T-034 - Networking.pdf

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ApplicationApplication EngineeringEngineering

T-034: Networking Application Manual

EnglishOriginal Instructions 4-2013 A044E523 (Issue 1


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Table of Contents

1. INTRODUCTION............................................................................................................................ 1

1.1 Overview ................................................................................................................................. 1

1.2 About This Manual.................................................................................................................. 2

1.3 Application Manuals................................................................................................................ 3

1.4 Safety...................................................................................................................................... 3

2. NETWORKING BASICS ................................................................................................................ 5

2.1 Overview ................................................................................................................................. 5

2.2 Simple Network Introduction................................................................................................... 5

2.3 Common Terminology............................................................................................................. 8

2.3.1 Protocols ...................................................................................................................... 8

2.3.2 Topology ...................................................................................................................... 9

2.3.3 Bus ............................................................................................................................... 9

2.3.4 Channel........................................................................................................................ 9

2.3.5 Media ......................................................................................................................... 10

2.3.6 Node........................................................................................................................... 11

2.3.7 Bandwidth .................................................................................................................. 11

2.4 Evolution of Networks ........................................................................................................... 11

2.4.1 Discrete Communication............................................................................................ 11

2.4.2 Serial Communication................................................................................................ 11

2.5 Early Protocols...................................................................................................................... 12

2.5.1 RS-232....................................................................................................................... 12

2.5.2 RS-485....................................................................................................................... 13

2.6 Addressing ............................................................................................................................ 14

2.7 Media Access Controls ......................................................................................................... 14

2.8 Determinism.......................................................................................................................... 15

2.9 Full-Duplex Switched Ethernet.............................................................................................. 15

2.10 Open Systems and Object Oriented Data Structures......................................................... 17

2.11 Multiple Networks in a Single System ................................................................................ 17

3. NETWORK DEVICES .................................................................................................................. 19

3.1 Overview ............................................................................................................................... 19

3.2 Input/Output Devices ........................................................................................................... 19

3.2.1 Digital Inputs .............................................................................................................. 20

3.2.2 Digital Outputs............................................................................................................ 20

3.2.3 Analog Inputs ............................................................................................................. 21

3.2.4 Analog Outputs .......................................................................................................... 23

3.3 Intelligent Devices................................................................................................................. 23

3.3.1 Network Interface Card .............................................................................................. 23

3.4 Network Management Devices............................................................................................. 24

3.4.1 Hub/Repeater............................................................................................................. 24

3.4.2 Switch......................................................................................................................... 24

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3.4.3 Bridge......................................................................................................................... 24

3.4.4 Routers....................................................................................................................... 25

3.4.5 Gateways ................................................................................................................... 25

3.5 Web Servers ......................................................................................................................... 26

4. NETWORK CHARACTERISTICS................................................................................................ 29

4.1 Overview ............................................................................................................................... 294.2 Network Topology ................................................................................................................. 29

4.2.1 Bus Topology ............................................................................................................. 30

4.2.2 Ring Topology ............................................................................................................ 30

4.2.3 Star Topology............................................................................................................. 31

4.2.4 Other Topologies........................................................................................................ 31

4.3 Bandwidth ............................................................................................................................. 31

4.4 Optimizing Network Speed ................................................................................................... 32

4.5 Distance ................................................................................................................................ 32

5. MEDIA CONSIDERATIONS......................................................................................................... 35

5.1 Overview ............................................................................................................................... 355.2 Cable Concepts .................................................................................................................... 35

5.2.1 Attenuation................................................................................................................. 35

5.2.2 Characteristic Impedance and Network Termination................................................. 35

5.2.3 Electromagnetic Interference ..................................................................................... 36

5.2.4 Crosstalk .................................................................................................................... 36

5.2.5 Shielding .................................................................................................................... 37

5.2.6 Insulating Jacket - Plenum vs. PVC .......................................................................... 38

5.3 Cable Types ......................................................................................................................... 38

5.3.1 Twisted Pair Cable..................................................................................................... 39

5.3.2 Fiber-Optic Cable....................................................................................................... 40

5.3.3 Coaxial Cable............................................................................................................. 405.4 Twisted Pair Cable Standards .............................................................................................. 41

5.4.1 NEMA Level IV Cable ................................................................................................ 41

5.5 Physical Layer Protocols ..................................................................................................... 41

5.5.1 Ethernet...................................................................................................................... 41

5.5.2 Modbus over Serial Line ............................................................................................ 42

5.5.3 PCCNet ...................................................................................................................... 43

5.5.4 LonWorks ................................................................................................................... 43

5.5.5 Wireless Media........................................................................................................... 44

6. PROTOCOLS............................................................................................................................... 47

6.1 Overview ............................................................................................................................... 476.2 Protocols ............................................................................................................................... 47

6.2.1 Proprietary vs. Open Protocols .................................................................................. 47

6.3 The OSI Reference Model .................................................................................................... 48

6.3.1 Layer 1 - Physical Layer ............................................................................................ 48

6.3.2 Layer 2 - Data Link Layer .......................................................................................... 50

6.3.3 Layer 3 - The Network Layer ..................................................................................... 52

6.3.4 Layer 4 - Transport Layer .......................................................................................... 55

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6.3.5 Application Layer Protocols ....................................................................................... 57

6.4 Ethernet Frame Construction................................................................................................ 57

6.5 The TCP/IP Model ................................................................................................................ 58

6.6 Common Protocols ............................................................................................................... 59

6.7 Data Link and Physical Layer Protocols ............................................................................... 59

6.7.1 Ethernet...................................................................................................................... 59

6.7.2 RS-232....................................................................................................................... 606.7.3 RS-485....................................................................................................................... 60

6.7.4 Application Layer Protocols ....................................................................................... 60

6.7.5 HyperText Transfer Protocol (HTTP) ......................................................................... 60

6.7.6 Simple Mail Transfer Protocol (SMTP) ...................................................................... 61

6.7.7 Simple Network Management Protocol (SNMP)........................................................ 61

6.7.8 Modbus ...................................................................................................................... 61

6.7.9 LonWorks ................................................................................................................... 61

6.7.10 BACnet..................................................................................................................... 62

6.7.11 Profibus .................................................................................................................... 62

6.7.12 Generic Object Oriented Substation Events (GOOSE) .......................................... 62

6.7.13 CAN (Controller Area Network) .............................................................................. 63

6.7.14 PCCNet .................................................................................................................... 63

6.8 Protocol Conversions............................................................................................................ 63

6.8.1 Ethernet Conversions................................................................................................. 64

6.8.2 Application Layer Protocol Conversions .................................................................... 64

7. MONITORING SYSTEMS............................................................................................................ 65

7.1 Overview ............................................................................................................................... 65

7.2 Purpose of Monitoring........................................................................................................... 65

7.2.1 System Reliability....................................................................................................... 65

7.2.2 Operating Cost........................................................................................................... 65

7.2.3 Agency Requirements................................................................................................ 66

