AC500 PLC and Modbus RTU Communication

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AC500 PLC and Modbus RTU Communication

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Table of Contents

1. Introduction .......................................................................................................................................... 2

2. Device Details ....................................................................................................................................... 2

3. Procedure to Configure ........................................................................................................................ 3

3.1 Creating a Project ......................................................................................................................... 3

3.2 PLC IP Address .............................................................................................................................. 3

3.3 PLC COM Port ............................................................................................................................... 4

3.4 PLC FBP Port.................................................................................................................................. 5

3.5 800xA Profibus Configuration ...................................................................................................... 6

4. Procedure to Program .......................................................................................................................... 7

4.1 Creating a Function Block............................................................................................................. 7

4.2 ModBus Communication.............................................................................................................. 9

4.3 Compiling and Downloading ...................................................................................................... 10

4.4 Transmitting Data to DCS ........................................................................................................... 11

4.5 Creating a Task for the Program ................................................................................................ 12


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1.

Introduction

PLCs have been in use since very long and have always had the

advantage of being small and inexpensive. They are widelyused in small scale independent operations. The use of PLCs as

an intermediary communication device is also fast growing

because of the cost advantages.

A PLC can communicate in various protocols like Ethernet,

ModBus, Profibus, DeviceNet, CAN, etc.

This document describes the process to configure a PLC to

communicate with Multi-Function Meters (MFM) used for

Energy Monitoring over Modbus RTU and Profibus to

communicate with 800xA DCS.

2.

Device Details

The details of the Hardware, Communication Protocols and the Software used during this document is

as follows:

PLC AC500 PM573-ETH

PLC - Device Protocol Modbus RTU (PLC as Master)

PLC - DCS Protocol ProfiBus DP (PLC as Slave)MFM Model Schneider Conzerv EM6400

PLC Configuration Software PS501 Control Builder Plus Rev 2.3.0

PLC Programing Software CodeSys Rev 2.3.9.40

PS501 Control Builder Plus is the complete software suite and automatically installs CodeSys and the

Gateway OPC.

Control Builder Plus is used to configure the PLC and the connected hardware modules whereas CodeSys

is used for configuring the software. It is worthwhile to mention that CodeSys is completely integrated

into Control Builder Plus.


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3.

Procedure to Configure

Install PS501 Control Builder Plus from the installation media or by downloading the setup from Solution

Bank.

3.1

Creating a Project

Open Control Builder Plus. Go to File --> New Project --> Select AC500 Project from Templates and give

your project a Name and Location and press OK.

This will open a new window where you can select the PLC model. Once selected, press Add Device.

This will create a project with the AC500 PLC selected and its associated modules.

3.2 PLC IP Address

If the PLC is not previously configured, you will have to configure its IP Address. Connect a LAN Cable

from your PC to the PLC. On your PC, go to the LAN Network Adapter IPv4 Settings and select Obtain an

IP address automatically.

Now come back to Control Builder Plus and double click on IP Settingson the left. This will open the

properties. Select IP-Configuration on the right. This will open a Device Scanner. Press Scan to find

the PLCs IP address.


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Once the PLC is found, click on it and scroll down to enter the new IP Address of the PLC. Click on send

to set the IP address. Make sure the PLC is in STOP mode before sending the new IP address.

3.3 PLC COM Port

The PLC has two COM Ports, both of which can be configured to communicate independently on the

same or different protocol.

For the purpose of this document, we will configure COM Port 1 for ModBus. The hardware connection

details are as follows for COM Port:

RS-485 POSITVE RS-458 NEGATIVE

COM 1 Pin 2 Pin 3

COM 2 Pin 3 Pin 8

To configure, go to Interfaces --> right click on COM 1 --> Plug Device. Now select COM 1

ModBus and select Plug Device and close.

Double click on the configured COM Port to open properties. Select RTS Control as Telegram and

Operation Mode as Client. This is done to configure the PLC as ModBus Master. Other parameters can

also be changed if required by the Slave Device.


