Expression Tag - Error_ExpressionEval

Ignition
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2

I can't see the problem (yet) but here are a few tips:

  1. See Wiki - how to post code on this forum. Yours is almost OK except for the first line.
  2. system.tag.readBlocking() expects a list of tag paths so even when there is only one it should be in [ ] brackets.
  3. Since system.tag.readBlocking can handle a list you do them all at once.
    tagPaths = ['xx/ATS-01-PR-A/POS',
                'xx/MPE',
                'xx/MPE_Previous',
                'xx/PE'.
                'xx/RUN'
    ]
    tagValues = system.tag.readBlocking(tagPaths)

Your code would then need to reference tagValues[0], etc.

  1. You also need [ ] lists on the writeBlocking line.
    system.tag.writeBlocking([tagPaths[2]], [mpe])

In current Ignition version the functions work when only one parameter is supplied (not in a list) but it's undocumented and could be "fixed" in the future.

A further thought - and this might be the problem: The script needs to be on the gateway, not in your project, otherwise the tag event can't find it. Where have you stored the script?