7.3 Monitoring System Functions................................................................................................ 66

7.3.1 Status Display ............................................................................................................ 66

7.3.2 Alarm Notification....................................................................................................... 69

7.3.3 Data Logging.............................................................................................................. 69

7.3.4 Reporting.................................................................................................................... 72

7.4 Monitoring System Architecture............................................................................................ 72

7.4.1 SCADA Systems........................................................................................................ 73

7.5 Web-based Monitoring Systems........................................................................................... 75

7.5.1 Web Server Location ................................................................................................. 77

7.6 Which Monitoring System to Choose.................................................................................... 78

8. COMMUNICATION SYSTEMS IN POWER GENERATION APPLICATIONS ............................ 79

8.1 Overview ............................................................................................................................... 79

8.2 Web-based monitoring – Alarm Notification ......................................................................... 79

8.3 Wireless Web-based Monitoring........................................................................................... 80

8.4 Local Utility Service............................................................................................................... 80

8.5 Large Scada Networks.......................................................................................................... 81

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9. APPENDIX ................................................................................................................................... 85

9.1 Glossary................................................................................................................................ 85

9.2 Acronyms .............................................................................................................................. 93

9.3 Codes and Standards ........................................................................................................... 96

9.3.1 Related Product Standards........................................................................................ 96

9.3.2 Modification of Products............................................................................................. 97

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WARRANTY

This manual is published solely for information purposes and should not be considered all inclusive. If further information is required, consult Cummins Power Generation. Sale of product shown or described inthis literature is subject to terms and conditions outlined in appropriate Cummins Power Generation sellingpolicies or other contractual agreement between the parties. This literature is not intended to and does notenlarge or add to any such contract. The sole source governing the rights and remedies of any purchaser of this equipment is the contract between the purchaser and Cummins Power Generation.

NO WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING WARRANTIES OF FITNESS FOR APARTICULAR PURPOSE OR MERCHANTABILITY, OR WARRANTIES ARISING FROM COURSE OFDEALING OR USAGE OF TRADE, ARE MADE REGARDING THE INFORMATION,RECOMMENDATIONS AND DESCRIPTIONS CONTAINED HEREIN. Each customer is responsible for thedesign and functioning of its building systems. We cannot ensure that the specifications of Cummins Power Generation products are the proper and sufficient ones for your purposes. You must satisfy yourself on thatpoint.

In no event will Cummins Power Generation be responsible to the purchaser or user in contract, in tort(including negligence), strict liability or otherwise for any special, indirect, incidental or consequentialdamage or loss whatsoever, including but not limited to damage or loss of use of equipment, plant or power

system, cost of capital, loss of power, additional expenses in the use of existing power facilities, or claimsagainst the purchaser or user by its customers resulting from the use of the information, recommendationsand descriptions contained herein.

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1 Introduction

1.1 Ov ervi ew

The purpose of this Application Manual is to educate engineers, system integrators, distributors,and interested users in the fundamentals of networks, as they apply and are used in on-site

power generation systems.

Communication networks have long been used to make equipment and processes operate more

reliably and efficiently in the following ways:

• Networks improve system reliability by communicating fault or potential fault conditions to

equipment or personnel that can take appropriate action to prevent failures or equipment

damage.

• Networks improve serviceability of a system as technicians are able to assess a system’s

service requirements remotely and in some cases allow for download of software upgrades

over the internet.

• Networks minimize operating costs by making service more efficient and simplifying status

indication and reporting requirements.

• Networks minimize installation costs by reducing point to point wiring and the associated

wiring errors.

• Networks enable more efficient operation of a system as operating and performance

information enables a system controller to cycle equipment on or off and make other

adjustments of operating parameters.

• Networks allow building management systems to monitor and control equipment. This

includes heating and cooling, lighting, transportation and security systems as well as power

generation and distribution equipment.

• Data provided over a network is often used for trend analysis which can optimize systemperformance over time or be used to diagnose problems.

Networks will play an expanded role in power generation, distribution and transmission systems

moving forward. Demand for power is growing in excess of existing generation capacity at the

same time tolerance for interruptions in our power supply is decreasing. Developing additional

centralized generation capacity is capital intensive and fraught with political difficulties as fuel

sources are perceived as non-renewable, harmful to the environment and pose health and

safety risks. This results in a demand for distributed rather than centralized power generation

sources and for improvements to power transmission systems in terms of supplying energy

needs more efficiently and more effective fault isolation and fault tolerance systems.

As power generation systems migrate from centralized to distributed generation and control, the

communications infrastructure will need to become more comprehensive and standardized sothat equipment from multiple suppliers will be able to communicate with each other seamlessly.

Seamless communication is the enabling technology that allows the potential advantages of

distributed power generation to be realized. Examples of this include

• Switching loads off of the grid when there is a danger of a utility transformer becoming

overloaded or to do maintenance on utility distribution equipment

• Exporting power to the grid from distributed sources to serve loads that have been

interrupted due to a fault in the distribution system.

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1. Introduction 4-2013

• Supplementing alternative energy sources with diesel gensets to mitigate the intermittent

nature of the alternative source

• Implementing load demand schemes across multiple distributed sources in order to use the

available capacity most efficiently.

Even equipment that is not connected to a grid will be expected to communicate with other

equipment for applications ranging from simple remote alarm notification to full monitoring

control and analysis. Networking capability is becoming an enabling technology for power

generation equipment across the entire range of power generation applications.

1.2 About This Manual

The intent of this application manual is to describe communication networks in general rather

than explain Cummins Power Generation networking equipment. However, there are several

instances where Cummins Power Generation equipment will be used as an example to illustrate

a concept and give guidance to engineers and integrators on the scope of work that may be

involved with implementing certain communication functions within such products.

The manual is divided into 8 chapters.

Chapter 1 discusses objectives and structure of the manual.

Chapter 2 defines purposes of having networks in power generation and describes a brief

history of how networks have evolved in control systems over the years. Chapter 2 also explains

some of the basic concepts of networks which will be used throughout this manual.

Chapter 3 identifies the types of devices that may be found in a network, from simple

input/output (I/O) devices and controllers to devices that manage traffic and communication over

a network, such as routers and switches, to devices that connect local networks to wide area

networks and the internet. This chapter will refer to Cummins Power Generation devices as

examples.

Chapter 4 defines core network characteristics, topology, bandwidth and distance, the

interaction between these characteristics, and provides a baseline for comparison of variousmedia and protocols.

Chapter 5 discusses media over which communication signals travel. General application

considerations such as termination, grounding, and shielding are covered as well as commonly

used media and how and where they are used.

Chapter 6 defines and describes protocols using the open systems interconnection (OSI) model

as a framework for the discussion. Protocols used by Cummins Power Generation products as

well other commonly used protocols are defined in the context of the OSI model and how and

where they are typically used. For protocols that are not used by standard Cummins Power

Generation products, a description of how these products may interact with equipment using

that protocol is included.

