access to the network,

the token is returned to sender with the acknowledgement. The sender then

releases the token to the next node on the network. If this node has nothing to

say, the node passes the token on to the next node, and so on. When the token

reaches a node with a packet to send, that node sends its packet. Physical ring

networks are rare, because this topology has considerable disadvantages.

6.17.3 Star Topology

A star topology, as shown in Fig. 6.22, is a physical topology in which

multiple nodes are connected to a central component, generally known as a

hub. The hub of a star is usually a wiring centre; that is, a common termination

point for the nodes, with a single connection continuing from the hub. In some

cases, the hub may actually be a central computer that contains a centralised

file and control system, with all its nodes attached directly to the server. As a

wiring centre, a hub may in turn be connected to the file server or to another

hub. All signals, instructions, and data going to and from each node must pass

through the hub to which the node is connected. The telephone system is the

best-known example of a star topology, with lines to individual customers

coming from a central telephone exchange location. The advantages and

disadvantages of star topology are shown in Table 6.8.

Table 6.8 Advantages and Disadvantages of Star Topology

Advantages

It is easy to add or remove nodes, and to modify the cable layout.

Troubleshooting and fault isolation is easy.

Failure of a single node does not isolate any other node.

Each node can regenerate the signal.

The inclusion of a central hub allows easier monitoring of traffic for

management purposes.

Star topology is deterministic, which means that its performance can be

predicted.

Disadvantages

A star topology requires large lengths of able.

If the hub fails, the entire network fails. To cope up with this possibility in

a sensitive network, redundancy is enhanced by incorporating another

hub in parallel.

Fig. 6.22 Star Topology of a Network

6.17.4 Tree Topology

Fig. 6.23 shows a tree topology that is constructed out of smaller bus

networks. Repeaters are used to boost the signal strength and make the

network larger. In this type of topology, the disadvantages of other types are

partially overcome.

The advantages of tree topology are given below.

It is easy to tree since tree is into sub-units and it is easier to add new

units.

It is easy to disconnect a sub-unit and hence fault isolation is easier.

The major disadvantage of this network is that it is dependent on the root

device and if this fails to operate, the reliability of the entire network will be at

stake.

For a factory environment the bus topology is popular. The large

member of wires required for a star configuration can be expensive and

confusing. The loop of wire required for a ring topology is also difficult to

connect.

6.18 Media Access Methods

Once the topology of a particular network is chosen, a user faces the

problem of accessing a particular node in a multi-LAN multi mode network. A

common and important method of differentiating between different LAN types is

to consider their media access methods. Since there must be some method of

determining which node can send a message, this is a critical area that

determines the efficiency of the LAN. The common methods used in current

LANs are shown in Fig. 6.24.

Fig. 6.24 Different Media Access Methods

6.18.1 CSMA/CD (Collision Sense Multiple Access/Collision Detection)

This method is a first-come-first-served media accesses method. It

operates in a similar manner to polite human communication. A person listens

before he speaks allowing anybody else who is already speaking to finish. If

two persons start to speak at the same time, both stop a little later and after a

lapse of time, one of them starts to speak. Thus in this method of conversation,

one first ensures that the other person is not speaking before starting to speak.

In this media access method, which is in fact a contention based access

method, the operation is similar. The first node to seek access when the

network is idle will be able to transmit. Contention is at the heart of the Collision

Sense Multiple Access/Collision Detection (CSMN/ CD).

In this method, if two nodes start talking and detect a collision they will

stop, wait for a random time and then start again. The collision detection logic

ensures that more than one message on the channel will simultaneously be

detected, and transmission from both ends, eventually stopped. The system is

a probabilistic system, since access to the channel cannot be ascertained in

advance.

6.18.2 CSMA/BA (Collision Sense Multiple Access/Bitwise Arbitration)

If two nodes start talking at the same time, they will stop and use their

node addresses to determine which one goes first.

6.18.3 Master-Slave-One

Device One in the network is the master and is the only one that may

start communication. Slave devices will only respond to requests from the

master.

6.18.4 Token Passing

Token passing is a deterministic media-access method in which a token, or

permission to talk, is passed from node to node, according to a predefined

sequence. A token is a special packet, or frame, consisting of a signal

sequence, which cannot be mistaken for a message. At any given time, the

token can be available or in use. When an available token reaches a node,

that node can access the network for a maximum predetermined time, before

passing the token on.

This deterministic access method guarantees that every node will get

access to the network within a given length of time, usually in the order of a few

milliseconds. This is in contrast to a probabilistic access method (such as

CSMA/CD), in which nodes check for network activity when they want to

access the network, and the first node to claim the idle network gets access to

it. Because each node gets its turn within a fixed period, deterministic access

methods are more efficient on networks that have heavy traffic. With such

networks, nodes using probabilistic access methods spend much of their time

competing to gain access and relatively little time actually transmitting data

over the network. Network architectures that support the token passing access

method include Token Bus, ARC net, FDDl, and Token Ring.

To transmit, the node first marks the token as "in use", and then transmits

a data packet, with the token attached. In a ring topology network, the packet is

passed from node to node, until the packet reaches its destination. The

recipient acknowledges the packet by sending the message back to the sender,

who then sends the token on to the next node in the network.

In a bus topology network, the next recipient of a token is not necessarily

the node that is closest to the current token passing node. Rather, the next

node is determined "by some predefined rule. The actual message is broadcast

on to the bus for all nodes to "hear". For example, in an ARC net or token bus

network, the token is passed from one node to the node with the next lower

network address. Networks that use token passing generally have some

provision for setting the priority with which a node gets the token. Higher-level

protocols can specify that a message is important and should receive higher

priority.

6.19 Open System Interconnection (OSI) Network Model

The Open System Interconnection (OSI) model was developed as a tool

to describe the various hardware and software parts found in a network in a

systems. It is most useful for explaining a successful network application. The

model contains seven layers, with the hardware at the bottom, and the software

at the top. A system has been shown in Fig. 6.25 in which computer # 1 is an

originator computer and computer #2 is an application computer.

Fig. 6.25 A System Containing Two Computers

A message originating in an application programme in computer #1 is

sent to the application in computer #2. This message has to travel through

those seven layers. The name of each layer and its basic action have been

tabulated and explained in Table 6.9.

Table 6.9 Names and Action of OSI Layers

Name of Layer Basic Action

Application This is level software on the computer.

Presentation Translates application requests into network operations.

Session This deals with multiple interactions between computers.

Transport Breaks up and recombines data into small packets.

