Allen Bradley rack locking up

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Thread Starter

Jeff Groves

I have a process running with a PLC5 and a remote 1771-A4B rack, ASB module, and some digital I/O. Occasionally (once every 3-4 months) the rack seems to lock up, it stops updating the I/O image to the PLC. Recycling the power to the rack clears the problem and away we go again. Has anyone seen anything like this, and where would my best starting point be to troubleshoot this problem? It happens infrequently and usually during production, so I don't get a good opportunity to analyze what is going on. Thanks.

Jeff Groves
PLC/Electronic Controls Tech.
Hoffman Enclosures
2100 Hoffman Way
Anoka, MN 55371
763-389-5621
[email protected]
 
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Bob Desrochers

The indicating lamps on the ASB card should point you in the direction of the problem the next time it happens. Be sure to use the ASB manual for your specific rev. Another thing that you could do is observe the performance of the blue hose to
the ASB at the plc. A good installation will run for weeks without a single error or retry.
Something I would consider trying the next time it crashes is to initiate a "reset" of the ASB instead of cycling the power. If the reset works the ASB card is probably not "locking up".

Bob
 
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Jeff,

I too had this problem a few years back. I had a 5/40E with five remote racks. About once every three or four days the remote racks "disappeared". This caused all the outputs to turn off. When it happened at night, with the plant unattended, some major process problems resulted. Usually, cycling power on the
remote racks fixed the problem. A few times the PLC had to be reconfigured to see the remote I/O again.

Allen-Bradley told me the problem was probably bad power. But all the PLC equipment was running off an online UPS and we thoroughly re-checked all the grounding. The next suggestion from A-B was to run the remote I/O cable in its own
conduit. We did this at great expense, but it didn't help. The other A-B theory was that the remote I/O cable was acting like an antenna and picked up RF which caused the faults. ( If that's the case, this product is unsuitable for industrial applications ! ).

Leviton makes a surge supressor for the remote I/O cable ( I don't remember the model #) which we installed. We also installed surge suppressors on power to the 120VAC digital output
modules. After this, the remote racks locked out about one every other month, like yours. We ended up re-engineering the control panels so the plant would shut down safely on a rack failure.

Sorry, not much help here, because I too am still interested in finding a real solution to this problem.


Jay Kirsch
Macro Automatics
2985 E. Hillcrest Drive, Ste 101
Thousand Oaks, CA 91362
[email protected]
 
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Blake Burgess Averydennison

Is the remote I/O cable more than 10 feet in length.

We had a similar problem with a cable length of about

6 feet to the next rack...
 
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Robert R. Stephens Pennzoil Products

I had a very similar problem with another manufacturers PLC. I found the problem was the backplane and the problem cleared up when I replaced the backplane.

Good Luck
Rob
 
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Larry Lawver

I wonder what the LEDs on the ASB are telling you when this happens...

I have seen complaints like this when the wiring and grounding guidelines
aren't followed.

For 57.6 kbaud Remote I/O, here's all you need to do:

1.) Daisy-chain the blue hose from device to device, and put a 150 ohm resistor across blue and clear at each end. This means that every screw terminal in the RIO system has exactly two leads on it, except for the Shield at each extreme end.

2.) Keep the length less than 10,000 feet, and follow good practice by not running the blue hose with power and motor cables.

3.) Establish a common ground, and run green wire of sufficient gauge (12ga or larger) from each device's grounding lug directly, uninterrupted, back to the common ground. (I don't really care if your counterpoise goes
through the earth's crust, just establish that one common ground!)

This last item is the most common source of trouble, particularly with experienced electricians who have other ideas (and colorful language). The right ground meets code; a code ground is not necessarily good enough.

Hope this helps!

Larry Lawver
Rexel / Central Florida
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Juan De los Santos Naumov

Dear Mr. Groves,

We have the same problem. But the frequency was increasing and finally we change the chassis. since that (2 months ago) the problem disappears.

Good luck

Juan De los Santos Naumov <mailto:[email protected]>
Division Manager
Industrial Control Division
CIRCULAR S.A.
(598) 2 902 5785 (TEL)
(598) 2 902 9151 (FAX)
 
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Al Pawlowski

I don't know exactly what AB's instructions are for wiring up remote I/O comm links or whether a common ground is needed for their equipment (if
they say to connect the blue hose shield at each chassis, that could be the problem). However, I disagree that a common ground should be necessary, or even desirable, for linked equipment that designed to be separated by more
than 100 ft, or so. IMO, this kind of equipment should be connected to the individual local grounds and isolated from one another as needed. In general, I think it would be safer and a better system design (I like component independence). I also question whether isolating electrical equipment from the local ground is permitted by the NEC; those loud electricians may be more correct than not.

IMO, if a common ground is neededed for AB equipment, AB has not done its design "homework". Maybe the link transcievers have less common mode
rejection than they should. In any case, it is possible to put in locally grounded systems that are linked and do operate correctly. I have done it myself.
 