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3.4 PLC FBP Port

The PLC supports FBP which stands for Field Bus Plug. FPB

offers fieldbus neutral device connection and can connect to

any device, protocol or sensor module directly.

For our use, we connect to a ProfiBus DP Network with PLC

acting as a ProfiBus Slave to pass data onto the 800xA

System.

To configure, go to Interfaces --> right click on FBP Online

Access --> Plug Device.

Now select FBP Slave (Profibus) and click on Plug Device and then

Close.

This will configure the FBP Port as Profibus Slave. Now Double click on

configured Port to open properties. The Device Address can be set here.

Now that the PLC is configured as a ProfiBus Slave, the I/O Modules need to be added. Right click on the

FBP Port and select Add Device. In the new window, select 16 Word Input (IEC Output) to add a

module.

This single module can send 16 words of data to the ProfiBus Master (DCS). If more data needs to be

sent, add more modules. In case some data is to be received from DCS, add an appropriate module.


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Please note the maximum data a ProfiBus Slave can send/receive is limited to 246 bytes. So the addition

of these modules should also be limited to stay within the threshold.

For the hardware connection, a PDP22 type FBP connects to the PLC. It has an IN port and an OUT port.

If the ProfiBus loop is terminating at the PLC, a terminating resistor, PDR11 must be used at the OUT

port. The PDP22 module requires +24V DC supply to be connected in order to work. Once powered, thepower LED starts glowing on the PDP22. For more details on FBP, refer ABB FBP Documentation.

3.5 800xA Profibus Configuration

Import the GSD of the PLC in Control Builder.

Insert the PDP22-FBP Module under CI854.

Assign the node number and name.

Insert the same number of I/O Modules under

the PDP22-FPB module as done in the PLC. For a

single 16 Word Input (IEC Output) module in

PLC, add a single 16 word input module under the PDP22-FBP module.

Declare the variables on the channels in the module just assigned and use as per requirement.


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4.

Procedure to Program

Once the PLC has been configured, it has to be programed.

Save your project in Control Builder Plus. Double click on AC500 to open CodeSys. This uploads all

configuration data to CodeSys and opens the program editor. Make sure any configuration changes inControl Builder Plus that you do are done only when CodeSys is closed.

CodeSys will open with only a single program: PLC_PRG. This is the main program which is executed

continuously. Any other functions / programs / code blocks must be called in this program for execution.

4.1Creating a Function Block

The code which needs to be run may be defined in a Function Block (or another program). This function

block can then be called multiple times for execution.

To create a Function Block, right click on POUs --> Add Object --> Select Function Block and the

programing language. Give it a name and click OK. This will create an instance of the Function Block and

open the editor.

The editor is divided into Lines (in FBD). Multiple lines can be added and each line can have multiple

functions. These functions are called as Box. These lines are referred to as Network.


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Right click on the first line --> select Box. This will insert a function (AND block

by default). Other options include Network (after), Network (before), Assign,

Jump, Comment and others.

Network (before/after) will add a new line.

Assign will open a dialog box to assign new variable.

Jump can be used to jump to a different code block.

Once the AND Block is inserted, the function can be changed by manually

entering the name of the desired function in place of AND.

The function block pins will change depending on the selected function. The ???

symbol stays till some variable is assigned to the pin. Click on ???and enter the

variable name, if it is not declared, a dialog box will open to declare that variable

by selecting the data type and memory address.

In certain cases, the function block itself needs to be declared. Such blocks will have ??? on top of them

to declare a name for the function block. Simply enter a name and it will be declared.

This way multiple functions / function blocks can be added. If the language needs to be changed, right

click on the Function Block on the left and select Convert Object.

To execute these Function Blocks, go to PLC_PRG Program and enter the name of the ObjectFunction

Block. This will call the function block for execution in a continuous loop.


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4.2ModBus Communication

The inbuilt function block for ModBus RTU is COM_MOD_MAST and that for ModBus TCP is

ETH_MOD_MAST. These can be used to communicate with the Modbus Slaves.