Network Network address and routing added to make frame.

Data link The encryption for many bits, including error correction

added to a frame.

Physical and

interconnecting medium

The voltage and timing for a single bit in a frame.

The wires or transmission medium of the network.

6.19.1 Application Layer

This is the layer where the user programme resides. On a computer, this

might be a web browser, or a ladder logic programme on a PLC.

6.19.2 Presentation Layer

This layer acts as an application interface so that syntax, formats and

codes are consistent between the two-networked machines. This layer also

provides subroutines that the user may call to access network functions, and

perform functions such as encryption, compression and conversion of data

from one form to another.

6.19.3 Session Layer

This layer establishes the connection between applications. The session

layer deals with issues that go beyond a single block of data. In particular, it

deals with resuming transmission if it is interrupted or corrupted. It also

enforces dialogue rules, which specify the order and speed of data transfer

between a sender and a receiver. For example, the session layer would control

the flow of data between an application and a printer with a fixed buffer, to

avoid buffer overflows.

6.19.4 Transport Layer

This layer is essentially an interface between the processor and the

outside world. It generates addresses for session entities and ensures that

packets of data have been sent or received. The transport layer will divide

small amounts of data into smaller packets, or recombine them into one larger

piece. This layer also checks for data integrity, often with a checksum.

6.19.5 Network Layer

This layer performs accounting, addressing and routing functions on

messages received from the transport layer. If the message is lengthy, this

layer will break it up and sequence it over the network. This layer also uses a

network routing table to find the next node on the way to the destination

address.

6.19.6 Data link Layer

This layer establishes and controls the physical path of communication

from one node to the next, with error detection. This layer performs Media

Access Control (MAC) to decide which node can use the media and when. The

rules used to perform these functions also are known as protocols.

6.19.7 Physical Layer

This layer is the electrical and mechanical definition of the system. It

describes items such as voltage levels and timing for the transmission of single

bits. This layer does not add anything to the message frame. It simply converts

the digital message received from the data link layer into a string of ones and

zeros represented by a signal on the media. One example is RS 485, where a

binary 1 is represented by a mark or off state and a binary zero is represented

as a space or on state. A mark is a negative voltage between the terminals on

the generator while a space is a positive voltage. The interface or

interconnecting media can be categorised by mechanical, electrical, functional

and procedural aspects. Mechanical specification defines types of connectors

and the number of pins. Electrical specifications define line voltages and

waveforms as well as failtre modes and effects. Functional specifications

include timing, data, control and signal grounds, and which pins the functions

are to use. The procedural interface specifies how signals are exchanged. Fig.

6.26 shows how the originator computer and the application device are

interconnected through the OSI layers.

In the world of instrumentation this OSI model as shown in Fig. 6.26 is

often simplified to use only three layers as shown in Fig. 6.27

(i) Layer 1 Physical layer

(ii) Layer 2 Data link layer

(iii) Layer 3 Application layer

This simplifies the operation of the overall system significantly. There is

another layer mentioned in the three layer model entitled user layer. This is not

part of the OSI model but is a critical part of the overall system.

Fig. 6.27 Structure of Simplified OSI Model

Table 6.10 shows how these layers are applied in the standard interfaces.

Table 6.10 Applications of OSI Layers

Name of standard interface Example of a layer

RS 232 and RS 485

Modbus protocol

Ethernet

HART smart instrumentation protocol

Profibus and foundation bieldbus

Physical layer

Data link layer

Physical and data link layers

Physical, data link and application layers

Physical, data link and application layers

6.20 Network Components

In designing a network, hardware is needed. Table 6.11 gives a

description of most of the hardware needed in the design of networks.

Table 6.11 Network Components and their Uses

Name of component OSI layer Purpose

Computer Network enabled equipment

Network interface

hardware

Network interface may already be built into the

computer/PLC/sensor/etc.

Media Interconnecting

media

The physical network connection between network

nodes. 10baseT (twisted pair) is the most popular. It

is a pair of twisted copper wires terminated with an

RJ-45 connector. 10base2 (thin wire) is a thin-

shielded coaxial cable with BNC connectors.

10baseF (fiber optic) is costly, but signal

transmission and noise properties are very good.

Repeater Physical layer These accept signals and retransmit them so that

longer networks can be built.

Hub/Concentrator Interconnecting

media

A central connection point that network wires will be

connected to. It will pass network packets to local

computers, or to remote networks if they are

available.

Router Network layer This will isolate different networks, but redirect traffic

to other LANs.

Bridges Ata link layer These are intelligent devices that can convent data

on one type of network, to data on another type of

network. They can also be used to isolate two

networks.

Gateway Application

layer

A gateway is a full computer that will direct traffic to

different networks, and possibly screen packets.

They are often used to create firewalls for security.

6.21 Control Network Issues

A wide variety of networks are commercially available, and each has its

own particular strengths and weaknesses. The differences arise from the basic

designs. One simple issue is the use of the network to deliver power to the

nodes. Some control networks will also supply enough power to drive some

sensors and simple devices. This can eliminate separate power supplies, but it

can reduce the data transmission rates on the network. The use of network

taps or tees to connect to the network cable is also important. Some taps or

tees are simple passive electrical connections, but others involve sophisticated

active tees that are costlier, but allow longer networks.

The transmission type determines the communication speed and noise

immunity. The simplest transmission method is baseband, where voltages are

switched off and on to signal bit states. This method is subject to noise, and

must operate at lower speeds. RS 232 is an example of baseband

transmission. Carrierband transmission uses Frequency Shift Keying (FSK),

that will switch a signal between two frequencies to indicate a true or false bit.

This technique is very similar to Frequency Modulation (FM) radio where the

frequency of the audio wave is transmitted by changing the frequency of a

carrier frequency to about 100MHz. This method allows higher transmission

speeds, with reduced noise.

Broadband networks transmit data over more than one channel by using

multiple carrier frequencies on the same wire. This is similar to sending many

cable television channels over the same wire. These networks can achieve

very large transmission speeds and they also guarantee real time network

access.

6.22 Advantage of a Standardised Industrial Network

The advantages of using standardised networks in an industrial

environment are discussed in the subsequent sections.

6.22.1 Open Systems

It is difficult and costly to integrate systems with instrumentation from

several vendors because of the multitude of communication protocols. With

standard protocols, devices from many suppliers can coexist on the same

network and communicate with one another.

6.22.2 Cost Reduction in Wiring

Many systems still use 4-20mA analog instrumentation, requiring

extensive point-to-point wiring. Standard networking incurs lower installation

costs.

