Strange MPI comms issue on HMI (Siemens)

uptown47

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Feb 2008
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Hi all,

We have a system that comprises of a Siemens HMI talking to 5 Siemens 300 series PLCs via MPI.

The PLCs and the HMI are about 50m away from each other so we installed a 'repeater' at the PLCs end.

The system has worked fine for around 4 months but now, one of the PLCs signals are appearing as ### on the display (i.e. we are losing comms to just one of the PLCs).

There isn't anything that has changed with the PLC in question. And it seems to lose comms for ten minutes and then it might be ok for a few hours.

There is nothing in the Diagnostic Buffer about any issues.

The other 4 PLCs don't lose comms.

I implemented a count on the screen that goes from 1 to 9 cyclically and when the input fields turn to ### the count freezes and then jumps to another number so it is definitely a comms issue.

Can you think of what could cause this? I'm going to remake off the cable end at the PLC but wondered if there was anything else I could do to try and get to the bottom of this or if anyone had any ideas what might be going wrong?

Many thanks :)
 
Yep I considered that but it is strange that it's worked for a few months without an issue?

However, I'll certainly do that if remaking the connector doesn't do anything.

Thanks for digging that link out :)
 

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