J
Thanks all,

I think I am going to try swapping out the chassis and see what happens. The weird thing is that while the rack is "locked up" all indicators on the ASB show normal operation.

Jeff
 
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A few suggestions:

1) Make absolutely sure you are using the correct value of termination resistor, only at the ends of the RIO runs, and that it is actually making good connection with the cabling. Easiest way to test is to remove the connectors off all the ASB modules and then use an ohmmeter across the blue and clear leads and make sure that you read HALF the nominal value of each termination resistor. You may find that one of them is not making contact, or is the wrong value for the RIO link speed you are using, or even that someone has put a termination resistor in the middle somewhere.

2) Make sure that the correct type of cable is being used. I ran into a situation with intermittant RIO problems where someone had moved a cabinet and spliced in some regular old shielded cable cause the blue hose was no longer long enough to reach.

3) Make sure its one long run of cable, daisy chained. No star configurations, and make sure it is not in a loop configuration. Also check
for drop lines (bad idea) or short segments (also a bad idea).

4) Check the entire run of blue hose and make sure its run in its own conduit with no power/motor wiring anywhere near it. You might even want to go so far as to meg it out )after disconnecting it from the PLC of course). Could be an insulation nick somewhere along the way.

5) Distance can be a problem at higher RIO baud rates. Try running at the lowest RIO baud rate if its not already.

6) Some older remote racks seem to be prone to having problems with block transfers causing either the racks to lock up or BT modules to stop
communicating. If its an older rack, power supply, or ASB, replace them all (maybe from your existing spares) along with the power supply cable. Older ASBs seem to have more quirks, as do the older racks. Even if its not older stuff, replacing it rules out them as the problem.
 
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In response to Jeff Groves' troubleshooting request, I posted a breezy summary of the wiring rules for Allen-Bradley Remote I/O. In that post, I was presenting facts, not opinions; you really do have to follow those rules to have a successful RIO system.

In a following post, Al Pawlowski questioned these rules as if they were opinions. I'd rather not start a thread in this new direction, but some
readers might misunderstand or doubt my previous post if I didn't respond.


Al Pawlowski writes:
<I don't know exactly what AB's instructions are for wiring up remote I/O comm links or whether a common ground is needed for their equipment (if
they say to connect the blue hose shield at each chassis, that could be the problem). However, I disagree that a common ground should be necessary, or even desirable, for linked equipment that designed to be separated by more
than 100 ft, or so.>

The instructions are exactly as I posted them. An opinion does not change that.

< IMO, this kind of equipment should be connected to the individual local grounds and isolated from one another as needed. In
general, I think it would be safer and a better system design (I like component independence). I also question whether isolating electrical
equipment from the local ground is permitted by the NEC; those loud electricians may be more correct than not.>

I did not say anything like this. Properly installed, RIO components ARE grounded locally. The common ground for communications purposes enhances safety, in my opinion. Nowhere did I say anything about isolating equipment from the local ground, which would be a terrible idea. The
dissenting electricians are still wrong.

<IMO, if a common ground is neededed for AB equipment, AB has not done its design "homework". Maybe the link transcievers have less common mode
rejection than they should. In any case, it is possible to put in locally grounded systems that are linked and do operate correctly. I have done it myself.>

So have I--- but I didn't do it in 1978 and proceed to deploy a couple of million nodes over the next two decades with remarkable forward and
backward compatibility! Of course, the system would be designed differently today. That doesn't help Jeff Groves with his problem.

Just trying to help!

Larry Lawver
Rexel / Central Florida
 
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I read Larry's post to say that AB recommends a scheme calling for isolating remote equipment from the local grounds and he recommended that
as the way to go.

I guess I misunderstood the post and apologize if my remarks seemed like any kind of personal slight.

However, I don't see how remote equipment could be wired to a single common ground and not isolated from their own (local) grounds without wiring in ground loops. In my experience, ground loop currents are trouble (especially when running near signal lines, like comm links) and I assumed local isolation as the normal way they are avoided.

In my experience, the way to do it is to locally ground. Use a single signal common/ground each local system and isolate from remote systems as
needed. I do not recommend connecting local grounds together, through a single common point or otherwise. I would not recommend even if AB does.

And, I have put in working systems of this kind as early as 1974.
 
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I despair of getting Al Pawlowski to see the point of my original post to this thread. For everyone else, note that the NEC is clear about the grounding-electrode conductor that is the central ground for all electrical equipment and AC power within a facility. I'll also mention my own favorite reference on these matters, _Electromagnetic Interference and
Compatibility_, published since the 1970s by Don White Consultants, Inc.

If you want your Allen-Bradley Remote I/O system to be, in general, UL Listed and CSA Certified, Class 1 Division 2, then ground it the way the
documentation and I told you to do it, and it will work. If you want to follow Al Pawlowski's advice, you are on your own.

Hope this helps!

Larry Lawver
Rexel / Central Florida
 
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