Chapter 7 describes monitoring systems. On-site supervisory control and data acquisition(SCADA) systems and web-based monitoring systems are explored. Additionally, typical

applications for each type of monitoring system are explained. Considerations for

communicating between Cummins Power Generation’s equipment and 3rd party monitoring

systems will be explained.

Chapter 8 displays examples of several types of communication systems in power generation

applications.

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4-2013 1. Introduction

The Appendix contains a glossary of commonly used network and communication

terminologies and a list of acronyms commonly used in the industry and in this manual.

1.3 Application Manuals

Every standby generator set installation will require power transfer equipment, either transfer

switches or paralleling switchgear. The proper system for the job and its proper application arecrucial to reliable and safe operation. The following Cummins Power Generation application

manuals address related aspects of standby and emergency power systems. Because these

manuals cover aspects requiring decisions that must be taken into consideration early in the

design process, they should be reviewed along with this manual.

Application Manual T-011 - Automatic Transfer Switches. Many applications utilize multiple

power sources to enhance electric power system reliability. These often include both utility

(mains) service and generator set service to critical loads. T-011 covers the various types of

power transfer systems available and considerations for their use and application. Careful

consideration of the power switching system at the start of a project will enable a designer to

offer the most economically viable and most reliable service to the facility user.

Application Manual T-016 - Paralleling and Paralleling Switch Gear. Paralleling equipmentmakes two or more generator sets perform as one large set. This can be economically

advantageous, especially when the total load is greater than 1000 kW. The decision whether to

parallel sets must be made in the early stages of design, especially if space and the need for

future expansion are critical factors.

Application Manual T030 – Liquid Cooled Generator Sets. Generator sets may operate as prime

power sources or provide emergency power in the event of utility power failure. They may also

be used to reduce the cost of electricity where the local utility rate structure and policy make

that a viable option. Because of their important role, generator sets must be specified and

applied in such a way as to provide reliable electrical power of the quality and capacity required.

T-030 provides guidance to system and facility designers in the selection of appropriate

equipment for a specific facility, and the design of the facility, so that these common system

needs are fulfilled.

1.4 Safety

Safety should be a primary concern of the facility design engineer. Safety involves two aspects:

safe operation of the generator set itself (and its accessories) and reliable operation of the

system. Reliable operation of the system is related to safety because equipment affecting life

and health is often dependent on the generator set – such as hospital life-support systems,

emergency egress lighting, building ventilators, elevators, fire pumps, security and

communications.

Refer to the Codes and Standards section for information on applicable electrical and fire codes

around the world. Standards, and the codes that reference them, are periodically updated,requiring continual review. Compliance with all applicable codes is the responsibility of the

facility design engineer. For example, some areas may require a certificate-of-need, zoning

permit, building permit or other site-specific certificate. Be sure to check with all local

governmental authorities early in the planning process.

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1. Introduction 4-2013

NOTE: While the information in this and related manuals is intended to be accurateand useful, there is no substitute for the judgment of a skilled, experiencedfacility design professional. Each end user must determine whether theselected generator set and emergency/standby system is proper for theapplication.

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2 Networking Basics

2.1 Ov ervi ew

A network can be defined as a collection of devices or “nodes” that communicate with oneanother over a common medium (i.e. wire, fiber, wireless) where information is exchanged via

sets of rules known as protocols. In this chapter we will define some of the key terms and

concepts associated with networks and discuss how networks have evolved historically.

2.2 Simple Network Introduction

The purpose of a network is to efficiently provide control functions or transmit information, either

between multiple pieces of equipment or between equipment and people.

For example, most power generation equipment that is used in critical applications is required to

be provided with remote monitoring equipment so that an operator can be aware of whether or

not a generator set is running and if it has some fault condition present that might cause agenerator failure.

In Figure 1 you can see a control panel on a generator set that provides alarm and status

information to a remote annunciation point. Note that DC power (generally from the generator

starting batteries) lights a specific lamp when a specific switch is closed to indicate a specific

condition that is occurring. For example, the top switch closing might indicate that the generator

set is running by lighting a light. Note that there is a switch for each condition to be remotely

indicated and a lamp directly connected to each switch.

FIGURE 1. CONTROL PANEL WITH ALARM AND STATUS INFORMATION

As you can imagine, this type of system monitoring is pretty limited because it gets expensive

and complex as the number of conditions to be indicated grows and as the distance between

the generator and the remote monitoring point increases, or if the number of monitoring points

increases. While a system of this type could provide simple control commands, such as a

remote start capability, it can not handle analog information (for example indication of a range of

temperatures) but rather just provides discrete yes/no type of information.

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2. Networking Basics 4-2013

Even a simple generator set application could include some pretty complicated wiring, as

indicated in Figure 2, because every wire would have a specific purpose and would need to be

connected in exactly the correct location.

FIGURE 2. TRADITIONAL COMMUNICATIONS SYSTEM WIRING

Note that even for this installation, there are inputs (a mechanism to tell the system the status of

a specific piece of information), outputs (a mechanism to provide information to a remote person

or device), media (wiring between the devices), and a power supply to operate the system (in

this case DC battery power).

Network-based communication was developed to deal with some of the weaknesses of

traditional communication links between devices. There are a number of similarities between

network-based communications and traditionally designed systems.

Figure 3 shows an annunciator panel, with the same outward functionality as shown in Figure

1, which utilizes a design with network-based communication. In this example there are still

switch inputs (only one is shown for simplicity), and these switch inputs, instead of directly

carrying current to a lamp, provide an input to an input/output (I/O) chip. The I/O chip connects

to a microprocessor (uP) that reads data off the chip and then transfers it through media (in this

case a twisted pair of wires) to a matching microprocessor in the annunciator. This second

microprocessor operates an output switch in the annunciator I/O module, and this output switch

can operate the lamp when required.

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FIGURE 3. ANNUNCIATOR PANEL WITH NETWORK-BASED COMMUNICATION

Note that a power supply is still required to operate the remote annunciator, but it does not

necessarily need to come from the generator set control. There are still inputs, outputs, media,

and with this design, the microprocessors must communicate with each other in the same

language, or protocol. They also must accept the same type of media to transmit messages.

The major advantages seen in the network-based design versus the more traditional designs is

a huge simplification in the installation of the generator set and, because of that, a greatly

reduced installation cost in both dollars and time. A major advantage that may not be clearly

visible is that since all the operation of the equipment is managed by software in the

microprocessors, changes can be made with software manipulation rather than with hard wire

changes, again providing lower cost and also a greater degree of flexibility in design. Changes

and additions are also easy to accomplish with this design.

As can be seen in Figure 4 vs. Figure 2, there are many less connections to make (just control

power to each component and the media that interconnects them—note in this illustration the

twisted pair cable is represented by a dashed line).

