PLCnext, Radioline and Modbus RTU

PLCnext Engineer
1

I am experiencing some odd behavior with my AXC 2152 and the implementation of Modbus RTU using the Modbus_RTU_11 library.

I have two PLCs (AXC 2152s) communicating with one another over RAD-900-IFS radios. The radios are operating in the PLC/Modbus RTU dual mode. The PLCs both use SmartElement RS485 modules, and thus, I selected the appropriate block from the library.

I am seeing the message transmit from the master, through the radios, and hit the slave interface. I see the buffer for the module populate with the message bytes, and the block goes busy momentarily, but does not issue a response.

What am I missing in this implementation? It seems as though the slave device is not responding to the poll.


Master Block:
![](https://i.imgur.com/AkUFt06.png)

Slave Block:
![](https://i.imgur.com/wBPjGCS.png)
2

Hello Andrew,

do the baud rates etc. match ?
What request data are recieved?
Are there anny error messages ?
Maybe try recording the data transfer via the LogicAnalyser.

Also double check the cabling and termination.

For test purpose you could try attaching a PC as either Slave or as Master for easy tests.

kind regards,
Oliver