Henry Watson

M241 Modicon Modbus TCP Slave input register reading problem

2 posts in this topic

I have been trying to configure my Modicon M241 PLC to act as a Modbus TCP slave using the built-in 'ModbusTCP_Slave_Device' option. I am using Radzio Modbus Master as TCP Master Simulator on my PC. Problem is I am able to read and write the Holding registers but not able to read the input registers. The Modbus Master simulator returns the message : 'Modbus Timeout Error'. On sniffing the traffic using WireShark, the PLC Modbus slave seems to reply Illegal Function, error code = 1 in response to a read request made by the master for input registers. Can you guys suggest some solution for this?

Do we need to separately set the read permission for input registers of Modbus TCP map - if so, how?


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I normally write the inputs and outputs to %MW or %M. Then you shouldnt have a problem. Not all modbus softwares allow all the modbus functions. But most should allow reading x3 and x4 which will be %mw and %m

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