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FIGURE 4. NETWORK BASED COMMUNICATIONS WIRING

A network, then, can be loosely defined as a series of devices (in our illustration a transfer

control, generator set control, and annunciator) that communicate with each other over a

common media (twisted pair cable, wireless, fiber optic cable) where information is exchanged

via an established language (set of rules) which is called protocol. The devices in a networkhave inputs and outputs (I/O) that provide the necessary functionality for each system. Points

with network connections are sometimes called nodes.

2.3 Common Terminology

In this section we will define some of the more commonly used networking terms that are used

in this manual. See the Appendix, Chapter 9, for a complete glossary of terms.

2.3.1 Protocols

A protocol can be defined as a set of rules used mutually by two or more devices or software

applications to communicate. Protocols specify all of the characteristics of a communication

network from the physical and electrical qualities to the languages used to communicate.

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2.3.2 Topology

Topology refers to the physical shape and interconnection scheme of the network. The three

most common topologies used in control networks are known as "bus", "ring", and "star". A bus

topology is one in which all of the devices are connected in a line. A ring topolgy is similar to a

bus topology except the devices on the ends are connected forming a closed loop. A star

topology is one in which one device is in the center and connects to all of the other devices like

the hub and spokes of a wheel.

2.3.3 Bus

The terms media, bus, and channel are often used interchangeably to describe the physical

carrier of the communication. While the terms are related they do have separate meanings.

A bus refers to the "backbone" of a communication channel. When a bus topology is used all of

the nodes on the network are physically connected to the bus.

2.3.4 Channel

A channel refers to an uninterrupted physical communication path. Most PowerCommand

networks consist of a single channel. More sophisticated networks will have multiple channels. Asingle channel consists of only one protocol and one type of media. Devices such as routers

and gateways can combine separate channels consisting of different protocols and media into a

single network. Routers and gateways will be discussed in detail in Chapter 3.

Figure 5 illustrates the concept of a channel and a bus. Each of the routers separate the

network into two channels: a Lon channel and an Ethernet channel. Each of the channels is

connected in a bus topology with each device on the channel connected to a common pair of

wires. This pair of wires is referred to as the bus.

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FIGURE 5. CHANNEL BUS CONCEPT

2.3.5 Media

Media is the generic term for the physical carrier of the signal, whether it is copper wire, fiber

optic cable or, as in the case of wireless communication, air.

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2.3.6 Node

The term "node" is a generic term referring to any device connected to a network. A node could

be a generator set control, an annunciator, a router or a web server, or any device on the

network.

2.3.7 BandwidthBandwidth refers to the speed of communication over the network. It is the amount of data that

can be transmitted in a fixed amount of time, usually expressed as bits per second (bps or

bit/s). The term bandwidth is also used to express the finite amount of time in which certain

communication may occur, so phrases like “that’s not an efficient use of bandwidth” refers not

only to the speed of the propagation of the data but also to the total amount of time that a

transmitting device uses to access the communication channel.

2.4 Evolution of Networks

2.4.1 Discrete Communication

Prior to the introduction of microprocessors to control systems, devices communicated over

discrete wires with a separate wire representing each signal. Discrete wiring is not inherently

bad and is necessary when signals need to transmit significant levels of power such as when

activating a relay. Switchgear applications still include very large wiring harnesses. However,

when signals are used just to convey information, communication networks offer materials and

labor cost savings and advantages in scalability over bundles of discrete wire. As

microprocessors became widespread in control devices, network communication became a

viable and economical alternative to communication via discrete wires.

2.4.2 Serial Communication

As personal computers (PCs) became common, a demand arose for PCs to be able to

communicate with other devices, such as printers and modems. Simple serial communicationprotocols were developed to support communications between two devices. As communication

equipment became available PCs and Programmable Logic Controllers (PLCs) began to use

serial communication to monitor and control industrial networks.

Serial communication can be defined as the process of sequentially sending data one bit at a

time over a common communication channel. This allows a single wire or pair of wires to

transmit many different pieces of information. Data is transmitted by a series of electrical pulses

and translated into a series of binary digits or bits, each of which can have one of two values,

commonly represented as “0” or “1”.

Establishing timing between sending and receiving devices is necessary for the receiving device

to recognize where one character (meaning a number or a letter) ends and the next begins

within a string of pulses. There are two methods for achieving this, known as synchronous serialcommunication and asynchronous serial communication. With synchronous communication a

separate clock signal is transmitted between the two devices and a new message begins based

on a pulse of the clock signal.

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Asynchronous communication is the more commonly used method of communication. With

asynchronous communication there is no separate clock signal. Messages are broken into

packets of 7-8 data bits with a start bit at the beginning of the packet, followed by the data bits,

and then possibly a parity bit and up to 2 stop bits (see Figure 6). Devices need to be

configured to transmit at the same speed (known as baud rate) the same number of data bits

and stop bits and the same parity test. As implied by the term “asynchronous” a device may

initiate a message at any time. As the start bit is transmitted the receiving device knows how

many data bits will be transmitted in the packet at what speed and will recognize when the

transmission is complete.

FIGURE 6. AN ASYNCHRONOUS MESSAGE PACKET

A parity bit is often used as a simple form of error checking. When a parity bit is used the

sending device will set that bit so that it is transmitting either an even or odd number of 1s in the

packet based on a predefined setting in the devices for even or odd parity. For example, if the

packet consists of 7 data bits and even parity is selected, then if the 7 data bits are “1010001”

(3 “1s”) then the parity bit should be a 1 so that there are an even number of 1s. If the parity bit

is 0, the receiving device would recognize that there has been an error in communication.

2.5 Early Protocols

One of the earliest forms of serial communication is Morse code transmitted over telegraph

wires. The signals were simple electrical pulses transmitted over the wires, a short pulse

represented by a “dot” and a longer pulse represented by a “dash”. Each letter of the alphabet

was represented by a series of dots and dashes so a message could be broken down to this

serial data, transmitted, received, and re-assembled by the receiver, thus Morse code can be

considered one of the first communication protocols.

2.5.1 RS-232

As communication equipment became more widely used, there was a need to develop a

standard communication protocol so that equipment from several different manufacturers could

communicate with each other. The standard protocol was developed by the Electronic Industries

Association (EIA) and was known as RS-232. The RS-232 standard defines communication

between two specific types of equipment known as Data Terminal Equipment (DTE, typically a

PC) and Data Communications Equipment (DCE, typically a modem) over a distance of nomore than 50 feet (15 meters). RS-232 also specified the pin designations on the 9- and 25-pin

connectors that were most common at the time. RS-232 defines separate transmit and receive

wiring paths with the transmit pin on one device physically connected to the receive pin on the

other device. This means that both devices can transmit at the same time. This is known as full-

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duplex communication (see Figure 7). Although it was designed for a very narrow scope of

applications (communication between only two devices no more than 50 feet (15 meters) apart)

RS-232 is still commonly used in those applications today due to its simplicity and relatively low

cost to develop and deploy. Cummins Power Generation’s proprietary Mon protocol, which is

used by the InPower service tool to connect to controls, is built on the RS-232 physical layer.

