Case packer. Safety is on a 1756-L72S with 1734-IB8S modules, the line PLC is a 1769-L33ER with IQ16 cards, FactoryTalk View ME on a PanelView Plus 7.
Operators wanted an "E-STOP PRESSED" banner on the runtime screen, so I took a wire off channel A of E-stop 2 at the IB8S terminal and landed it on a spare IQ16 point. The tag comes into the L33ER fine, but it flickers. It drops out for a blink every second or so and the banner flashes on and off all shift. The e-stop itself never faults on the safety side.
I tried a new wire, a different spare point and tightened both ends. Same.
Is that terminal on the IB8S bad, or is the IQ16 card on its way out? A blink every second like clockwork doesn't feel like a loose wire to me, so could someone explain what would do that?
> took a wire off channel A at the IB8S input terminal
Is that point pulse tested, with T0 or T1 as its test source? Because then it isn't a clean 24V you tapped. TIA does the same with the F-DI test pulses and a standard DI card sees them.
Pulse tested, yes. Test source T0. So the flicker is the module doing something on purpose?
Yes. Put a scope on that wire, or a fast meter, and you'll see 24V with a short drop every so often. That's the IB8S pulling T0 low for a moment and checking the input follows. It's how the module proves the wire isn't shorted to 24V somewhere. The safety input is built to ignore its own pulse. The IQ16 is not, it sees a zero, and your banner flashes. Nothing is bad.
Two proper ways. One, a separate contact block on the e-stop for the standard PLC, most operators have a spare position. Two, don't wire it at all, let the L72S publish the e-stop status as a standard tag and read it over the network. What you must not do is leave a standard PLC hanging on a safety channel.
Bumped the input filter on the IQ16 as far as it goes. No luck, just a slower flicker. Not sure which of your two makes more sense, is there a third position free on that operator?
Third block if there's a free position, and don't let anybody suggest killing the pulse test on that point instead. On an e-stop channel that pulse is what catches a short to 24V, and switching it off is a safety task edit with a new signature and a conversation with whoever signs those. Otherwise a produced tag from the L72S. I'd do the tag, one produced DINT and no extra wire.
Went with the contact block for now, E-stop 2 had an empty position on the operator so I fitted an NC block and wired it to the IQ16 point and pulled the tapped wire off the IB8S. Banner solid all shift, safety side untouched.
The flicker was the IB8S pulsing T0 on the wire I tapped, and the IQ16 reading it as a real drop. Its own contact block for the standard PLC and it stopped.
Drawings show that third block as a stack light, which it isn't. Produced tag is on the list for the next outage. Thanks marekw and jcasey.
One contact per consumer. Same rule as a 4-20 loop.