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How to configure Read_var instruction to read 4x registers from Modbus slave using Unity Premium Processor.

Published date: 29 June 2020

Goals and Symptoms

This resolution is intended to be used as a guide to demonstrate how to configure the read_var instruction to read 4x (%MW) registers from Modbus slave over a RS485 Network.

Facts and Changes

This example uses a TSXSCP114 RS485 communication card installed in channel 1 of the TSXP57104 processor. The Modbus slave is a TSX Micro PLC with the TER port configured for Modbus salve address 1.


TSXP57104M Unity Processor
TSXSCP114 PCMCIA communication card.
TSXPACC01 RS485 Tap Link box
TSX372100 Micro PLC (Modbus Slave device)

Operation and Debug

1, Create an animation table, enter the variables below. Click the Modification button. select the value for %M1. Click the up arrow to force the vale high.
This will initiate the Read_var instruction. Turn the block off with the down arrow.


Management parameters:
%MW 10 - contains the Exchange number. This will change to an even number upon completion.
%MW11 - Should be viewed in Hex High byte - Operation Report, Low Byte - Communication Report
%MW12 - Time Out in 100ms
%MW13 - Length of data received

When on-line with the controller it is possible to see all slave devices connected to the Channel from the debug screen.

Upon each operation the COM LED will flash on the TSXSCP114 card.
The TER LED will also Flash on TSX Micro each time it is polled.


The zip file below contains both Unity and PL7 programs.

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