Case packer cell on a 1756-L72S with the safety I/O on 1734 IB8S modules in the operator panel, five E-stops on the cell. About once a week E-stop 3 throws a DCS fault, the FP bit comes on in the safety routine and the cell goes down. Reset clears it every time and it runs another week.
The button itself tests fine. I stood there online and pressed it twenty times, both inputs drop together and come back together, and measured 24VDC on both channels at the module with the button out.
The other four E-stops have never done this, same wiring, same module, same DCS block copied five times.
There's a spare on the shelf and I'm half tempted to swap it. Is the input module going bad on those two points, or could someone explain what else it could be?
Before you swap the module. What discrepancy time is in that DCS, and what input type, Equivalent Active High? And when it faults, is it after somebody actually pushed E-stop 3, or at random with nobody near it? Those are two different problems.
Discrepancy time is 500, all five the same. Equivalent Active High.
Random as far as anyone will admit, and always night shift, which is when forklifts run pallets past that end. One of the day guys wants to raise the 500 to 3000 like the filler line did.
Don't raise it, I reckon. A button that needs three seconds to agree with itself is a broken button, you would just be hiding it.
Sorry, so the discrepancy time is how long the two contacts are allowed to disagree before it faults? So one contact is opening and the other is not, some of the time?
That's the one, finn. DCS watches both channels and if one changes and the other has not followed within the discrepancy time, FP goes on. 500ms is already generous for a mushroom head, a good one changes both blocks within a few tens of ms.
Random, night shift, forklifts past that end. Somebody has hit E-stop 3 sideways. The holder cracks or one block comes loose on its clip, and then any bump opens channel B a few hundred ms after channel A. Enough to trip DCS, not enough to latch the button.
Pull the operator out of the panel and look at the back. Twist test each block. There's a writeup on the interlock side of this here: https://plctr.com/implementing-plc-safety-interlock-systems-in-industrial-environments/
Pulled E-stop 3 tonight. From the front it looks fine. Behind it the holder is cracked on one side and block B rocks on its clip, block A is solid. Ordered a new holder and two blocks, no luck confirming yet, won't know for a week if that was it.