Micrologix 1400 base outputs issue with Multihop

H2Otbaker

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Apr 2014
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somewhere over the mountain
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ML1400 base outputs are being controlled by instruction that is getting messaged from another ML1400. The messaging works because there are other outputs on an OW16 module that work just fine. Without a program in the processor and just by toggling the bits, the outputs will work. Once instruction is applied to the rungs, the instruction doesn't work. There's only two instructions in each rung. The rung is being scanned because i've inserted test rungs and can change the outputs state with a test binary bit. If the (xic) is toggled in the rung, the output in the rung doesn't change state. How can the (xic) instruction have a 1 in the data table but the output doesn't turn on? There is nothing else "controlling" the output. Again the only outputs that are not working, that is being "controlled" by the messaging are base outputs.
 
I'm unclear if your messaging instructions are writing directly to the outputs or to some internal registers (N or B registers). I would be too paranoid to ever write a message directly to outputs for reasons like you describe. If the message does write to internal registers, you should be able to view their state in the receiving PLC just by going online and that could help you understand what is going on.

I'm Assuming you have run a search for the output bit and it only shows up in one spot? If the XIC bit is the messaging bit, are you seeing the XIC bit changing when the other PLC messages a change? If so, have you tried an experiment of creating a test-only bit on the rung and forcing it to see how it affects the output (to eliminate the messaging as an issue)?
 
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