6.22.3 Increased Information Need

In the present day industrial environment, engineers are required to

gather more information about their processes and the instrumentation

connected to the processes.

Traditional 4-20mA instrumentation provides only one value, the process value.

On a digital network, instruments can provide maintenance and diagnostics

information for better tracking of instrument performance. If these are

standardised, efficient control of a process can be achieved.

6.22.4 Intelligent Devices

Now-a-days instruments have been developed with more intelligence to

satisfy consumers' demands for more functionality at lower costs. The

increased information available with a digital network is necessary for

capitalising on the extra capabilities made possible by intelligence in the

devices. Standardisation in information collection, transmission and control

helps in optimising the efficiency of a process.

On the basis of the points discussed above, it is clear that the use of a

standard network enhances the reliability and efficiency operation of an

industry.

6.23 Industrial Network

In industry, there are many networks in use and under development today.

Different types of networks exist because there is a wide range of industrial

process and manufacturing applications that use digital communications.

6.24 Bus Network

To accommodate efficient communication in a complicated process,

different types of bus networks are devised. For overall enhancement of

efficiency of an industrial process, these bus network configurations play an

important role. Fig. 6.28 shows a block diagram of the total bus network of a

complete process plant. Process I/O devices are connected to a process bus

network and discrete I/O devices are connected to a device bus network, both

of which are connected to a LAN through a PLC. Remote I/O devices send their

information through PLCs to the same LAN, which is interfaced to the

information network through supervisory PLCs. This information network is

connected to the overall plant computing system. Fig. 6.29 lists some of these

networks and their bus classification.

6.25 Device Bus Network vs. Process Bus Network

Device bus networks that include discrete devices, as well as small analog

devices, are called byte-wide bus networks. These networks can transfer

between 1 and 50 or more bytes of data at a time. Device bus networks that

only interface with discrete devices are called bit-wide bus networks. Bit-wide

networks transfer less than 8 bits of data from simple discrete devices over

relatively short distances.

The size of the information packet has an inverse effect on the speed at

which data travels through the network. Therefore, since device bus networks

transmit only small amounts of data

Fig. 6.28 Block Diagram of a Total Bus Network

at a time, they can meet the high-speed requirements for discrete

implementations. Conversely, process bus networks work slower because of

the large size of their data packets, so they are more applicable for the control

of analog I/O devices, which do not require fast response times. The

transmission speeds for both types of I/O bus networks can be as high as 1 to

2.5 megabits per second.

Fig. 6.29 Different Types of Industrial Networks

Since process bus networks can transmit several hundred bytes of data at

a time, they are suitable for applications requiring complex data transmission.

Two simple examples may be cited in support of the above discussions. In a

conveyor belt of a batch process unit, proximity sensors are used which can be

networked together to control the movement of products on the belt, as well as

to provide simple diagnostic information. Since a proximity sensor only

transmits an ON/OFF signal, it can be used to indicate the location of a product

on the belt through a signal accommodated in a few bits of data. The same

may also be used to indicate the condition of the belt by addition of a few bits.

Another example is of an intelligent pressure transmitter fitted in the pipeline of

a fluid. This transmitter, in addition to sending information about the pressure of

the fluid, may also send information about temperature and flow rate of the fluid

through the pipe. All this information requires a large data packet to be

transmitted. This sensor interface, therefore, requires a process bus network.

These simple examples justify the necessity of a wide variety of industrial

communication networks. These networks demand sophisticated protocols

rather than the simple command sets, which are commonly used with typical

serial instruments for efficient operation of the communication system.

Dedicated hardware and software drivers are required to provide robust

connections between computer platforms and each of these networks. Options

for using these networks today include DOE servers and DLL function libraries

for the Windows environment. Although anyone network may not satisfy all

industrial networking requirements, these buses will bring a more standardised

interconnection between computers and the devices used in industrial

automation applications.

6.26 Controller Area Network (CAN)

The Controller Area Network is a serial bus growing in popularity as a

device level network. CAN was developed by Bosch to cope with the needs of

in-vehicle automotive communications. It was further developed to provide a

digital serial bus system to connect controllers. CAN has been standardised

internationally (ISO DIS 11898 and ISO DIS 11519-2) and is already available

in a number of silicon implementations. The CAN protocol can detect and

correct transmission errors caused by electromagnetic interference. In addition

the network itself is relatively easy to configure and can perform centralised

diagnostics.

There are now many examples of CAN being the basis for networks used

in industrial manufacturing applications. CAN is particularly well suited to

networking smart I/O devices, as well as sensors and actuators, either in a

single machine or in a plant. Several industrial devicebus systems have been

built upon CAN. Allen-Bradley developed DeviceNet, a CAN-based protocol

now maintained by the Open DeviceNet Vendor's Association. Other such

industrial networks include CANopen, developed by CAN in Automation (CiA)

and the Smart Distributed System (SDS), developed by Honeywell Microswitch.

CAN is a communications protocol specification that defines parts of the OSI

physical and data link layers. CAN does not specify the entire physical layer or

the medium upon which it resides, or the application layer protocol used to

move data.

6.26.1 Characteristics of CAN Protocol

The OSI seven layer communication model for CAN protocol is shown in

Fig. 6.30, and the basic characteristics of CAN protocol are shown in Table

6.12.

Fig. 6.30 OSI 7-Layer Communications Model for CAN

Table 6.12 Characteristics of CAN Protocol

CAN communications are performed in a unit cal\ed a frame, which may

have a length of up to 8 bytes.

Access to the CAN network is carried out using a method called

nondestructive bitwise arbitration. In this system, when a CAN node

wants to send a frame, it waits for the bus to become idle, it then starts

its frame with an arbitration identifier (10). Because of the underlying

physical layer, a dominant bit always overrides any recessive bit. As a

node is writing its bits to the bus, it also reads the bus to determine

whether the bit on the bus is different from the bit written by the node. If

the bits are different, the node stops its write because some other node

has higher priority to the bus. Thus, the arbitration ID determines the

priority of messages on the bus, with lower IDs having higher priority.

CAN provides extensive error correction, including bit monitoring

(comparing transmitted bits to be received), bit stuffing, CRC checksum,

acknowledgement by all receivers, frame check (verify length),

automatic retry, and fault confinement (defective devices automatically

shut off).

Typical data rate are 125kbps to 1 Mbps, depending upon the distance

over which the network is operating. The allowable distance ranges from

40m at 1 Mbps to 500m at 125kbps.