FIGURE 7. TYPICAL RS-232 NETWORK

In the 1990s, control devices with microprocessors and networking capability were becoming

more popular. Networks consisted of several devices spread over large distances. A

communication channel that is shared by multiple devices presents several challenges beyond

what is required for communication between only two devices. It is necessary for signals to

travel over greater distances without distortion. The speed of communication becomes more

important. There is a need for identifying which device a message is intended for. When there

are many devices connected to a common communication channel it is not practical to have

individual transmit and receive lines for each device. This means that multiple devices cannot

transmit messages at the same time so there needs to be some method to determine which

device can access the channel. Protocols that support multiple devices on a network need to

address these concerns of distance, bandwidth, addressing, and media access.

2.5.2 RS-485

RS-485 was developed as the standard for connecting more than two devices on a network

over longer distances. Unlike RS-232, which has separate circuits for transmitting and receiving

signals, RS-485 specifies a single circuit for both transmitting and receiving signals, known as a

transceiver. The RS-485 transceiver is designed in such a way that multiple transceivers can beconnected to the network without affecting the one transceiver that is transmitting a signal (see

Figure 8). RS-485 was designed so that signals can travel long distances without being

attenuated by resistance and inductance in the wires and have a high level of noise immunity.

FIGURE 8. TYPICAL RS-485 NETWORK

RS-232 allowed two devices to send messages at the same time by having separate send and

receive paths between the two devices. Since RS-485 uses only a single transceiver for both

transmitting and receiving data, this is not possible. At any given time a device may either

transmit or receive data but the device cannot perform both operations. This is known as half-

duplex communication. There is also a 4 wire implementation available on the market which

supports full-duplex communication. These devices consist of two separate RS-485

transceivers, so that one can transmit and the other can receive at the same time.

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Cummins Power Generation’s PCCNet protocol, used to communicate between a generator set

and its accessories, such as human machine interfaces (HMIs) and annunciators, is built on the

RS-485 physical layer. As the RS-485 standard defines only the physical layer of

communication, it defines distance and bandwidth. Addressing and controlling access to the

communication channel is outside of the scope of the RS-485 standard and is defined by higher

level protocols.

2.6 Addres sing

Each device on a network must have a unique identifier often known as an address. Many

protocols will define both a logical and a physical address. The logical address is assigned

when the network is commissioned; if the same device is used in a different network in the

future, it will likely be assigned a different logical address. Also, if a device fails and needs to be

replaced, the new device will most likely be assigned the same logical address as the device

that it replaced.

Some protocols, such as Ethernet and LonWorks, will also specify a physical address. The

physical address is assigned by a standards body, is unique in the world, and is permanently

assigned to a specific device.

2.7 Media Access Controls

There are several methods for determining which device can access a channel to send a

message. A few methods are outlined below.

1. Master/Slave: One device is the master of the bus, initiates all communication, and controls

access to the bus. The master communicates to one device at a time. The addressed slave

device responds to the master per its request. Most early communication networks used

master/slave mode of communication and it is still common when there is a programmable

logic control (PLC) controlling several devices in a system. Modbus RTU uses master/slave

communication. This type of access control is not appropriate when peer-to-peer or

broadcast communication is required, meaning that any device will be able to initiatecommunication.

2. Token Passing: This is similar to master/slave, but a signal called a token is passed around

the bus from device to device giving each device its own chance to act as the master and

initiate communication for a short time until it has to pass the token to the next

device. Cummins Power Generation's PCCNet uses a token passing scheme.

3. Carrier Sense Multiple Access with Collision Detection (CSMA/CD): Each device “listens” to

the bus to see if anyone is “talking”. If not, then the device may transmit its message. If two

or more devices transmit at the same time, each transmitting device detects this, stops

transmitting, and then waits a random amount of time before attempting to transmit

again. LonWorks and CAN both use CSMA/CD for bus access. Earlier versions of Ethernet

also used CSMA/CD, although with the fully switched Ethernet networks that are commontoday collisions are avoided. See the discussion on switched ethernet in Section 2.9.

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2.8 Determinism

When networks are used to transmit time critical data for controllers, there is a need for the

communication to be deterministic, meaning that the amount of time it takes for a certain

message to travel between devices must be predetermined and consistent. In this case,

protocols will include some priority arbitration scheme so that critical messages will pre-empt

noncritical messages and devices transmitting these messages will get priority in accessing thebus.

2.9 Full-Duplex Switched Ethernet

Ethernet is a protocol defined by the IEEE 802.3 standard which specifies physical and data link

layer aspects of communication such as bit encoding, physical addressing, and media access. It

is currently used for approximately 85% of the world's LAN-connected PCs and workstations. Its

prevalence created a demand for control networks to communicate over Ethernet.

When Ethernet was standardized in 1985, communication was half-duplex, meaning a device

could not transmit and receive data simultaneously. All devices on the network communicated to

a central hub which would broadcast messages to all of the devices on the network. Ethernetused CSMA/CD for media access and collision detections. Due to the possibility of collisions,

the communication was not deterministic so it was not suitable for real time control applications.

Full duplex communication was standardized for Ethernet in 1998, which essentially doubled the

available bandwidth; however, it still did not make the protocol deterministic because collisions

still occurred. For a truly deterministic network, it is not sufficient to detect collisions. Collisions

must be avoided. The most common collision avoidance method is to divide the network into

segments consisting of only two devices communicating over a full-duplex connection. This is

accomplished by using Ethernet switches (see Figure 9).

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FIGURE 9. SWITCHED ETHERNET

In a switched Ethernet network the device will send messages to an Ethernet switch. The

Ethernet switch will check the message for errors, and then forward it to the intended recipient.

The message will also include a prioritization code so in the event that multiple devices are

attempting to communicate with the same recipient, the higher priority message will be

transmitted first.

A full duplex switched ethernet allows devices to transmit and receive data at the same time and

eliminates collisions by having only one device connected to each port of the switch. These two

features enable deterministic communication which makes Ethernet suitable for industrial control

networks.

Notice that Figure 9 shows two small networks with Ethernet switches connected by routers.

Routers are similar to switches in that they both manage network traffic. The difference is that

switches direct traffic within a local network while routers direct traffic between networks.

Routers enable networks using different types of media or different protocols to interact with

each other. The most common example of this is a control network using routers to connect to a

remote monitoring system over the internet. Switches and routers will be discussed in detail in

Chapter 3.

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2.10 Open Systems and Object Oriented Data Structures

As the communication infrastructure has evolved, from communicating between two devices that

are physically next to each other to communicating with devices anywhere in the world, the

nature of the data that is communicated has evolved as well. In both cases, the trend is toward

open communication to allow equipment from multiple suppliers to integrate seamlessly.

