AB PLC 5 problem. Software running but no inputs, outputs running.

Lee Wood

Member
Join Date
Jan 2004
Location
Lancashire. England.
Posts
5
Hope can get some help. Today we had a senior, very AB PLC competent engineer in our plant to look at a PLC 5 problem. The program can be uploaded, downloaded and modified online as you would expect but the PLC is not responding with acknowledgement of inputs or outputs changing state. THe software seems to be running but we can find no reason for the fact that the inputs/outputs are not changing in line with software that is running. Bet you have all heard this before "never seen this in 20 years of programming". We can't think what we are missing?
 
This may be stupid but did you check your key position? I know its weak but it happens.

I have seen a bad cpu and a back plane problem create similar conditions on a plc 5. I assume you reseated all cards in the rack and checked power supply etc etc al

I am sure I am restating things you have checked but doesnt hurt to make sure.
 
Have seen it !

I have seen it several times, and others that I have spoken to have also seen it:
One module or all modules in a PLC5 rack doesnt respond or responds erratically. A very frightening experience :eek:

Cause: A design fault of the PLC5 module backplane connector causes the force that makes the connection to be too little. Some or all connections can come loose from vibrations or aging, even if the module seems to be properly inserted.
If only one module in a rack is behaving strange, then it is that module that is wrong.
If all modules in a rack are behaving strange, then it is the CPU or scanner in slot 1 that is wrong.

Remedy: Remove and insert the module several times to give the module connector "a massage". This clears the problem most of the times. If not, exchange the rack !
 
i've seen it!

after the system had been running for some time correctly.one day,one of the input modules did not respond.then i changed the slot and it works well.but unluckly,this did not last long.the module did not respond again.at last i changed the rack,and it is working well.
i took the "bad" rack back and tested it,finding it good.so strange~!
maybe the problem will be resolved by reinstalling the module again and again to make the module and the slot contacted richly.:)
 
If your I/O is in the local rack, the suggestions that have already been posted are what I would try!

If your I/O is connected via some adaptor - make sure the adaptor is correctly configured and healthy. Typically, you will see something to guide you in the right direction when you look at the diagnostic indicators, but not always.


I had a recent problem where a ControlNet Flex I/O adaptor had a bent pin and as a result, didn't recognize the I/O modules plugged into it and failed to work. The diagnostic lights told me "On-Line, but Not Connected" (everything was green, though one indicator was flashing). While this didn't immediately lead me to the bent pin, it did get me to look much closer at the adaptor itself!

Marc
 
Hi Lee,
Just a thought, have you checked the controller I/O properties? I seem to remember an Inhibit bit that can be set, this will prevent the PLC from updating it's I/O. It actually is called Inh and when set it appears to have a x in the box. There is also a res bit that will hold the outputs off.

Hope this helped.

Bitmore
 
re:i/o updating

I had a similar problem. it was a remote rack and someone had installed a new cable in the top of the panel. one of the drilling slivers got behind the rack and intermittently caused the rack to look fine but not respond. Also check that its not inhibited in the rack configuration... Les
Is it a local or remote rack?
 
We recently had a problem with a pump which would not shut off.
The cause ended up being a loose neutral wire at terminal block. The neutral wire went from the input card to a terminal block and then continued to Q.O. panel. We were showing some inputs on which should have been off, and others off which should have been on. Located the loose wire, tightened and all ok.
 
Thankyou to all who replied to my problem. We had tried the "cards in / out" approach amongst a number of usual fixes with no change. Ditto for all possible loose or poorly contacting cables and connectors. The problem was solved when we got to doing "irrational" changes to the PLC just to see if any response from the PLC was seen. The node was correct, we changed it and nothing happened, as expected. Then we changed it back and the PLC began working normally again. Couldn't make it fail or repeat the no input / ouput original problem. System has now run for 36 hours OK. Obviously we will monitor but I can't see it coming back. Maybe just one to remember for the future.

Regards to all.

Lee Wood
Enersys Ltd.
 
"The node was correct, we changed it and nothing happened, as expected. Then we changed it back and the PLC began working normally again."

What is "The node" and "it" ?
Did you change the DH+ address for CH1A via DIPSW1 ?
If so, then I guess that the problem was the connector between the CPU and the chassis backplane. This because you had to remove and reinsert the CPU in the rack in order to change the switch settings.
 
The "node" that I refered to was the address of the specific PLC within the network of PLC's running the system. "It" was the address. No we didn't remove the PLC from backplane to resolve. Only software change. Hope this clears up your question. If not come back to me.

Regards
Lee
 
PLC-5 controllers can be connected for online monitoring to three different networks; Ethernet, Data Highway Plus, and ControlNet.

You cannot change the Node Address of a PLC-5 on DH+ or on ControlNet without shutting down the controller and removing it from the backplane and changing DIP or rotary switches.

This means you must be talking about changing the IP address of the controller in RSLogix 5 software. This would require an "apply pending changes" operation to the controller, which might have something to do with "kick-starting" a firmware process that might have been suspended or locked up.

If you're serious about determining what caused this unusual state of controller operation, you're going to need substantially more detail about circumstances and symptoms.
 
Ken
Correct me if I'am wrong but isn't channel 1A the only
one controlled by the DIP switch setting? Channels 1B,2A,and 2B
are all software configured?

Mickey
 
Point well taken, Mickey.

I seldom configure more than one DH+ channel on a PLC-5 controller, so I always use Channel 1A as DH+, and usually run the other configurable channels as RIO scanners.
 
The problem was solved when we got to doing "irrational" changes to the PLC just to see if any response from the PLC was seen. The node was correct, we changed it and nothing happened, as expected. Then we changed it back and the PLC began working normally again.
...
Obviously we will monitor but I can't see it coming back. Maybe just one to remember for the future.

It has been my experience that any problem that inexplicably 'fixes' itself, will just as inexplicably 'un-fix' itself.
 

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