Industrial protocols built upon CAN add further specifications in the

areas of wiring types, connectors, diagnostics indicators, configuration

switches, and hot-swapping capability.

CAN bus covers the first two layers of the OSI model. The network has a

bus topology and uses bit wide resolution for collisions on the network

i.e., the lower the network identifier, the higher the priority for sending.

6.26.2 CAN Data Frame and Communication

In a CAN network, the transmitted messages are called frames. The CAN

protocol works with two frame formats, the essential difference between two

formats is the length of the arbitration 10. In the standard frame format, the

length of the ID is 11 bits, in the extended frame format, the length of the 10 is

29 bits. The four different frame types include: (1) data (2) error (3) remote and

(4) overload.

The data frame begins with a start bit. This is then followed with a

message identifier. The ready to receive it bit will be set by the receiving

machine. If the receiving machine does not set this bit the remainder of the

message is aborted, and the message is resent later. While sending the first

few bits, the sender monitors the bits to ensure that the bits that are sent are

heard in the same way. If the bits do not agree, then another node on the

network has tried to write a message at the same time - there was a collision.

The two devices then wait for a time period based on their identifier and then

start to resend. The second node will then detect the message, and wait until it

is done. The next 6 bits indicate the number of bytes to be sent, from 0 to 8.

This is followed by two sets of bits for a Cyclic Redundancy Check (CRC) error

checking, this is a checksum of earlier bits. The receiving node sets the next bit

ACK slot if the data was received correctly. If there was a CRC error this bit

would not be set, and the message would be resent. The remaining bits end

the transmission. The end of frame bits is equivalent to stop bits. There must

be a delay of at least 3 bits before the next message begins.

6.26.3 CAN Error Detection and Confinement

One of the most important and useful features of CAN is its high reliability,

even in extremely noisy environments. CAN provides a variety of mechanisms

to detect errors in frames. A frame with an error is retransmitted until it is

received successfully. CAN also provides an error-confinement mechanism

that is used to remove a malfunctioning device from the CAN network when a

high percentage of its frames result in errors. Error confinement prevents

malfunctioning devices from disturbing the overall network traffic.

6.27 DeviceNet

Originally developed by Allen Bradley, DeviceNet is managed by the

Open DeviceNet Vendors Association (ODVA http://www.odva.org).an

independent supplier organization. DeviceNet is a low-level network designed

to connect industrial sensors and actuators to higher level devices like

controllers. DeviceNet focuses especially on the interchangeability of low-cost,

simple devices often used in manufacturing applications - such as limit

switches, photoelectric sensors, motor starters, bar code readers, variable

frequency drives, and operator interfaces. One goal of DeviceNet was to

achieve the same level' of interchangeability for 120/240V AC and 24V DC

discrete devices using digital communications as is possible with hardwired

I/O.

This network is made noise resistant and robust. In this network, the PLC

chassis can be eliminated and the network can be connected directly to the

sensors and actuators. This will reduce the total amount of wiring by moving

the I/O points closer to the application point. This can also simplify the

connection of complex devices, such as HMls.

6.27.1 Physical Layer Features

Table 6.14 shows the key parameters of data rates and wire lengths for

the DeviceNet physical layer.

Table 6.14 DeviceNet Data Rates and Wire Lengths

Data Rate Trunk Distance

Drop Length

Maximum Cumulative

125kb/s 500m (I 640ft) 6m (20ft) 156m (512ft)

250kb/s 250m (820ft) 6m (20ft) 78m (256ft)

500kb/s 100m (328ft) 6m (20ft) 39m (I 28ft)

Key characteristics of the DeviceNet physical layer are as shown in Table 6.15.

Table 6.15 Characteristics of DeviceNet Physical Layer

Basic trunkline-dropline physical topology.

Separate twisted-pair buses for both signal and power distribution with

signal and power carried in the same cable.

Optional opto-isolated design so that externally powered devices can

share the bus cable with bus-powered devices.

Hot insertion of devices without removing power from the network.

6.27.2 Communication Features

DeviceNet builds on the CAN protocol. Using the OSI model, CAN

specifies only portions of the physical layer and data link layer (layers 1 and 2),

while DeviceNet adds the remainder of these layers, plus the media layer and

application layer (layers 0 and 7). Two way communications inputs and outputs

allow diagnosis of network problems from the main controller. Fig. 6.31 shows

the OSI model of DeviceNet and the relationship between the communications

stack for DeviceNet and CAN.

Fig. 6.31 DeviceNet Communications Stack including Contribution of CAN

Protocol

As a general statement, CAN defines the form of data movement while

the DeviceNet Application Layer defines the meaning of the data moved.

Some specific characteristics of DeviceNet communications are shown

in Table 6.16.

Table 6.16 Specific Characteristics of DeviceNet

Prioritised, peer-to-peer communication based on the nodestructive

bitwise arbitration scheme of CAN protocol.

Up to 64 node addresses on a single network.

Lengths of 500m/250m/100m for speeds of 125kbps/250kbps/500kbps

respectively.

Addressing includes peer-to-peer, multicast, master/slave, polling or

change of state.

A single bus cable that delivers data and power.

Producer-consumer model for data transfer.

Data packet size of 0-8 bytes.

Devices can be added/removed while power is on.

6.27.3 A DeviceNet Network

The network cable is important for delivering power and data. The two

basic types are thick and thin trunk line. The cables may come with a variety of

connections to devices such as: bare wires, unsealed screw connector,

usealed mini-connector, sealed micro-connector, vampire tapes. Each node on

the network will have its own address between 0 and 63.

6.28 ControlNet

ControlNet is complimentary to DeviceNet. It is also supported by a

consortium of companies, (http://www.controlnet.org), who also conduct some

projects in cooperation with the DeviceNet group. The standard is designed for

communication between controllers, and permits more complex messages than

DeviceNet. It is not suitable for communication with individual sensors and

actuators, or with devices off the factory floor. ControlNet is a more complicated

method than DeviceNet.

Some of the basic features of this network are given in Table 6.17.

Table 6.17 Specific Characteristics of ControlNet

Multiple controllers and I/O on one network.

Deterministic.

Data rates up to 5Mbps.

Multiple topologies (bus, star, tree).

Up to 99 nodes with addresses, up to 48 without a repeater.

Multiple media (coax, fiber, etc.).

Data packets up to 510 bytes. Unlimited I/O points.

