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BRX Modbus communication with Barix X8

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  • BRX Modbus communication with Barix X8

    Is anyone using a BRX with a Barix? I'm trying to get the PLC to talk to it but even though the X8 appears to receive the message it doesn't send a response. I'm using the @IntModbusClient device with the Barix defaults (19200,8,E,1), I've set RS485 with the termination resistor and I'm attempting to read Holding Register 257. I've tried messing with CTS and RTS control settings but the best options there are the defaults (CTS: Unconditional, RTS: Follow Transmitter). I've tried swapping A and B without success.

    Any help would be appreciated.
    Thanks, Andrew.


  • #2
    Have you triied to get this working with a modbus master program and your PC? I would work through that first to verify wiring, etc, etc. Then move to the PLC. You can find download many different free modbus master utilities.
    A few of my favorite:
    modpoll - https://www.modbusdriver.com/modpoll.html
    RMMS - http://en.radzio.dxp.pl/modbus-master-simulator/

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    • #3
      Thanks for the response. I've been trying to get it working with a PC but without success so far. I think I've found the issue though - by default the unit has a modbus client-id set to 255 which isn't a valid id (valid range is 1-247). I need to find a way to change its client ID to a usable number.

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      • #4
        Originally posted by Andrew S View Post
        Thanks for the response. I've been trying to get it working with a PC but without success so far. I think I've found the issue though - by default the unit has a modbus client-id set to 255 which isn't a valid id (valid range is 1-247). I need to find a way to change its client ID to a usable number.
        255 is the valid default for Modbus/TCP.

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        • #5
          Problem solved. In addition to having to change the client ID, I had to put a 5ms transmit-control delay into the port config. Thanks again for the assistance.

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          • #6
            Originally posted by BobO View Post

            255 is the valid default for Modbus/TCP.
            That's what I read too Bob, however my master test software (qModMaster) only allowed me to use 1-247, and this website also said valid IDs are 1-247: http://www.simplymodbus.ca/FAQ.htm

            Anyway, it's working now.
            Thanks, Andrew.

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            • #7
              Originally posted by Andrew S View Post

              That's what I read too Bob, however my master test software (qModMaster) only allowed me to use 1-247, and this website also said valid IDs are 1-247: http://www.simplymodbus.ca/FAQ.htm

              Anyway, it's working now.
              Thanks, Andrew.
              I think with Modbus/TCP the Unit ID can be used to route though Ethernet to a Modbus/RTU connection, but when talking to a TCP endpoint the ID is basically unused. I think 255 can also be used for RTU broadcast, but I don't think that is very common.

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              • #8
                Originally posted by BobO View Post

                I think with Modbus/TCP the Unit ID can be used to route though Ethernet to a Modbus/RTU connection, but when talking to a TCP endpoint the ID is basically unused. I think 255 can also be used for RTU broadcast, but I don't think that is very common.
                I believe both of those are correct, about the TCP -> RTU routing as well as RTU broadcast. The only case for broadcast that I remember ever hearing is using it with slave that could have its ID set that way.

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                • #9
                  Originally posted by ControlsGuy View Post
                  The only case for broadcast that I remember ever hearing is using it with slave that could have its ID set that way.
                  That's exactly what I ended up doing. It worked like a charm once I had figured out the delay problem and used a Modbus Master that supported ID 255 (thanks for the tip with RMMS).

                  Andrew.


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                  • #10
                    I have had to use either 255 or 0 for the address on a few Modbus/TCP connections to get the slave to pay attention as well. Seems to be an oddity in the way the master is configured... Different vendors using different styles or whatever?

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