Traditional data, communicated by a piece of equipment, has no structure. It is simply a list of numbers that has no meaning by itself. It is up to the system integrators who are programming

or configuring the network devices to provide the context, to tell the equipment what the data

actually means. This is known as a flat data structure. For example, a generator may have the

value 2773 stored in its Modbus register number 40026, but it will provide no information on

what that number represents. In order for this to be useful, the generator manufacturer must

publish a register map or some other document explaining that register 40026 contains the line

1 to neutral voltage and it is scaled at 0.1 volts per count. A system integrator will need to take

that information and program their device so that when the value for the line 1 to neutral voltage

is required, the device polls the generator for the value stored in register 40026 and then divides

that value by 10. In general, there is no standardization between manufacturers on this.

In an object oriented data structure, related data is bundled into an object. A generator would be

defined as an object with certain properties such as Line 1 to neutral voltage and functions it

can perform, such as starting and stopping. If all generator manufacturers define the generator

object the same way, it would make it very easy for a system integrator to design systems with

equipment from many different manufacturers.

The trend in networks is toward open systems and interoperability. The expectation is emerging

that all kinds of equipment will be interconnected and power generation and distribution

equipment will be no exception. With the trend toward distributed generation and control, and as

SmartGrid technologies are applied and matured, the demand for interoperability will only get

stronger.

2.11 Multiple Networks in a Single System

With the prevalence of communications networks in all types of control systems it is not

uncommon to have several networks within a single system. Some Cummins generator sets, for

example, simultaneously support three networks, each using a different protocol (each of the

protocols mentioned will be discussed in more detail in Section 6.2).

1. A Modbus network for communication with monitoring and control systems. Many of these

systems are designed and commissioned by 3rd parties who use Modbus protocol in their

system. Because Modbus is commonly used open (meaning available to everyone)

protocol it is well suited for communicating with monitoring and control systems.

2. A PCCNet network for communicating between a generator set and its peripherals such asa display (commonly known as an HMI or Human Machine Interface). PCCNet is a

Cummins proprietary protocol designed specifically for use with Cummins equipment. It is

very inexpensive and simple to maintain and requires no configuration. The fact that it is

only used by Cummins means that we don't have to worry about unknown equipment trying

to communicate which simplifies ongoing support of devices that use the protocol and

enables "plug and play" configuration.

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3. A CAN network for communicating between the genset controller and the Engine Control

Module (ECM). Can is a commonly used protocol in the automotive industry used to

communicate between the engines and other control systems in a vehicle. It is the standard

protocol used by Cummins ECM's in all kinds of applications and in a genset is used only

for communicating between the genset control and the ECM.

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3 Network Devices

3.1 Ov ervi ew

In this chapter we will identify the types of devices that are commonly found in a power generation system network. We will refer to Cummins Power Generation’s products as

examples.

We will discuss five types of devices: I/O devices, intelligent controllers, network interface

cards, network management devices (hubs, switches, routers, gateways), and web servers.

3.2 Input/Output Devices

Input/Output Devices allow controllers to get information from the physical world and to control

physical components. I/O devices convert data between physical states or values and logical

signals communicated over a network. For example, the position of a switch, a physical state, is

converted to a logical value and communicated to a controller, or a controller will send a logical

command to activate a relay, or change its physical state. Defining input and output

requirements for system components is a key requirement in the overall system design.

An input device will convert some physical parameter, such as a position of a switch or a

voltage level, to a logical value, which will be communicated over a network. An output device

will receive a logical signal over a network and change the state of an output, which will result in

some physical change to a piece of equipment, such as grounding a relay coil, which will cause

the relay contacts to close. An output from one device can be an input to another device.

Examples of Cummins Power Generation’s network I/O devices include the LonWorks DIM (see

Figure 10), CCM modules, and the Aux101.

FIGURE 10. CUMMINS POWER GENERATION'S AUX101 I/O MODULE

I/O can be either digital (discrete) or analog. Digital refers to signals that can be in one of two

states, for example on or off, or open or closed. Analog I/O refers to a value that is continuously

variable. Voltage, amperage, and temperature are examples of analog signals.

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3. Network Devices 4-2013

3.2.1 Digital Inputs

Digital inputs, also known as discrete or switch inputs, are used to indicate switch positions in a

power system. Oil pressure switches, temperature switches, status of generator run/off/auto

switches, and auxiliary position indicating switches on circuit breakers and transfer switches are

the most common uses for digital inputs. Digital inputs are inexpensive and simple to implement

compared to analog inputs so it is also common to use digital switches for analog parameters

such as oil pressure or coolant temperature, which will activate when the parameter hasbecome higher or lower than some threshold value. This method does have a disadvantage with

respect to using an analog input in that with a switch a control is not able to recognize a failed

switch. With an analog signal the control is able to differentiate a failed sensor from a parameter

which has passed some fault threshold because a failed sensor will yield an "out of range"

value.

Digital inputs are typically activated by a contact closure between the input line and either

ground or a control voltage reference provided by the control device. Inputs that are activated by

a closure to ground are called “active low”; those that are activated by closure to a control

voltage are called “active high”.

In some cases, a “pull up” or “pull down” resistor (see Figure 11) is required on a digital input to

positively connect the input when it is in its passive state. For example, if a customer activatesan input by switching it to the control voltage (active high), a pull down resistor is required

between the input and ground so that the I/O device will read the input as a “low” value when

the customer’s switch is open. This is the case with the digital inputs on Cummins Power

Generation’s CCM boards when they are used as “Active high” inputs. This is not a requirement

with the Aux101, DIM devices, or CCM inputs when they are used in an active low

configuration. In these cases, the pull up or pull down function is implemented on the control

boards.

FIGURE 11. PULL DOWN RESISTOR

3.2.2 Digital OutputsDigital outputs are used to start or stop generators, operate equipment, such as fans or louvers,

close motor starting contacts, and indicate conditions based on some event that occurs in a

control system. For example, with the Aux 101, generator events are mapped to relays using

the service tool. With some generator set controls the Human Machine Interface (HMI) is also

capable of mapping events to output relays. With the CCM and DIM, the signal is initiated by

some controller and mapped to outputs on the CCM or DIM using configuration software.

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4-2013 3. Network Devices

A common method for implementing a digital output is to use a form C contact (see Figure

12). A form C contact consists of one normally open and one normally closed contact and a

common line between the two contacts, which ensures that one contact is always open and one

is always closed to the common line and activating the output will change the state of both

contacts. The digital outputs on Cummins CCM, DIM, and Aux 101 are form C contacts.

FIGURE 12. FORM C CONTACTS

The amount of current that a digital output circuit can source or sink is a key consideration in

determining how to apply the device. If the current rating of the digital output is not sufficient to

operate the piece of equipment it is connected to, an external pilot relay will be required.

3.2.3 Analog Inputs

Analog inputs measure continuously variable signals. The parameter to be measured is

converted to a direct current (DC) voltage and read by an input device. The DC voltage is

converted to a numerical value using an analog to digital (A/D) converter and that numerical

value is used by a control or communicated over a network. The sensing circuit will be scaled

so that the maximum current or voltage that the input device can accept will correspond to the

maximum value that the signal can have. For example, 0-5V DC is a common standard for

analog input devices. 0V would correspond to the minimum value and 5V would correspond to

the maximum value that the sensed parameter can have. Analog inputs are used to measure

generator output voltage and current, temperatures of oil, exhaust, ambient air, and alternator

windings, oil pressure, fuel level and voltage, and speed bias lines.