Maximum length may be: 1000m with coax at 5Mbps - 2 nodes; 250m

with coax at 5Mbps - 48 nodes; 500m with coax at 5Mbps with

repeaters; 3000m with fiber at 5Mbps, 30Km with fiber at 5Mbps and

repeaters.

5 repeaters in series, 48 segments in parallel.

Devices can be removed while network is active.

Devices powered individually (no network power).

This control network is unique because it supports a real-time messaging

scheme called Concurrent Time Domain Multiple Access (CTDMA). The

network has a scheduled (high priority) and unscheduled (low priority) update.

When collisions are detected, the system will wait for a time of at least 2ms, for

unscheduled messages. But scheduled messages will be passed sooner,

during a special time window.

6.29 Ethernet

Ethernet was originally developed by Xerox, Digital, and Intel in the

1970s, but now it is under the IEEE Standard 802.3. It has become the least

expensive, most popular high-speed LAN alternative in use. There are several

common definitions, which help to describe the various types of Ethernet media

as shown in Table 6.18.

Table 6.18 Different Types of Ethernet Media

Type Description

Standard Ethernet It is a thick coaxial cable and is called Thick Ethernet. It

can run for as

(10Base5) much as 500m (1,640ft.), without using a repeater.

Connection to this cable is carried out through a vampire

clamp that has a 15-pin connector called an AUI port on

the other end.

Thin Ethernet

(10Base2)

It uses a less expensive, thin coaxial cable and connects to

nodes by way

of a T type BNC connector.

Twisted-pair Ethernet

(10BaseT) It uses telephone wiring and hence is very economical.

Standard RI-45 connectors are used in the system. This

type of Ethernet is wired in a star configuration and

requires a hub.

Fiber-optic Ethernet

(10BaseF)

It is used to extend Ethernet segments to distances of

1.2km.

Fast Ethernet is essentially the same as the original Ethernet except that

the transfer rates at 100 Mbps., are 10 times faster Another difference is that

Fast Ethernet includes a mechanism for auto-negotiation of the media speed.

This means dual-speed Ethernet interfaces can be installed and run at either

10 or 100Mbps. There are three forms of Fast Ethernet as defined in the IEEE

Standard 802.3. These are shown in Table 6.19.

Table 6.19 Types of Fast Ethernet

Type Description

100BaseT4

100BaseTX

100BaseFX

It uses four pairs of telephone-grade twisted-pair wire.

It uses two pairs of data grade twisted-pair wire.

It uses two stands of fiber-optic cable.

6.29.1 Protocol

Ethernet defines only the physical layer and not the protocol. Ethernet

defines the data link layer and functions at layers 1 and 2 of the OSI model as

shown in Fig. 6.32.

6.30 Proprietory Network

Allen Bradley has developed the Data Highway II (DH +) network for

passing data and programmes between PLCs and to computers. This bus

network allows up to 64 PLCs to be connected with a single twisted pair in a

shielded cable. Token passing is used to control traffic on the network.

Computers can also be connected to the DH+ network, with a network card to

download programmes and monitor the PLC. The network will support data

rates of 57.6Kbps and 230Kbps.

Fig.6.32 OSI Model and its Relation to Ethernet

6.31 Smart Distributed System

The Smart Distributed System, developed by Honeywell's Micro Switch

Division, is an advanced bus system for intelligent sensors and actuators. This

CAN-based network is useable at both the control and device levels. This

optimises machine applications. Combining the power of CAN technology,

computer control and .intelligent I/O devices, the Smart Distributed System

provides a truly integrated solution.

A Smart Distributed System can interface up to 64 nodes, with a

maximum of 126 addresses, over a single 4-wire cable. These intelligent

sensor and actuator devices do more than just turn on and off. Smart

Distributed System devices have advanced device-level functions, system and

device diagnostics. The Smart Distributed System can interface with both PC

controllers and PLCs. It is ideally suited for industrial applications replacing

older, discretely wired sensors and actuator systems.

6.32 Interbus-S

Interbus-S, introduced in 1984 by Phoenix Contact, is a ring-based,

distributed device network for manufacturing and process control. An Interbus-

S system consists of a controller board installed into a PLC or computer (PC,

VME, etc.) that communicates to a variety of I/O devices. .

6.33 Seriplex Bit-wide Device Bus Network

The Seriplex device bus network can connect up to 510 field devices to a

PLC in either a master/slave or peer-to-peer configuration. The Seriplex

network is based on the Application Specific Integrated Circuit, or ASIC chip,

which must be present in all I/O field devices that connect to the network. I/O

devices that do not have the ASIC chip embedded in their circuitry (i.e., off-the-

shelf devices) can connect to the network via a Seriplex I/O module interface

that contains a slave ASIC chip. The ASIC I/O interface contains a 32 built-in

Boolean logic function that is used to create the logic necessary for the

communication, addressability, and intelligence to control the field devices

connected to the network bus. Fig. 6.33 shows a Seriplex bus network with a

controller. A Seriplex network can span distances of up to 1,515m (5,000ft.), in

a star, loop, tree, or multidrop configuration. This bit-wide bus network can also

operate without a host controller. Unlike the ASI network, the Seriplex device

bus network can interface with analog I/O devices; however, the digitised

analog signal is read or written one bit at a time in each scan cycle.

Fig. 6.33 Seriplex Bus Network with a Controller

6.34 AS-I Interface

AS-I, the abbreviation of Actuator/Sensor Interface, is a connection system

for the lowest process level in automation systems. This system uses a single

electrical cable called AS-I cable. Using the AS-I cable and the AS-I master, the

simplest binary sensors and actuators can be connected to the control devices

at the field level via AS-I modules. AI-I is the SIMATIC product name for this

AS-I technology, which has been proposed for international standardisation.

Under the name AS-interface, Siemens produces master interface modules for

industrial PCs and programmable controllers. The range of available master

interface modules is being developed continuously.

The main characteristics of the AS-interface are presented in Table 6.20.

Table 6.20 Main Characteristics of AS Interface

AS-interface is optimised for connecting binary sensors and actuators.

AS-I cable is used for data exchange between the sensors/actuators and

a master and between power supply and sensors and actuators.

Simple and cost-effective wiring having high flexibility, which can be

easily installed using the "penetration" technique.

It has a fast reaction time, e.g., the AS-I master requires a maximum of

5ms for cyclic data exchange with up to 31 stations.

Stations on the AS-I cable can either be sensors/actuators with an

integrated AS-I connector or AS-I modules to which up to four

conventional binary sensors/actuators can be connected.

