RS5000 weird message tag substitution

My fellow PLC tech just found something weird, as in there are only two of us who could have done it but we didn't. And we know we would not have done this. We have a ControlLogix L61 that communicates with three PLC-5s, a read and write message to each one. We are using 100 element dint arrays as the source and destination tags. The messaging is working fine, but when we open the configuration box, on 4 of the 6 messages, the source (or destination) is listed as the alias for the first element instead of the base tag. Neither one of us would have programed it like that, so we blamed each other of course.

Has anyone else ever seen this happen before, or is one of us just very forgetful of what we did? Maybe a strange compiling error?

The first element of each array is the heartbeat, and the alias is only a single dint. I would not have thought the message would work correctly like it is, but it does. I actually hope one of you has had this happen before because then we are not going senile.

My guess is that you're right about it being related to how the compiler handles aliases.

I remember something similar years ago where if you lost the offline program and performed an upload, all your references to aliases would appear as references to the base tag, because that's how they got compiled.

Maybe the code that shows the Source and Destination tags in an MSG instruction control block works the other way around.

Thanks Ken, I knew I couldn't be that nuts. After I wrote my first post and was getting ready to leave for the day I went back and saved the current program. Then the other two messages did the same thing. My colleague was already gone for the day and I know I didn't do it. Before I went home I changes the source and destination tags back to the base array tag and thus far has not changed on us.

We do have some occasional drops in comms with rslinx for no apparent reason, so maybe it has something to do with that. Wasn't a major problem but it was just one of those weird things that happen with Rockwell Software.

I believe this is compiling "feature" as you guessed. I recall an incident similar to what Ken described a few years back.

In that case, there were several instances of a base tag with two aliases. The original logic had alias #1 used throughout. The uploaded logic had alias #2 instead. Since that episode, I've tried to keep no more than one alias for each base tag.

I actually hope one of you has had this happen before because then we are not going senile.

Click to expand...

Maybe we're all going senile!