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Productivity PLC for MODBUS Simulation

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  • Productivity PLC for MODBUS Simulation

    I am new to the PLC game, and need a little help.

    I am having to develop a MODBUD simulator to test a monitoring system. I will be using MODBUS RTU to communicate with a master device. I am trying to determine if the Productivity PLC's can be programmed to change a MODBUS value on a timed basis. I need it to hold a value for 30-60 minutes and then change to another value.

    Any advice?


  • #2
    Since not many folks are around this evening, the short answer is yes, it can. I don't do Productivity yet myself, but it should be more than capable. Do-more and Click PLCs are also capable, given a processor with the right ports. Maybe someone will chime in later.

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    • #3
      Mike, I agree - on all counts.
      1. not many folks around
      2. the short answer is yes.
      3. someone will chime in later

      One issue is OPs usage of the word 'MASTER' in conjunction with RTU!
      From the wiki:
      In serial and MB+ networks, only the node assigned as the Master may initiate a command.
      On Ethernet, any device can send out a Modbus command, although usually only one master device does so.


      If PAC is to spit out values on an interval, will the 'MASTER' be listening?
      Maybe the MASTER is querying the PAC. If so, great - this would NOT require any Modbus comms code in the PAC.
      We don't yet know.

      given a processor with the right ports
      I think that all PAC CPUs have 1 - RS-232 port and 1 - RS485 port.


      Mr. Moose, please provide some more info. Thanks.

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      • #4
        Yes, the master device will be polling. Sorry for not providing enough info, still new at this.

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        • #5
          Should not be a problem then.

          You will need to assign a Modbus register address to your PAC TAG.
          You will have code to change this tag's value, but no code to send, since this will be a RESPONSE to the MASTER's query.

          In your MASTER, you may have to add or subtract 1 from slave bit or register address.

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          • #6
            Both the P2 and P3 will have no trouble with MODBUS RTU -- the PAC can be either the master or the slave -- it cannot be both. If the master -- set up the polling in the task box. Ignore the busy and ready bits unless you want to get into a fist fight in a broom closet with the comms UART -- I have found that just cycling the query on a timer works best for some reason. The largest network I am presently running is 26 slaves off of a single master PAC. I am polling a 485 net on a sequencer at 192K and dumping about 5K of data on each flash message both world wide and specific. Total time on the net is about 1/2 second (it is very quick). Remember -- if sending bit instructions -- they go on and stay on until you reset them either internally or by the net -- so to turn on a bit in a slave takes only one command pulse. It will stay that way until you come back to it. You can do the same on the Ethernet port if you want -- in my case the Ethernet port was busy talking to the plant master computer and data base system. If you have a small network, the auto poll command will work fine.

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