must type faster ... must type faster ...
darn ... while I was typing away, Tom beat me to it ... but I hate to waste all of this so I’m going to post it anyway ... maybe the repetition will be beneficial ...
Greetings jrs2563,
this is just a wild guess based on what you’ve posted:
something else (PROBABLY located in the PLC-5/40’s program) might be overwriting the binary word ...
recommended plan of attack for troubleshooting:
temporarily change the “write-to” address in the message command to aim it at a new UNUSED binary word in the PLC-5/40 ... see if the bit settles down ...
or MAYBE the actual input in the PLC-5/20 really and truly IS turning off and on (you know that you can’t always trust the ON/OFF status on the screen display) ...
so temporarily change the “write-from” address in the message command to take the data from a new UNUSED binary word (specifically not an “I:” Input word) in the PLC-5/20’s memory ... see if the bit settles down ...
let’s suppose that the last step above DOES settle the binary bit down ... but you can’t come up with any logical reason for the actual “I:” input bit to be going on and off ... here’s a REALLY long shot to look for ... suppose that someone has entered a typographical addressing error on an OTE (Output Energize) instruction ... and instead of “controlling” something like “O:123/0” they’re “controlling” your input bit “I:123/0” ... again that’s a bona fide LONG SHOT ... but I HAVE seen it happen once or twice ... basically that type of error will hijack the ON/OFF status of the input bit and cause it to act erratically ...
another thing that could be causing the binary bit in the PLC-5/40 to jump around is an HMI ... sometimes (rarely but it DOES happen) the HMI programmer makes a typographical error and hijacks (and therefore controls) a bit in the PLC ...
and just for completeness ... based on what you’ve posted, I do NOT think that this next one has anything to do with your current problem ... but do you realize that in a PLC-5, the MSG (Message) instruction is an “asynchronous” instruction? ... that means that even though the MSG rung might be EXECUTED right !NOW! in the processor’s scan, the actual transfer of information between the PLCs might not occur until some time !LATER! in the scan ... how much later usually depends on network traffic, etc. ... once again, I don’t think that this has anything at all to do with your problem ... but ... it MIGHT be something to think about if nothing else turns up ...
finally ... is there any reason why you can’t zip your .RSP files and post them on the forum? ... we’d be glad to take a look at them for you ...