With AS-I modules, up to 124 actuators/sensors can be operated on the

AS-I cable.

A simple 2-wire cable without shielding or twisting can be used to

transfer both the data and the power supply.

Tree structure network consists of a cable of length up to 100m. The

"tree structure" of the AS interface allows any point on a cable section to

be used as the start of a new branch. Loops are not permitted. The total

length of all subsections can be up to 100m.

AS-I uses constant message lengths. Complicated procedures for

controlling transmission and identifying message lengths or data formats

are not required. This makes it possible for a master to poll all connected

slaves within a maximum distance of 5ms and to update the data, both

on the master and slave.

6.34.1 AS-I-Open Standard for Network Systems at the Process Level

Eleven companies active in the field of binary sensors and actuators

compiled the electrical and mechanical specifications for AS-I. The

specifications are available for companies who are interested in this field. This

makes AS-I an open standard independent of the manufacturer. With the AS-

interface, Siemens provides a system complying with the AS-I standard. The

"Association for Promoting Interfaces with Bus Capability for Binary Actuators

and Sensors" (AS-I Association) is responsible for promoting the application

and dissemination of the AS-I system; in particular the specification,

standardisation, certification and general user information.

6.34.2 AS-Interface/ AS-I System Operation

The AS-interface/AS-I system operates in two steps, namely, (j) Master-

Slave Access Technique and (ij) Electronic Address Setting. The AS-interface

is a "single master system" having only one master per AS-I network, which

controls the data exchange. It sends data to all slaves one after the other and

waits for a reply.

The address of the slave is its identifier, which can be set by using a

special programming and diagnostic device. Once the address for a particular

slave is set, it is stored permanently on the slave. When the data reaches a

slave, it checks whether it is meant for it by decoding the address to which the

master has sent, and then only decodes the actual data.

6.34.3 Operating Reliability and Flexibility

The transmission technique used (current modulation) guarantees high

operating reliability. The master monitors the voltage on the cable and the

transferred data. It detects transmission errors and the failure of slaves and

sends a message to the PLC. The user can then react to this message.

Exchange or addition of slaves during normal operation does not impair

communication with the other stations.

6.35 Foundation Fieldbus

Foundation Fieldbus is a sophisticated industrial network specially meant

for complex distributed control in process plants. The Fieldbus specifications

were developed by the Fieldbus Foundation, a group representing over 80% of

the world's suppliers of industrial automation systems, devices, and services.

Foundation Fieldbus is based upon existing standards and is supported by the

research and development results of the International Society for Measurement

and Control (lSA), the International Electrotechnical Committee (lEC), Process

Fieldbus (Profibus), a German national standard, Factory Instrumentation

Protocol (FIP), a French national standard, and Highway Addressable Remote

Transducer (HART), a widely-used process instrumentation protocol.

The Foundation Fieldbus has two speeds, namely, (i) a low speed of

31.25kbaud, referred to as HI and (ii) a high speed of 1Mbaud or 2.5Mbaud

(depending upon the ac current or dc voltage mode), called H2. A summary of

the characteristics of H1 and H2 is given in Table 6.21.

Table 6.21 Characteristics of H1 and H2

Low speed bus (H1) 31.25 kbaud 3-32 devices that are not bus powered.

2-12 devices that are bus powered.

2-6 devices that are bus powered in an

Intrinsically Safe (IS) area.

High speed bus (H2) 1 mbaud 27 devices, ac current mode (16khz

frequency) powered from a bus in an

Intrinsically Safe (IS) area.

1 mbaud 127 devices, dc voltage mode, not

powered from bus, and no Intrinsically

Safe (IS) area.

2.5 mbaud 127 devices, dc voltage mode, not

powered from a bus, and no Intrinsically

Safe (IS) area.

The Foundation Fieldbus communication protocols are based on the OSI

seven layer communications model. It has optimised the OSI architecture for

process control by removing the middle layers that are generally associated

with non-time critical applications. A comparison of the Foundation Fieldbus

model and the OSI 7-layer communication model is given Fig. 6.34.

Fig. 6.34 Foundation Fieldbus Model Compared to the OSI 7 Layer

Communication Model

As it is seen from the figure, the Foundation Fieldbus system consists of 0)

the physical layer [Layer-l of OSI model], (ii) the communication "stack,"

[consisting of Layer 2 (data link layer)] and Layer 7 (application layer) of the

OSI model] and (iii) user application.

6.35.1 Physical Layer (Layer 1)

The physical layer is based upon standards created by ISA/IEC (ISA

S50.02-1992, IEC 1158-2, which also specifies the capability for intrinsically

safe operation.

6.35.2 Communication Stack (Layers 2 and 7)

The communication stack portion of the Fieldbus process bus network

consists of layer 2 (the data link layer) and layer 7 (the application layer). The

data link layer controls the transmission of messages onto the Fieldbus

through the physical layer. The layers above the physical layer are together

often referred to as the "stack" for the Fieldbus. There are certain specific

characteristics of the data link layer as mentioned in Table 6.22, which are the

key to distributed real-time control actions.

Table 6.22 Characteristics of Data Link Layer

Data link layer is based on a token passing protocol

Link Active Scheduler (LAS) is a centralised device that acts as the

arbitrator of the bus.

LAS executes a schedule that makes deterministic communication

possible.

LAS distributes time to the network to permit all devices to share the

same sense of time.

The application layer contains the Fieldbus Messaging Specification

(FMS) standard, which encodes and decodes commands from the user layer.

The FMS is based on the Profibus process bus standard. Layer 7 also contains

an object dictionary, which allows the Fieldbus network data to be retrieved

either by a tag name or an index record.

The Fieldbus process network uses two types of message transmissions:

cyclic (scheduled) and acyclic (unscheduled). Cyclic message transmissions

occur at regular, scheduled times. The master network device monitors how

busy the network is and then grants the slave devices permission to send

network transmissions at specified times. Other network devices can listen to

and receive these messages if they are subscribers.

Acyclic, or unscheduled, messages occur between cyclic, scheduled

messages, when the master device sends an unscheduled informational

message to a slave device. Typically, acyclic messages involve alarm

acknowledgment signals or special retrieving commands designed to obtain

diagnostic information from the field devices.

6.35.3 User Layer (Layer 8)

The user layer implements the Fieldbus network's distributed control

strategy. It contains three key elements, namely, function blocks, device

description services, and system management. The user layer, a vital segment

of the Fieldbus network, also defines the software model for user interaction

with the network system.

