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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.