PLC Output Issue

Thread Starter

Graham7853

Joined Sep 3, 2021
2
Hi All,

Quick question, has anybody had an issue before where an Output will operate intermittently then seems to sort itself out eventually?

We have a machine where each morning this week, we seem to have lost an Output. The corresponding LED is illuminating but there is no output from the card. BUT, eventually the output will appear and the longer the machine runs, the more it corrects itself. The strangest part of the issue is the problem will reoccur the next day upon start up and the same scenario will play out. The problem first occurred on Tuesday morning and has happened each day since.
 

strantor

Joined Oct 3, 2010
5,677
Usually these problems are not intermittent. More commonly it works or it doesn't. But it is obvious you have a bad output. I assume a relay output, and probably it only works after its coil has warmed it up. You can usually solder in a new relay easily. If I'm wrong about it being a relay, still possible to replace the output triac or transistor, but it may be less obvious which component needs replacing.
 
The kind of scenereo your prescribing could be caused by a cold solder joint. They can occur inside an IC too.

It's a thermal failure. Examination and freeze spray are usual tools to isolate the fault. Freeze spray enhances the cracks in components. Not sure if a thermal camera can help.
 

Thread Starter

Graham7853

Joined Sep 3, 2021
2
Thanks to you both for the responses. I just wondered if I was missing something as I hadn't come across this before. As Strantor said, usually they either work or they don't. I suspected that it "warming up" could be the reason for working after a while and just wanted to check
 

GetDeviceInfo

Joined Jun 7, 2009
1,897
Certainly I have come across failed output ports, but one must be curious as to the port configuration and the method of determining its functionality. If it’s configured as a sink, it of course needs something to sink, same can be said for sourcing.
 

MisterBill2

Joined Jan 23, 2018
9,539
It might also depend on the load that the output is connected to. Is it sink or source from an external supply, or an internal connection? The very first diagnostic will be to remove and re-insert y=that output module. If you have more than one output module of that type, swap them and see what happens. If you have staff qualified to open and resolder the connections that could be a quick fix occasionally. Probably it is temperature driven somehow.
 

panic mode

Joined Oct 10, 2011
1,891
are you sure that supply to your output is not disabled by something else such as safety circuit? not all outputs are powered from steady source....
 

MisterBill2

Joined Jan 23, 2018
9,539
There is another possibility, sometimes called "the Monday Morning Syndrome", in which a solenoid air valve always hangs up on startup after being off all weekend. Some valves with some lubricants will do that very consistently. So the question there is "Has the voltage output been checked?"That is definitely a possibility, oif the output is to a valve. The quick fix is "Marvel Mystery Oil"
 
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