Apprentice, second month in a plant that makes plastic tubs. Small CompactLogix 1769 L24ER on a trim station, Studio 5000, whatever version the laptop's got on it.
The reject solenoid on Local:2:O.Data.4 chatters. You can hear it buzzing at the valve and the output LED flickers with it. When I go online and find the rung it's green, so the output should be on, but the valve isn't properly on.
I swapped the solenoid coil with the one next to it and the chatter moved with the output rather than the coil, so it's not the valve. Then the electrician reckoned the scan was too fast for it, so I changed the card RPI from 20ms to 10ms and that did nothing either.
I think the output card might be on its way out, it's the only output on that card doing it. Before I ask for a new card, is there anything else that makes an output buzz while the rung sits green?
Card's probably fine. Before anything else, right click the output tag and do Cross Reference. How many OTEs come back for Local:2:O.Data.4? Not XICs, OTEs.
And put the RPI back to 20ms, that's got nothing to do with it.
Cross reference shows two OTEs. One in MainRoutine rung 14, the other in a routine called Reject_Old rung 3. I'd only ever looked at the MainRoutine one. The Reject_Old one's red, that rung is false.
So one rung says on and one says off? Not sure how that's even allowed.
Yes. Same thing on STEP 7 with two assignments to one Q address, the cross reference list shows both and the last one in the cycle wins. On a 300 you'd simply never see that output come on at all. Why yours buzzes instead I'll leave to the Logix people.
sergio74 has it. Two OTEs on one tag is a double coil. Rung 14 writes it on, Reject_Old rung 3 writes it off later in the same scan. The buzz is a Logix thing, outputs don't wait for end of scan, the card takes whatever the tag holds at the RPI, so sometimes it catches the on and sometimes the off.
One output, one rung. Either delete Reject_Old if it's dead code, or move its conditions into rung 14 as a parallel branch so one OTE owns the tag. I'd say Reject_Old is left over from whoever did the last change. There's a writeup with the double coil case in it here: https://plctr.com/control-logix-plc-ladder-troubleshooting/
Ok. So two OTE rungs on the same output, MainRoutine rung 14 and Reject_Old rung 3, and the second one kept turning it back off.
Asked the controls guy about Reject_Old and he said it was from before the station got rebuilt and should have been unscheduled years ago. It was still in the JSR list. I've taken the JSR out and left the routine there for now, and the valve is solid on when it should be.
What I don't get is why the flicker only started last week if that routine's been there for years. Nobody admits to changing anything. Thanks rsdave and sergio74.
Cross reference on the output tag, first thing, every time. Marking solved.