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CPU talking with another CPU?

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  • CPU talking with another CPU?

    I'm new to all of this and I'm trying to get 2 seperate CPU's (P3-550) to communicate with one another. The application is having each unit being able to take over full command at each hub in case of an outage. I'm not sure if I'm missing a setup piece in the Productivity Suite Programming Software or if I need to set up certain parameters.


    Any help you could give would be appreciated.


  • #2
    I dont have any ideas for a good way to implement failover. Normally you would use a remote slave CPU and the slave rack would talk to the main CPU, however if the main rack went down the slave wouldnt be able to execute the whole program on its own. And normally the slave rack has its own IO anyway, not a duplicate of the main rack.

    My guess is you would have to implement some sort of communication to each PLC every 25-50 milliseconds or so and if a good communication signal is not returned then it would set a bit on that the secondary was now in "run mode". Obviously it is always in run mode, but the bit would be like a run command bit. This bit would go to a "call task" instruction and your whole program would be in that called task. That way the program on the secondary PLC only runs when it is the primary unit. The primary PLC unit would have to have a check in it so that before each scan it sees if the secondary is "running" things and if so, the main unit wouldnt run. Instead, good communication would now be restored so you would then have it hand off the program execution at a suitable point back to the main unit and the secondary would stop executing the program.

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    • #3
      What method are you going to use for communicating between the two CPUs?

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      • #4
        Originally posted by LWgreys View Post
        What method are you going to use for communicating between the two CPUs?
        It would be through a network over a switch.

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        • #5
          So either Modbus TCP or EtherNet/IP then.

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          • #6
            Do you wish to have two PLCs perform the same logic?
            Why worry? If you've done the best you can, worrying won't make it any better

            - Walt Disney

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            • #7
              I think I understand what you are trying to do. Are you having a master PLC control a few process and if comunications fail the remote racks take over for them selves? If so I would use Ethernet IP implicit and in the remote plc set up heart beat detection to check for good coms. If the coms fault the remote plc will call sub routines with the Masters program for that rack. If the cons are ok it will call a subroutine that maps the Ethernet IP words to IO. I hope this makes sense to you.

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              • #8
                Take a look at this Automation Direct video:
                https://www.automationdirect.com/vid...at3=10&q=ether

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