Greetings mdeltat ...
and welcome to the forum ... I’ve got a hunch that my distinguished colleague OkiePC has come up with the right idea on your first question ... although my colleague Ken Moore (also distinguished) could of course be correct too ... anyway ... one possible source of confusion is that just because you see “blue hose” connecting two PLCs does NOT necessarily mean that DH+ (Data Highway Plus) is being used ... specifically, the same “blue hose” can also be used for RIO (Remote I/O) too ...
if OkiePC has guessed correctly, then the answers that I recently posted in this thread on another forum about running a PLC-5 in “adapter” mode should help you understand what’s going on ...
as for your second question ... finding the “control” for something like bit B10:9/3 can be more involved than many people think ... here are some ideas that might help ...
first of all (and simplest) bit B10:9/3 could be controlled at the “bit level” by something like an OTE instruction ... IF (big IF) it is, then the software should show you an “X” when you access the “Usage” feature for the B10 data table ...
next ... bit B10:9/3 could be controlled at the “word level” by something like an MOV instruction ... IF (big IF) it is, then the software should show you a “W” when you access the “Usage” feature for the B10 data table ...
next ... bit B10:9/3 could be controlled at the “file level” by something like a COP instruction ... IF (big IF) it is, then the software should show you an “F” when you access the “Usage” feature for the B10 data table ...
those are the most common ways that bit B10:9/3 MIGHT be controlled - and the simplest indications of how each of those control methods MIGHT be shown on the data table ...
in any of the cases listed above, using a “Find All” search with the “*” as a “wildcard” MIGHT help you track down the controlling instruction ...
going further ... “indirect addressing” might also be involved ... here are some of the things we normally look for in this situation ...
first notice that bit B10:9/3 can also be referred to as bit B10/147 ... here we’re using N7:0 and N7:1 as “pointers” to “indirectly address” the bit ... and this method might be quite tricky to track down ... for one thing, “indirect addressing” does NOT register on the data table when you select the “Usage” feature ... personally I always do a “Find All” search for ONE (and only ONE) of the “[]” square bracket characters when I suspect that “indirect addressing” might be involved - since those characters are always included in an “indirect address” ... then you’ve got to determine just what range of values MIGHT be entered into the “pointers” in order to nail down which bits are being addressed ...
finally (at least for now) in some systems it is also possible that bit B10:9/3 is being controlled by an HMI/SCADA system ... for example: RSView32 or Wonderware running on a networked computer might be controlling the bit ... or a PanelView system might be involved ...
I hope that some of these ideas will help you on your way ... wish I had more time ...