Parameter Data Type Description

EN BOOL EN pin is used to enable the communication

COM BYTE COM pin is used to indicate whether its on COM1 or COM2

SLAVE BYTE SLAVE pin is used to indicate the address of the slave

FCT BYTEFCT pin is used to indicate the Function Code of the Slave,

generally the Holding Register

TIMEOUT WORD (in ms) TIMEOUT is used to give the maximum timeout before error

ADDR WORDADDR is for the Slave register address from where the data is

to be read

NB WORD NB is the number of data registers to be read

DATA DWORD DATA is the address / location where the data is to be stored

DONE BOOL DONE indicates that the Read Cycle is completed

ERR BOOL ERR indicates an error in communication

ERNO WORD ERNO indicates the error code.

In case of multiple slaves, multiple COM_MOD_MAST blocks must be used. In such a case, the EN of

succeeding block should be connected to the previous blocks DONE bit. This will allow a successive

execution, block after block.

Also important to note here is that the ADDR should be entered as Slave Data Address1.


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A Conzerv EM6400 MFM Slave can be configured as follows:

MFM on COM 1 and Slave ID 11. The FCT is 03 for Holding Registers. The Data Register at 3001 has a

set of 20 registers with the data as shown (from EM6400 Manual). These registers have to be read in a

single cycle. Therefore, NB will be 20. The DATA pin expects the address of a memory location. This can

be given by using the Function ADR.

The function block to communicate will therefore be as shown below:

This way the data can be read from the slave device.

4.3Compiling and Downloading

Once the code is ready, compile the code to check for any errors. This can be done by going to Project

menu on the top --> Rebuild All. This will compile all the programs / function blocks and list the errors

in the window below.

Once the code is free of any errors, download the project onto the PLC. But before downloading, the

communication parameters need to be defined.


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Go to Online menu on the top --> select Communication Parameters. In the new window, press

Gateway and select TCP/IP and other properties as

default.

Now select New --> TCP/IP and give a name to the

connection. In the address field, enter the IP address of thePLC which has been set previously.

Once these parameters have been set, connect a LAN cable

from your PC to the PLC. Go to Online menu and select

Login. This will download the project. Login is similar to

the Download and go Online option in AC800M. To go offline, go to Online --> Logout.

Once the download is complete, go to Online menu and select Create Boot Project. This will save the

program to EEPROM. Make sure you are in Login/ Online mode when you Create a Boot Project.

To start the execution of the code, a RUN command has to be given to the PLC. This can be done bygoing to Online menu --> Run.

Once all configuration is completed, Logout and save the project in CodeSys and exit CodeSys.

4.4Transmitting Data to DCS

Once the ModBus Data is received at the PLC, it needs to be sent to DCS via ProfiBus. To do this we need

to declare Global Variables.

Close CodeSys and go back to Control Builder Plus. Expand the FBP Slave Module declared previously and

double click on the 16 Word Input(IEC Output) module. On the right assign new variables on the desired

channels. If the new variable is a WORD then assign it directly in the word channel or else merge two BYTE

type variables and assign them on a single channel as HIGH BYTE / LOW BYTE.


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Once you have declared these variables, save the project and open CodeSys again. The variables

assigned in Control Builder Plus become Global Variables and can now be used in the Program to send

values.

4.5Creating a Task for the Program

The PLC will executes only those Programs / functions which are assigned a Task. To do this, open

CodeSys and navigate to Resources from the bottom menu.

In Resources go to Task Configuration. By

default, the Task Configuration will have

only System Events.

Right click on Task and select Append

Task. Give a name to the task and set the

propertiesPriority / Interval / Watchdog.

Right click on this new Task and select

Append Program Call. Select the PLC_PRG(); program and save.

Its is advisable to assign task to a single program, preferably PLC_PRG() and call all other function blocks

from PLC_PRG itself.

AC500 PLC and Modbus RTU Communication

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