(a) Function Block

The user layer defines "blocks" that represent the functions and data

available in a device. Rather than interface to a device through a set of

commands as commonly used with communication protocols, a Foundation

Fieldbus user interacts with devices through a set of blocks that define device

capabilities in a standardised way. Function blocks are the core components

with which a user specifies the behavior of a control system. Foundation

Fieldbus defines standard sets of function blocks. There is a set of 10 basic

controls and I/O functions as shown in Table 6.23.

Table 6.23 Control and I/O Functions of Foundation Fieldbus

Function Block Name Symbol

Analog Input

Analog Output

Bias

Control Selector

Discrete Input

Discrete Output

Manual Loader

Proportional/Derivative

Proportional/Integral/Derivative

Ratio

AI

AO

B

CS

DI

DO

ML

PD

PID

RA

The inputs and outputs of individual function blocks can be connected to

specify communication of data on the bus. Even more importantly, the

execution of a function block can be precisely scheduled. This is a key

capability of Foundation Fieldbus because it allows execution of control loops

directly over the network. The function blocks themselves reside in individual

devices but the overall scheduling of execution is specified and executed

across the network. Fig. 6.35 shows a simple control loop with three function

blocks – AI, PID, AO.

Fig. 6.35 A Feedback Control Loop with AI, PID and AO Function Blocks

The function blocks shown in Fig. 6.35 could be implemented on the

fieldbus in several different ways. The Al, PID, and AO could reside in separate

devices, such as a transmitter, loop controller and valve. Alternatively, the PID

itself could reside in the control valve. In the second case, there is no explicit

controller device. In either system, the user's view is the same - a series of

connected function blocks and an execution schedule. The second system,

however, shows the true potential of Foundation Fieldbus - distributed control,

where the control function exists in the field rather than being concentrated in

larger controllers.

(b) Device Description Service (DDS)

The user layer of the Foundation Fieldbus has another important feature

i.e. Device Description Service (DDS). A key objective for Foundation Fieldbus

is the ability to build systems comprising devices from a variety of

manufactures, and take full advantage of both the standard and unique

capabilities of every device. Function blocks go a long way in ensuring a

consistent model of a control system. From a system point-of-view, however, a

mechanism is needed to document, in a standard way, the types of functions

available in any given device. To achieve this end, Foundation Fieldbus

defines the Device Description (DD). This is a standardised description of the

functions available in a device. Using the DD, the host in a control system, for

example a Windows NT-based Man Machine Interface (MMI) can obtain the

information necessary to create the human interface for interacting with the

device to configure parameters, perform calibration and diagnostics, and other

functions. DD is a mechanism for describing the functions in a device. This is

the key to field bus interoperability. This can be contrasted to a more simplistic

and common approach to the problem of compatibility and interchangeability,

namely by specifying that only a given set of functions can be used in a device

to ensure that a given system can always talk to a new device. This would

severely restrict the ability of a device manufacturer to innovate by adding new

device features, and there would be a never ending contention about the

"right" set of functions upon which to standardise. With the DD, developers can

add new features and be confident that host systems can learn about and take

advantage of these features in standard way.

(c) System Manager

The system management portion of the user layer schedules the

execution of function blocks at precisely defined intervals. It also controls the

communication of all the Fieldbus network parameters used by the function

blocks. Moreover, the system manager automatically assigns field device

addresses.

6.36 Profibus

Profibus is the leading' open fieldbus system in Europe

(http://www.profibus.com). It is used worldwide in manufacturing, process

control and automation. Profibus is standardised in the German standard DIN

19245 and European fieldbus standard EN 50170. The Profibus User

Organization is engaged in continuous research and development on Profibus

technology.

Fig. 6.36 shows the hierarchy in a Profibus network. In the upper level

the host computer controls the overall system. It is connected to the application

range at cell level and field level through an information network TCP/IP

gateway. At cell level, Profibus FMS is networked with sensors, application

equipment and through PLCs to the field level bus e.g. Profibus DP, Profibus

FMS and sometimes through segment couplers to Profibus PA also. At the field

level, bus, sensors, actuators, transmitters, I/O etc. are connected.

Fig. 6.36 Profibus Network Hierarchy

The general features of Profibus are given in Table 6.24.

Table 6.24 General Features of Profibus

Straight bus topology.

Maximum of 126 nodes.

A token passing between up to three masters.

Length from 9600m/9.6Kbps with 7 repeaters to 500m/12Mbps with 4

repeaters. With fiber optic cables, the lengths can be over 80Km.

Fig. 6.37 shows the protocol architecture of Profubs protocols using the

OSI 7-Layer model. This technology can be used for both high-speed time-

critical data transmission between controllers and I/O, and complex

communications between PLCs. The Profibus family consists of three

compatible versions – DP, FMS, and PA. These three protocols have been

described briefly in the following sections.

Fig. 6.37 Protocol Architecture of Profibus

6.37 Profibus-DP

Profibus-DP uses layers 1 and 2, and the user interface of the OSI model

and layers 3 to 7 are not defined. The Direct Data Link Mapper (DDLM) allows

access between the user interface and layer 2. The user interface specifies

both application functions and device behavior. RS-485 and fiber-optic are

available physical media for Profibus-DP.

This is designed for high-speed, cost-effective communication between

industrial controllers and distributed I/O. Parallel signal transmission with 24V

or 0 to 20mA can be replaced. On such a network, central controllers, such as

PLCs or PCs, communicate with distributed field devices (such as I/O, drives

and valves) via a high-speed serial link. Most of the data communication with

these distributed devices is done in a cyclic manner.

6.38 Profibus-FMS

In Profibus-FMS, layers 1, 2, and 7 are defined. The application layer

consists of Fieldbus Message Specification (FMS), and Lower Layer Interface

(LLI). FMS contains the application protocol and provides the user with a wide

selection of communication services. LLI implements communication

relationship and provides FMS with device-independent access to layer 2.

Layer 2 (FDL Fieldbus data link) implements bus access control and data

security. Rs-485 and fiber-optic physical layers are available for Profibus-FMS.

In general, this is meant for communication primarily between

programmable controllers such as PLCs and PCs. However, its application

layers provided with a user communication service make it possible to access

variables, transmit programmes, and control programme execution, as well as

transmission of events. Profibus-FMS defines a communication model in which

distributed application processes can be unified into a common process by

using communication relationships.

