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UAC-CX-01RS4 Modbus problem


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I'm having a really hard time getting UAC-CX-01RS4 attached to a US5/7-x10-TR22 working. I have followed the manuals as close as I can and the software is not giving any warnings or anything but for some reason there is no activity on the RS485 bus. I have checked the bus with a USB-RS485 cable and the PLC is not causing any traffic on it. I have only 3 devices on the test bus - the PLC, the USB cable and an RS485 IO module from Wellpro. The IO module works fine when I send it requests from the USB cable. And I can see the responses arrive over USB as well. But when I run he PLC nothing happens. What I need the PLC to do is act as modbus master and periodically poll the IO module for data. Can you please share some example code of modbus master on RS485 bus?

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Hi Ratsept;


Download the UniStream Examples Zip file from the UniStream Downloads section.

There are Modbus examples under Communications.

The Modbus is set up under protocols and creates a Struct  in Global Variables that has data in there about how the 

communications are working - Attempts, successes, fails, and status codes, and more.

Optional -  Post your Code  and we can review it.




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  • 8 months later...



I reviewed the program.


You are not properly triggering the Modbus   Read

The  Tag  TAGASITEMP  is set up as both the data Tag and the Trigger -- will not work.

  Change the trigger name  to ( say)   TAGASITEMP-TRIGGER     in the Active Periodic  loaction 

    then in the ladder turn that    Bit   ON  to read the coil data   each time you want to   read the coil.




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