Output stays on when rung is false

chorn46562 · Mar 15, 2005

Hello everyone.
I am having an issue with a system. Specifically what happens is the output on a rung after it is activated will stay on even when the rung goes false. The instruction is just a simple output energize. The instructon address is not found as an OTE anywhere else in the program. It is occuring in the 5000 Allen Bradley PLC and it is using an RSView32 application for the GUI. I am just wanting to know if anyone has come across something similar and if so is there a software patch or firmware update to correct something like this.

Thanks everyone in advance.

just_lionel · Mar 15, 2005

It is hard to say. Can you post the code so we can look at it and troubleshoot it?

elevmike · Mar 15, 2005

My guess is that there's some other instruction turning on the output. Remember ONE instruction per output. You cant have two seperate rungs that feed the same coil.

Post the program so somebody can take a look at it.

kamenges · Mar 15, 2005

Are you sure the routine that the output is in is being called? Is it possible the routine is conditionally called based on the input driving the output? Can you explicitly write a zero to the output (or toggle it by right-clicking on the output and selecting toggle)? If you can the output isn't being written to in the program and is not being scanned.

Keith

bgtpha · Mar 15, 2005

Maybe you've got an electrical problem. Watch out if you're using some output adaptors for easy wiring... they use to be weak and can create some shortcircuits. Try to remove the adaptor, watch for wrong contacts in it and then put it in your PLC again.
Sometimes you can find some problems if you're using old relée outputs. When the relée reachs the limits of its use life, it can create shortcircuits too. Try to re-address the output to a free one.
Hope this can help you

Mtn_Bkng_Dave · Mar 15, 2005

Make sure the HMI isn't doing a write to that address. That could be turning the output on even though the rung logic is false....

Dave

akreel · Mar 15, 2005

Keith and Mike suggested the first two things I'd look at:

Is the rung being executed? Are you using jumps or subroutines?

And is the output used twice?

Also, are you sure you're using the OTE and not the OTL instruction?

AK

LJBMatt · Mar 15, 2005

I have to also agree with Dave if you're sure the output isn't referenced elsewhere in the logic and routine is always scanned I'd say your RSView could be affecting the output coil. I had an RSView application where the tag = 1 and tag == 1 is 2 different things in RSView. tag = 1 sets the tag to 1 in the PLC tag database, whereas tag == 1 is a compare statement checking to see if the tag "is equal to" 1.

Kent Hostetler · Mar 15, 2005

If the program uses buffered addresses that are copied to the output bits by words, the buffered address is another address to check.

For example, in our PLC-5s, my company prefers to use buffered addresses for outputs (e.g. OTE N11:0/0, another as OTE N11:0/1, etc.) then copy those buffered addresses - in this case the N11:0 word (comprised of 16 bits)gets copied to the output word as a whole word.

In this were true in your program, your false rung would want the output OFF via your OTE, but another rung might be causing it to turn ON although you can't readily find it because it might be turning ON an OTE with a different address that's mapped or copied to your output address.

I have to watch out for this in the PLC-5s, but I don't know what addressing format is used in other A-B platforms such as the 5000.

DonsDaMan · Mar 15, 2005

Then again, it could just be a bad discrete output that's shorted.

I've also seen solid state relays (triacs) that are unable to turn off due to an inductive load pulling the current even with the gate turned off.

Lastly, try lifting the wires to see if the output isn't welded shut (as in relay contacts holding in the armature).

If the DO is showing "on," try the force as others have suggested.

So, is the output device closed, or is the actual PLC output pin still high with the LED lit? Please give us a bit more to work with, such as voltage levels and other indications.

leitmotif · Mar 15, 2005

Is it possible you used the wrong contact instruction ie XIO vs XIC or vice versa with NC or NO switch??

Dan Bentler

REZAR · Mar 15, 2005

Chorn46562,
Any good progress?

CaseyK · Mar 16, 2005

Don (who I often call Dan) mentioned the bad output device.

Another possibility if you are using a NON-relay output. There is some leakage. If the output is connected to a very low current consuming device, leakage could keep it energized. This is remote, but possible. IF this is the case, increase the output load with a resistor or pilot light or other device.

Also, is your output a normal "coil" or is it retentative?

regards.....casey

I haven't seen a relay output "short" on, but I suppose that it could happen. I have only seen overloaded relay contacts blow an internal fuse, or release the "Resident Smoke".

darrenj · Mar 16, 2005

Been there and done that!! with the same set up, 5000 and panel-view plus..

as kamenges said if the subroutine isn't being called the outputs stay in there last state..I fixed that with an MCR command that shut the whole routine down when it wasn't called for..I meant to post something here about it because why would this happen? If the routine isn't being run then why turn the output on?..Think i just figured it out..i should not think if the routine being run..i should think of it being scanned..if the output was on when i stopped the routine then it isn't being scanned anymore and leaves everything in its last state...would that be a fair statement?

Darren

kamenges · Mar 16, 2005

Yes, that would be a fair statement.

Keith

Output stays on when rung is false

chorn46562

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