6.39 Profibus-PA

Profibus-PA is specifically designed for process control and automation,

using the international fieldbus standard physical layer, IEC 1158-2, for bus-

powered sensors and actuators to be operated in 'intrinsically safe areas. It

uses the extended Profibus-DP protocol for data transmission. Profibus-PA

devices can be integrated in Profibus-DP networks using segment couplers.

The Profibus's medium access protocol is a hybrid communication method

that includes a token-passing protocol for use between masters, and a master-

slave protocol for communication between a master and a field device. Through

this hybrid medium access protocol, a Profibus network can function as a

master-slave system, a master-master system (token passing), or a

combination of both systems as shown in Fig. 6.38.

Fig. 6.38 Master-slave and Master-master Profibus Communication

6.40 Application of Profibus for Real PLC Communication

(a) Overcoming Language Barriers between Devices with FMS

Data transmission via a configured FMS connection is suitable for the

transmission of structured data between two Profibus nodes that support the

FMS standard.

The great advantage of this FMS protocol is that data structures can be

transferred in a neutral format, in other words in a format that is not dictated by

any particular end device. This data is then converted to the format required by

the end device. This means that one can communicate with all devices that

understand the FMS protocol. In the user programmes of the end devices, one

can therefore use "Device Language", for example Statement List for SIMATIC

S7/SIMATIC M7 PLCs and C for the PC application.

(b) FMS Interface and FMS Master System

Data is transmitted on an FMS connection when triggered by the user

programme. Special SIMATIC S7 Function Blocks (FBs) form the interface to

the user program on the SIMATIC S7 programmable controller. Function blocks

are available for the following tasks as shown in Table 6.25.

Table 6.25 Function Blocks in 51MATIC 57 Programmable Controller

Task Function Block (FB)

Read variable READ

Write variable WRITE

Report variable REPORT

Coordination ACCESS

General VFD services IDENTIFY

STATUS

On the Profibus, devices are divided into masters and slaves. The right of

access to the bus is known as tile token, and it is passed on from master to

master. The slaves can only react when requested to by a master. In terms of

the functionality of an FMS device, a further distinction is made between the

following:

FMS client: The FMS client requests a service; assuming that it is a

master on the Profibus.

FMS server: The FMS server provides the requested service; both a

master on the Profibus as well as a slave on the Profibus can act in the role of

a server.

An FMS master system is formed by all the devices with FMS

functionality on the Profibus Subnet. This means that several FMS masters can

access the same slaves.

In contrast to this system, with distributed peripheral I/Os (DP) there are

additional assignment criteria with which all, or a subset of the DP slaves on

the subnet can be assigned to a DP master. In other words, several DP master

systems are possible.

(c) Virtual Field Device (VFD)

A device operating on Profibus and complying with the FMS norm is

generally known as a Virtual Field Device (a field device with an open

communications interface).

Fig. 6.39 FMS Connection of an S7 VFD to any Device with an FMS Interface

The FMS services implemented on the Profibus Communication

Processor (CP) ensure that the data are converted from the PLC format to the

neutral FMS data format and vice-versa.

(d) Properties of the FMS Connection

An FMS connection allows programme-controlled communication between

two nodes on the Profibus with the following properties:

Data transfer is bi-directional, in other words, it is possible to send

and receive simultaneously on the FMS connection.

Data is transferred using the FMS services complying with the

standard. The services are negotiated between the communication

partners automatically when the connection is established. The user can

specify the services required on the Profibus Communication Processor

(CP) during configuration.

The data are transmitted on the FMS connection in the FMS format

as specified in the standard.

Depending on the services used on the FMS connection, a VFD

functions as the FMS client, as the FMS server or performs both roles.

FMS client: The FMS client requests a service; this assumes that

the device is a master on the Profibus.

FMS server: The FMS server provides a requested service; both

masters and slaves on the Profibus can act as servers.

(e) Tasks of the Profibus CP

The Profibus CP is responsible for the following tasks when handling the

data transfer on an FMS connection:

Receiving data from the Profibus, converting the data from the FMS

representation to the particular representation required by the device and

passing on the data to the user data area on the CPU.

Accepting data from the user data area of the CPU, converting the data

to the FMS representation and sending the data on the Profibus.

The Profibus CP must first be entered in the hardware configuration of the

PLC and must be connected to the subnet.

(f) The FMS Interface in the User Program

When the user writes the user programme, the user must start with

configuration of FMS connections. The FMS connections are established when

the Profibus CP starts up; the user programme is not involved in the way that

the connections are handled.

The return parameters on the FMS interface (FBs) give information on

the status of the FMS connection.

(g) Principle Writing, Reading and Reporting Data with Function

Blocks (FBs)

The Function Blocks (FBs), given in Table 6.26, are available for

handling communication on FMS connections for Siemens S7-300/400 PLCs.

Table 6.26 FBs in S7-300/400 PLCs

FB Function/Method of OperationsWRITE The user data referenced in the cell are converted to the FMS

representation and transmitted. The conversion is made

according to the variable description stored on the partner

and read during connection establishment. The data transfer

is confirmed by the FMS server.

READ The data area referenced by the FMS client in the job is

converted to the FMS representation in the FMS server and

transferred to the FMS client as the response.

The data is reconverted on the FMS client according to the

variable description read from the FMS server during

connection establishment.

REPORT The user data referenced in the call are converted to the FMS

representation on the FMS server according to the configured

variable description and transferred.

The user program does not receive confirmation of the data

transfer.

The data are reconverted on the FMS client according to the

variable description read from the FMS server during

connection establishment.

The schematic diagram in Fig. 6.40, illustrates how these function blocks

work; the arrows indicate the direction of flow of the user data.

The performance of control systems is no longer simply determined by

the PLCs, but is influenced to a great extent by the environment in which the

PLCs are located. Apart from plant visualization, operating and monitoring, this

also means a high performance open communication standard, which can

converse with the products of almost all vendors. In process automation,

complex control tasks are subdivided into smaller tasks with different

distributed systems. As a result, efficient communication between the

subsystems is an absolute necessity. Such structures have some unique

advantages, which are listed below.

1. Independent and simultaneous startup of the individual sections of a

plant.

2. Parallel processing by distributed automation systems.

3. Reduced load on individual processing units.

4. Increased plant availability since the rest of the system can continue

to operate if a subsystem fails.

Fig. 6.40 Service Request and Data Flow Between FMS Client and FMS

Server

This chapter gives a vivid picture of open communication standards for

devices and processes. In the next chapter, Programmable Logic Controller

based solutions of some industrial automation problems have been attempted.