How-to Test & Troubleshoot: 1785-BCM Back-Up Communications Module Allen Bradley PLC-5

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Today we’re going to be testing a 1785-BCM Back Up Communications Module. I have two
here, the test will be basically testing the modules ability to pick up if one fails.
So, if this rack fails which is currently in primary, then this rack, or back up communications
module should pick up and take over and vice versa. This way I know that they’re able to
pick up number one if they’re supposed to and number two that both modules are communicating
with each other. And we’ll probably have a little bit of troubleshooting maybe a couple
problems that I see in the field. So, right now the way I have this set up is I have my
parallel wiring between the two BCMs and I also have my wiring going from Channel 1A
and Channel 1B on the processors. I have two racks set up as just my test lab here this
is probably a smaller version of what you have in the field, but there are two 1771-A1B
four slot chassis, they’re being powered by 1771-P4S power supplies and the processors
are 1785-L20B series E They’re both L20B- series E they don’t have to be if in the field
you have an L20 on one side or an L40 or L60 or L80 on the other side it doesn’t matter.
What’s important is that they’re both communicating on the same Data Highway Address, that’s one
tip for you. So, now that the system is up and running,
it is in remote run mode for this test procedure I want to simulate a power failure on my primary
rack, now that’s important it must be on the primary rack ro make sure that the secondary
rack moves itself into primary and knows what to do and when to do it.
I’m going to do that by shutting the power off so please follow. This light here should
move to primary when I shut this off and it has, exactly what I want to see. Similarly,
when I turn this power back on, my rack that was my primary should jump into secondary
and it has. I’ve turned it back on and as you can see at the tip of my finger here,
the LED has moved itself into secondary position. So, these two modules know exactly what they’re
supposed to be doing and to even further the test I’ll simulate a power failure test on
this side by shutting this down now and my rack on the left has jumped into primary so
little bit of back and forth there but this is telling me that these two processors are
communicating properly, they’re doing what they’re supposed to be doing when they’re
supposed to do it. Okay, so a couple common failures I see in the field, a little bit
of troubleshooting is somebody may have changed the information that’s in the processor and
it’s not communicating properly so some of the troubles we see range anything from communication
to module failure and you usually see at the tip of my finger there’s the fault light and
it will turn red if you have a problem and like I said it could be anything ranging from
communication failure or distraction maybe you don’t have something terminated or you
resistor terminator has broken loose and it’s creating a fault and not communicating properly
all the way to just basically a bad module. another thing is you want to make sure they’re
communicating at the same b rate So a couple things for you to look for pretty
easy and I hope you found that this is helpful these are again 1785-BCM is a module that
we carry we do have them in stock and we do offer same day shipping so if this is something
that interests you please see us at PDFsupply.com Thank You

Bernard Jenkins

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