So some questions here are how do you use a message to toggle these bits? I need to use an input on one PLC and have that generate a signal that is messaged to the other PLC where it can be read to toggle and input on that program.
Your best bet is to mark off a block of integers (N file or B file) in each PLC to be reserved for this transfer of data.
Map the other data points to these reserved areas. By reserved, I mean you have made notes and commented the bits and words for your use...strictly documentation.
It is always my preference to use the same addresses on each end of a RSLogix 500 message. It is nice if N20:0/3 is a copy of N20:0/3 from the other end, rather than some other file and word location. Then your comments can be copied between the two projects with a minor edit notating the direction of the data transfer.
Then set up the MSG instruction to read (and/or write) from that area. I prefer to trigger MSGs that need to be frequently repeating to run from timer that gets reset when the previous message is ERrrored or DoNe. Some folks like to just MSG over and over again as fast as possible but that is rarely a good idea.
If the process needs to know for sure that the data from the other PLC is fresh and that the other PLC has not faulted out, send a heartbeat integer along with your control bits and use that in the other PLC to monitor for stale data. A copy of S:4 is like a random number. If it stops changing, then the MSGs stopped happening.
Sometimes it is best for the process to fire the message only when the data changes so that the information is updated as soon as it changes, but not more often than necessary. If you do this, then the "heartbeat" logic may need some rethinking...maybe send data every 5 seconds and instantly upon change...maybe something different.
The MSG instruction can't do single bits, the smallest amount of data would be a single 16 bit "word". But the overhead to do a single word is large enough that I typically do at least ten integers even if 9 and a half of them are spares for future use.
You cannot edit a MSG instruction online in a Micrologix (but you can with a SLC).
The way the message instruction behaves is pretty much the same for all RSLogix 500 processors, but the details of setting them up vary widely when you move from a SLC to a Micrologix, so we need to know exactly what CPU types you're connecting to advise further.