Some sensors, such as temperature and pressure senders, have a resistance that is

proportional to the sensed parameter. A fixed current is passed through the resistive elementwhich will create a voltage for the I/O device to read. With Cummins Power Generation’s Control

Communication Module (CCM) and Aux 101 boards, the current source is provided on the

board. Since oil pressure and different temperature senders all have very different

characteristics, they will require different levels of current to generate voltages in the appropriate

range to be measured by I/O devices. With the CCM there are different current sources for each

analog input and two of the inputs have two different levels of current available, which can be

selected using an on-board switch. With each of the Aux 101 inputs, the operator uses the HMI

to select what parameter is measured and the current source for that input is configured

appropriately.

Some controls can read voltage and speed bias inputs. These are simply customer supplied

potentiometers (variable resistors) through which a fixed current is passed to create a voltage

which can be read by the A/D converter.

Control and communication devices can read phase voltages and load current. Phase voltages

are stepped down through transformers to a level that is suitable for rectification to create a DC

voltage and then read by the A/D converter. Load current is passed through a current

transformer (CT) for sensing and the output current of the CT is run though a burden resistor

that creates a voltage that can be measured. With Cummins Power Generation’s equipment

there are typically two sets of CTs in the sensing circuit, one set right at the generator output or

load terminals of a transfer switch and a second set of smaller CTs mounted on a circuit board.

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There are also devices on the market that create a current which is proportional to a measured

parameter. A common standard is 4 – 20 mA devices. To monitor this with a CCM, the current

is passed through a known resistance (off of the CCM board) and the CCM can read the voltage

across the resistor.

To make use of the values created by the A/D circuit, it is necessary to know how the

parameters are scaled. Scaling will depend on the physical properties of the sensing circuit and

the resolution of the A/D converter. For example, a temperature sensing circuit will converttemperature to voltage depending on the resistance/temperature characteristic of the sender

and the current source. The A/D converter will convert that voltage to a number of “counts”

based on its resolution. For instance, a 10 bit A/D converter will convert the measured voltage to

a number of counts between 0 and 1024 (2^10). When the conversions between the measured

parameter and analog voltage, and between analog voltage and A/D counts are known, the

conversion between the measured parameter and counts can be calculated.

In many cases, the I/O device, or the control that gathers input from the I/O device, will make

this calculation or there will be a tool to make this calculation. In other cases, it will be easier to

establish the conversion through a test if it is known that the relationship between the measured

parameter and the analog voltage is linear. Simply reading the A/D output for 2 known inputs

will allow a simple calculation of the conversion factor. For example, consider a temperature

sender. Through test it is determined that at a temperature of 197 °F the sender had a voltageof 3.23V and at 172 °F the voltage was 2.9 V. With these 2 points we can determine the sender

gain and the offset for the sender. The gain defines how much the voltage changes for each

degree of temperature change. It is simply the ratio of the difference in temperature to the

difference in voltage. In this example it is (197-172)/(3.23-2.92) = 80.65 °F/V. This means that

for every 80.65 degrees that the temperature changes, the voltage will change by 1 volt. The

fact that the gain turned out to be a positive number indicates that as the temperature increases

the voltage will increase. Some types of sensors have a negative gain, meaning that as the

sensed parameter increases the corresponding voltage decreases.

Some senders do not have an output of 0 volts when the parameter being sensed is at 0. In

those cases we need to calculate an offset to fully define the sender. To continue the example

above, we will calculate the offset for that sender, which is the temperature that would

correspond to 0 volts. We can calculate this using the sender gain and one point. We'll use the

2.92V, 172 degrees point. The equation is 172 - 2.92*80.65 = -63.5. This means that 0 volts

would correspond to a temperature of -63.5 degrees. A plot of the linear relationship between

temperature and voltage is shown in Figure 13. Note that different analog devices have

different calibration tools and methods. This is just one example.

FIGURE 13. SENDER CHARACTERISTIC

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3.2.4 Analog Outputs

There are analog output devices available on the market which are typically controlled by

programmable logic controllers (PLCs). These devices commonly source a 4 – 20 mA current as

their output.

In general, analog outputs are not very common in network products. When a process needs a

controlled, variable analog voltage it typically uses a customized controller rather than an off-the-shelf networked product. While there are standard digital to analog converters (D/A)

available for integration into a circuit board, most analog outputs are generated by using a

pulse-width modulated signal (PWM) which is filtered to yield a steady analog signal. Cummins

Power Generation does not produce any analog output devices for use on a network but uses

them in system control products that interface with 3rd party equipment.

3.3 Intelligent Devices

Intelligent devices have a microprocessor and are capable of making decisions based on

information received over the network and interpreting and processing the information it

receives from its inputs before sending the information over the network. Generator and transfer

switch controls and HMIs are examples of intelligent devices. To illustrate, an HMI will translatea series of button clicks by an operator (digital inputs) and send the information to a controller,

which it will then process and generate a digital output to start the generator. Intelligent devices

are often capable of operating as a standalone piece of equipment; they do not always need the

network communications to function.

3.3.1 Network Interface Card

Network Interface Cards, or NICs (see Figure 14), are used to connect a device to a network.

Cummins 2100, 3100, and 3201 generator set controls and OTPC, BTPC, CHPC and PLT

transfer switch controls have NICs, known as Network Communications Modules (NCM), which

connect these controls to LonWorks networks. The NCMs are sold as a feature with the

generator or transfer switch and are physically mounted on the main control board.Market trends have evolved to where communication capability is expected on even the most

basic controls. Controls have long had integral network capabilities when a standard interface is

required on all controls such as CAN interface from a genset controller to an engine. With the

market expectation of communications capabilities on most controls it is becoming more

common to have the network interface resident on the control rather than as a separate

component. Cummins Power Generation’s 1.X, 2.X, and 3.X generator controls are examples of

controls with integral network capabilities.

FIGURE 14. NETWORK INTERFACE CARD

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3.4 Network Management Devices

There are several types of devices that manage traffic on a network. These devices are

commercially available from a number of suppliers. With the exception of the ModLon Gateway,

Cummins Power Generation does not produce any of these devices.

3.4.1 Hub/Repeater Hubs and repeaters are simple devices that repeat all of the network signals to all network

devices to which they are connected. They do not process data or make any type of decision

regarding where to send the data.

A repeater has two ports. It connects two segments of a network. Since they are able to amplify

a weak signal, repeaters are able to extend a network over large physical distances.

A hub has multiple ports and allows devices to attach to a network in the star configuration.

(See Section 4.2 on page 29 for discussion on network topology.) A hub receives data and

broadcasts the data to all of its ports.

Hubs and repeaters are becoming less common as they are not able to meet demands for

faster networks with more devices. More often than not, they are replaced by Ethernet switches.

