Puzzle for Siemens S5 EXPERTS!!

[Kusmaul]

I have an S5 135U PLC that is loosing inputs to the CPU in as small a time frame as one scan, I.E., the E-stop input should always be on,
I 0.0, unless of course the E-stop circuit is open, I have ran a pigtail from the E-stop controller to another PLC to make sure it is indeed a PLC problem and NEVER loose it on the other PLC, an S5 115U.
I am loosing inputs from different input cards, not just one, as I set up test inputs from spares. The card order in the main rack is CPU, a WF706 encoder card, 4 input cards, 4 output cards (3 regular output cards and 1 relay card, then an expansion card for an ET200 unit that runs the control panel. Is there a way to troubleshoot the problem to the defective component, or is it a swap solution till it goes away, I have already replaced the CPU, power supply, and the PLC rack, and the first input card. Help!!

[Mike Williams]

I had a problem close to the same with our s5 system. Make sure you record the voltage level at the input that you are missing.

When the problem here became mine, the cards, cpu etc... had been replaced.

I recorded voltage levels at the inputs and found it dropping from 24 to 6vdc. Turns out it was a shorted prox that was located on a elevator that moves up and down. When the wire decided to, it would short to the frame of the press.

I wrote code also to capture and make sure the suspected inputs were going from 1 to 0. My 2 cents.

[Kusmaul]

Mike,

Thanks, but this is a problem about a month old, and it had all the symptoms of a voltage problem when we first saw it, which led to me running the pigtail to another PLC, because I had put my meter on the same pigtail and read the min max, it never dropped below an average of 23.8 vdc, which I do not know if it is possible for a dc voltage from a ac/dc trasformer to drop and rise in a few milliseconds, so the pigtail was run to another plc with watchdog code to pick it up, and was never lost.

[Mike Williams]

I also had a problem close to this with a AB plc5. When you get into these kinds of problems it sometimes turns into a parts swap and wait thing.

The problem with the ab turned out to be a faulty input card, but the input/outputs it affected were AFTER the card in the rack.

One other thing, sometimes these digital meters WILL NOT see a voltage drop like this. A good ole simpson analog may be worth watching or get yourself a good scope.

Good luck

[BarryT]

Something else to check. Siemens use IO in the ET100/ET200 ranges for reporting back errors. It could be that the system has been configured with IB or IW 0 for the error code and as the fault is detected in the ET IO the error is written back which leaves input 0.0 off.

Similar could be said for the WF Card, but I am not familiar with this.

You will need to check the configurations and see if the error is written to Input 0.

[ganutenator]

just my two cents

I am not familiar with symens, but it sounds as if it updates its inputs asynchronously to the plc program scan (because of your within one scan comment). You may want to think about synching your inputs to your program scan by copying your inputs to registers at the beginning of the program scan, and use the copies throughout your program. This will be good, even after you fix your problem (not just a patch). And, may help you to write code to determine the problem.