Case sealer. Light curtain on the infeed, OSSD1 and OSSD2 into a 440R safety relay. Standard PLC is a 1769-L33ER with a PanelView Plus 7 on the front.
Operators see a "CURTAIN BLOCKED" banner blink on and off for a fraction of a second every few seconds, all shift, nobody near the curtain. The curtain itself never trips, the relay never drops, the line keeps running. It's just the screen, but the operators have stopped trusting it.
That banner comes off an IQ16 input, where somebody before my time teed OSSD1 at the terminal strip into a spare point so the standard PLC knows the curtain state.
So I power cycled the cell twice and moved the tee to a different IQ16 point, same blink either way.
Is the curtain on its way out? It's ten years old. Any ideas before I price a new one?
How often exactly, and what's the input filter on that IQ16 point? "Every few seconds" sounds like a schedule, not a fault.
watched it with a stopwatch, every four or five seconds, same each time. Filter I've no idea, whatever the card does out of the box.
The curtain is checking itself, and it's fine. OSSD outputs drop low for a very short pulse every so often, that's how the curtain proves the output stage and wiring aren't shorted. The 440R input is built to ignore that pulse. The IQ16 isn't, it sees a zero for a blink, and your banner follows it.
Two things. Most curtains have a separate status or aux output that isn't pulse tested, meant for a standard PLC wanting the state. Use that into the IQ16. And take the tee off OSSD1. A standard PLC wire on a safety channel is a place for a fault to sit that nobody tests. A longer input filter would probably hide the blink, but hiding isn't the fix.
Curtain connector has an "aux" pin, the drawing says unused. So the aux goes to the IQ16 and the tee comes off, I think. Do I need anything between?
Nothing between. Aux to the IQ16, tee off, banner tag stays the same.