Micro820 PLC - Modbus RTU issue - Code 55 - Messages fail

[jk0b]

Hey everyone, I am seeking assistance on finding and solving potential cause of a Modbus RTU issue, from a Micro820 PLC.

We are unable to successfully establish any Modbus RTU messages from the PLC. Any messages almost immediately fail, with error code 55, which appears to relate to a time-out of the message. We are only attempting to read, not write.

We have checked over the configuration time and time again, baud rate, parity and the rest of it, and everything is certainly matching, and the parameters all appear to be correct in CCW.

What's important to note is that the Modbus device connection works just fine when established from a MicroLogix 1400 PLC, which I assume would likely rule out any failures in the cabling or the Modbus slave device itself.

The PLC in question is more specifically a model 2080-LC20-20QWB, we are using the embedded serial port, and the program on the PLC is also handling completely unrelated Modbus TCP messages in another routine, and those Modbus connections are working perfectly fine.

Is there anybody else who has experienced similar issues to this, and is there anything else we should check or consider in order to resolve this issue?

Thanks very much in advance.

 

[TheWaterboy]

I know these are elementary but bear with me...
Have you tried swapping RX and TX wires?
Have you verified the device addresses and verified the zero vs one based register addresses?

I only say these things because I trip over them far too often to not double check.

 

[jk0b]

Hi TheWaterboy, thanks for the reply.


Yes - we have checked those things many times over.
All of which unfortunately continued to return the error code.


Would using a 2080-SERIALISOL plug-in card, as opposed to the embedded serial port, be likely to make a difference?

 

[jk0b]

Update - adding a "2080-SERIALISOL" add-on serial card, and 120 ohm resistors solved the issue. All good now.

 

[TheWaterboy]

that sure makes it sound like a cabling/termination/grounding issue.

 

[danw]

If using a separate serial port solved the problem, I would think that the problem was with the "model 2080-LC20-20QWB, we are using the embedded serial port", which was not talking Modbus for some reason.

 

[TheWaterboy]

But he added resistors as well, could have been that. I don't know a lot about the 800s.