3.4.2 Switch

A switch is similar to a hub except instead of broadcasting all the messages it sees, it checks to

see that there are no errors in the message packet and if there are no errors, it will transmit the

message only to its intended recipient.

By sending only “good” messages to their intended recipient, switches increase the speed of the

network by eliminating a lot of unnecessary and erroneous traffic. Switches can also improve

network speed by eliminating collisions.

Ethernet switches divide a network into segments called “collision domains”. A collision domain

is defined as a set of network devices whose packets can collide with each other. As discussed

in Chapter 2, if two devices on a network try to “talk” at the same time there is a collision. Whena collision occurs, all the data is lost and each device will delay its message before attempting

to transmit again.

Because a switch manages traffic between ports, there will be no collisions between devices on

separate ports. Only devices that are connected to the same port of a switch can experience

collisions. Devices connected to the same port on a switch are said to be in the same collision

domain. To completely eliminate the possibility of collisions, only a single device is connected to

each port of a switch. There is only one single device in each collision domain.

To maximize speed of network communication, full duplex switching is employed. Full duplex

communication allows devices to both transmit and receive messages at the same time. This is

accomplished by each device having separate transmit and receive lines. By using full duplex

communication with only one device on each collision domain the speed of communication isoptimized. It is limited only by the characteristics of the transceiver and the media.

3.4.3 Bridge

A bridge is similar to a switch except that a bridge has only two ports where a switch may have

many ports. A bridge can connect two network segments separated by large distances or two

segments that are communicating over different types of physical media.

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3.4.4 Routers

Routers are similar to switches in that they both manage network traffic. The difference is that

switches direct traffic within a local network; whereas, routers direct traffic between

networks. Where a switch communicates between network segments that are physically

connected to that switch, routers can connect two devices that are physically separated by great

distances with multiple routers in between them. Routers operate based on a device’s logical

address rather than physical location. To illustrate, one can connect his/her laptop to their internet service provider’s server from their home, their office, an airport or their favorite coffee

shop. The physical location does not matter; it is the IP (Internet Protocol) address, the logical

address, that matters.

Routers enable networks that use different media, access, and addressing protocols to

communicate with each other. (They can translate between different Physical, Data Link, and

Network Layer protocols, and layers 1, 2 and 3 of the OSI model. The OSI model will be

discussed further in Chapter 6.) They do not translate between higher level languages

(Application Layer), such as LonWorks and Modbus.

A common example of this is in a campus application where generators, transfer switches, and

switchgear are all located in an equipment room and are to be monitored in a control room a

great distance away. There is an Ethernet connection over fiber optic cable between the tworooms. The equipment communicates via LonWorks over twisted pair wire. In each of the two

rooms there would be a router with one channel connected to the twisted pair wire and the

second channel connected to the fiber. The router does not translate the high level language

(LonWorks), it just manages the transmission of data from the twisted pair network, over the

fiber optic cable, and back to the twisted pair network at the other end. See Figure 15.

FIGURE 15. ROUTERS IN MULTIPLE NETWORKS

3.4.5 Gateways A gateway is a device that enables two network segments of different protocols to communicate

with each other. A gateway manages all layers of the OSI protocol stack from the physical

media to the high level language. Cummins Power Generation’s ModLon Gateway (see Figure

16), which converts LonWorks to Modbus data for Cummins products, is a good example of a

gateway. It should be noted that gateways can slow down communication and in some cases

can compromise reliability as they introduce single points of failure into the system.

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FIGURE 16. MODLON GATEWAY

In some cases, gateways that connect serial protocols, such as RS-232, to Ethernet are known

as Device Servers. They are typically used to connect devices such as printers and fax

machines which have only a serial connection to an office LAN so that the equipment can be a

shared resource.

3.5 Web Servers

Web servers are devices that reside on a network and serve web pages consisting of data

acquired from equipment on that network. In most cases web servers will be capable of sending

emails based on alarm conditions. Typically, with a web server in a power generation

application, the owner of the system needs to provide internet access at the site where the

generator is and will need to provide the appropriate IP and email server addresses to connect

to the internet.

Cummins’ PowerCommand 500 and 550 and iWatch 100 are examples of web servers (see

Figure 17). The PowerCommand 500/550 and the iWatch 100 communicate with generator and

transfer switch controls over LonWorks or Modbus and serve web pages that display the currentstatus of the equipment. Operators can view this data with their own web browser. If operators

have the proper security access they can also start the generator set or initiate a test of the

transfer switch through the web page. Web servers will also send text messages or emails on

alarm conditions so that service personnel can be alerted that there has been some event in

their power system. Cummins Power Generation’s DMC200 and DMC300 digital master

controllers also have integral web servers. Components in some equipment may also have web-

servers. For example, it's physically possible for a generator set control to have an on-board

web server. However, it is more practical to have a web server for a system than a single

component in a multiple component system.

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FIGURE 17. NETWORK INCLUDING IWATCH WEB SERVER

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4 Network Characteristics

4.1 Ov ervi ew

This chapter defines physical network characteristics, topology, bandwidth, and distance. Itdescribes the interaction between these characteristics and provides a baseline for comparison

of various media and protocols.

Selection of topology, bandwidth, and distance impose tradeoffs on a system. Typically,

bandwidth and distance conflict with each other. Increasing the physical distance between

devices decreases the maximum speed of data transmission. Topology choices will affect the

other capabilities as well, with the simplest topology allowing for the greatest distance and

bandwidth, while more complex topologies will impose limitations.

4.2 Network Topology

Topology refers to the physical shape and interconnection scheme of the network. The threemost common topologies used in control networks are known as “bus”, “ring”, and “star”. Some

of the more complex topologies such as “tree” and “mesh” topologies are combinations of the

three basic topologies. Echelon Corporation, creators of the LonWorks network platform, uses

the term “Free Topology” to position LonWorks as flexible enough to be configured in any

topology. Some topology examples are shown below (Figure 18).

FIGURE 18. COMMON NETWORK TOPOLOGIES

The main criteria for choosing a network topology are:

• Distance/bandwidth capability of the media and hardware,

• Degree of difficulty in wiring it,

• Robustness in the event of a break/failure,

• Capability for redundant communication paths,

• Ability to segment traffic (does all traffic have to pass through one spot? are there

dedicated paths between individual devices?).

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4. Network Characteristics 4-2013

4.2.1 Bus Topology

With a bus topology, all of the devices are connected to a common backbone or bus. A bus

topology is the simplest and least expensive to install. A bus topology will allow for the greatest

distance and bandwidth. With Cummins Power Generation networks, we always recommend

using a bus topology except where redundant communication paths are required.

Bus topology is very similar to a “daisy chain” topology. The terms “bus” and “daisy chain” areoften used interchangeably but there is a difference. With a bus topology, each device is

connected to the bus, and with a daisy chain, each device is connected directly to the adjacent

device. With a bus network,

T-034 - Networking.pdf

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