We put two DI 8x24VDC ST modules on the end of an ET 200SP rack last month for a new guard on the stacker. IM 155-6 PN ST, twelve modules now, CPU is a 1516.
Door closed, everything reads fine. Open the door and all eight inputs on each of the last two modules go to 0 at the same time. The other ten keep working, the LEDs stay green and the diagnostics buffer is empty. I swapped module 11 with module 5 and the fault stayed with the position, and the door switch makes and breaks clean on a meter. One thing I noticed, the last two base units are a lighter grey than the rest, if that means anything.
Is the door switch pulling something down that I can't see? Or is that lighter grey on the last two base units telling me something, I think it has to be more than a colour.
What feeds the L+ terminal of those two bases, and is it the same 24 VDC as the rest of them? Trace it in words for me: from which supply, through which contactor, to which terminal on the base. Could be it goes through the safety relay's K1 K2 contacts.
Traced it. L+ on base 11 comes from terminal strip X4, and X4 is fed from the output side of the safety contactor K2, the one the door drops out. The other bases up to 10 come off X2, which is straight off the 24 VDC supply. The electrician who added them took the nearest terminal he could reach. Photo of X4 attached.
Two things it could be, a dead module or the feed, and your trace answers that. The lighter bases matter as well, the colour isn't a styling choice on these. Post the device view screenshot if you want it checked against the configuration.
The light coloured BaseUnit starts a new potential group. Dark ones carry the supply along from the group before, a light one breaks it and takes its own L+ and M from its terminals. Modules 11 and 12 sit on a light base fed from X4, X4 hangs off K2, door opens, K2 drops, the group loses supply, inputs read 0. The module logic runs off the backplane so the LEDs stay green.
Two fixes: move the X4 feed to the unswitched supply, or swap base 11 for a dark BaseUnit so it stays in the group before it. The ET 200SP system manual, potential groups chapter, has the drawing. Someone did a decent article on the module side: https://plctr.com/understanding-the-basics-of-input-output-modules-in-automation/
Moved the X4 feed over to X2 this morning, so both groups sit on the unswitched 24 VDC now. Door open, inputs stay on.
So the new light base started its own potential group and the electrician fed that group off K2, which means the door was taking the supply to modules 11 and 12 with it every time. Unswitched 24 VDC on X4 and it behaves.
What we still don't know is whether those guard inputs were meant to sit on a switched supply on purpose, the drawings say nothing either way. The safety guy is going through it before we call it closed, and I haven't touched the door interlock itself, that's still the relay contacts. Thanks hkraus and ronh55.
Guard inputs on a supply the guard switches off. Seen that circle before.