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DL06 master to P2-550 Modbus

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  • DL06 master to P2-550 Modbus

    Has anyone used a DL06 as a master to communicate to a P2? If so I need some help. I am using a DCM on the 06 with the MWX and MRX instructions. It seems like the MWX works great. The MRX doesn't though. So I am able to write to the P2, but the read is very unreliable. It seems like it only works here and there, then when it does turn a bit on in the 06 it will not shut the bit off.
    My thought is if I can write, I should be able to read?

  • #2
    Should work in both directions.

    If this is an existing project, is there anything else in the P2 program that can modify that bit?
    If not, try NO program in the P2 except for an END instruction. (You may be able to have the P2 in STOP mode, if you cannot clear an existing project.)

    Ensure that nothing else is on the comms line - and that nothing else can change the bit.
    Write a single bit, wait a little bit (10 - 20 mS) then try to read the bit.

    I have not used the DCM, so maybe it has some processing time overhead that you are not accounting for.
    Maybe the wait described above can help.

    I browsed the DCM manual, I remember seeing the phrase 'read a block' and 'write a block', so maybe it doesn't work at the 'bit' level, and needs to read a 'block'- size of bytes range. Whatever your 'block'-size is.
    Last edited by kewakl; 11-04-2019, 02:59 PM.


    • #3
      It sounds like a timing issue.
      There is a sample program in the manual for multiple writes/ read instructions. 4-17
      I like using a shift register to keep the timing of the communication.

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      • #4
        Thanks for the input, I have tried everything using the DL06 as the master and still have no luck. I did however reverse the master to the P2-550 and it works like a champ.
        I am not sure why the 06 doesn't work as the master.


        • #5
          What port on DL?
          I think that one ofthe DCM ports is slave only.


          • #6
            from the p2000 manual:

            Modbus RTU Slave connections: The RS-232 port is intended to be used for point-to-point
            connections but it is possible for the RS-232 port to be used on a Modbus RTU network by using
            a RS-232 to RS-485/422 converter. The port is addressable in the Hardware Configuration in the
            Productivity Suite programming software. It is important to note that the RS-232 port cannot be
            a Modbus RTU master and slave concurrently. If the port is set to Modbus RTU and there are no
            communications instructions (MRX, MWX, RX, WX) in the project, the CPU will automatically
            respond to Modbus requests from a Modbus master. See Communications Connectivity section for
            more information.


            • #7
              I am using port 2 on the DCM, RS-485. I am using this currently in my plant as a master, without any issues. I have found a solution to my issue. I am taking the P2 (Master) to port2 of the DL06, then using a separate network to do the communications to the rest of the 06 PLC's.
              I would still like to know if anyone has had luck using the P2 as a slave to an 06 though. When I have more time I will get back into this issue.