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I have set up the communications IAW the A-B manual for the SLC, using MSG blocks. The system was set up several years ago by someone else. I just duplicated their work to add more sites. It has worked OK, until this last upgrade, when I attempted to program in a site that was not up yet. What to my surprise, when I ran the program, all the previous sites failed communications. What appears to be happening is that the message queue is getting scrambled. If I disable the downed site the other sites will come back up within 2 minutes.

The user manuals usually give very simplistic examples of automatic re-triggering or periodic triggering of MSG instructions.

A robust multi-node SCADA system requires significantly more attention to port configuration and MSG instruction management. You can't just throw MSG instructions at the SLC port and expect it to manage them for you.

Without seeing your logic and knowing more about your system, I can't do more than guess what's going wrong.

I've seen systems where the radio modem buffers provided just enough collision prevention to allow three or four slaves to simultaneously reply, but not five or six.

I've also seen systems that simply placed too many MSG instructions into the execution queue, and when one of them didn't complete, the logic just re-triggered the others, filling the queue and requiring a PLC reboot.

You'll find quite a few MSG instruction experts over at www.plctalk.net/qanda