Bench rig at school, Micro820 (2080-LC20-20QBB) in Connected Components Workbench, a 24VDC contactor on output O-00 for a little conveyor motor. Start button is NO on I-00, stop is NC on I-01.
Press Start and the contactor pulls in and the motor runs. Let go of Start and it drops out straight away.
```
I-00 Start ---] [--- I-01 Stop ---] [--- ( ) O-00 Motor
```
I swapped the start button for the spare on the rig and got the same thing, so then I metered it, 24VDC when pressed and 0 when released, and the button's fine.
Is there something wrong with the way the Micro820 handles the output, or am I missing something obvious? The instructor's off this week and the two others on the rig have exactly the same.
Nothing wrong with the output. Your rung is only true while your thumb's on the button.
Confirm one thing, the stop input reads 1 when nobody's touching it? NC button, so it should.
Yes, I-01 shows 1 in the I/O monitor with nothing pressed and goes to 0 when I press Stop. That's why I used the normal contact on it, the notes said examine if closed.
Someone in the class reckons I should use OTL on Start and OTU on Stop instead. I think that would hold it, but is that the proper way?
OTL and OTU would run, but you'd end up with two rungs deciding one output and nothing on the rung telling you why the motor's on. Same rule in every brand, an OTE is on only while its rung is true. So the rung has to stay true after the button's released, and the only thing that can do that is the output itself.
> the only thing that can do that is the output itself
That. Put a branch around the Start contact with a contact of O-00 in it. Start OR motor already running, then Stop in series, then the coil. The moment the contactor pulls in the branch holds the rung, and it stays held until Stop opens.
```
I-00 Start ---] [---+--- I-01 Stop ---] [--- ( ) O-00
|
O-00 Motor ---] [---+
```
Every trainer rig I've seen has this rung missing on day one. There's a writeup on the basic rung patterns here: https://plctr.com/control-logix-plc-ladder-troubleshooting/
Added the branch this morning. Start, let go, keeps running. Stop drops it. All three of us on the rig had the same rung, so it was never the button.
There was nothing holding the rung once the button opened, which is what a seal-in is for and mine didn't have one. The parallel branch with an O-00 contact around the Start contact is all it needed and Stop stays in series.
One thing I'm still not sure about. If the PLC loses power and comes back with Start not pressed, the branch is open so it stays off, right? I'll test that before the instructor's back. Thanks carlh and carav9.
Right, it stays off. That's the point of doing it with an OTE.