Seal-in
A momentary Start turns an output on, and the output keeps itself on. Stop turns it off.
Step by step
Pick a step. The rungs are drawn by the editor after Core scans the program up to that step.
1. Nothing pressed. Neither branch passes power, so OTE writes 0 to Motor.
2. Press StartPB. The StartPB branch passes power. XIO StopPB passes because StopPB is 0. OTE writes 1 to Motor.
3. Release StartPB. The StartPB branch is open now, but the XIC Motor branch reads the 1 that OTE wrote last scan. The rung stays true and Motor stays on.
4. Press StopPB. XIO StopPB blocks power. OTE writes 0 to Motor, so the XIC Motor branch opens too.
5. Release StopPB. Both branches are open, so Motor stays off until StartPB is pressed again.
6. Press both. XIO StopPB is in series with both branches, so Stop wins. Motor stays off.
| Step | StartPB | StopPB | Motor |
|---|---|---|---|
| 1. Nothing pressed | 0 | 0 | 0 |
| 2. Press StartPB | 1 | 0 | 1 |
| 3. Release StartPB | 0 | 0 | 1 |
| 4. Press StopPB | 0 | 1 | 0 |
| 5. Release StopPB | 0 | 0 | 0 |
| 6. Press both | 1 | 1 | 0 |
Why it works
- OTE writes the rung's result to
Motorevery scan. - The XIC on
Motorsits in parallel withStartPB. OnceMotoris 1, that branch passes power on the next scan by itself. This branch is the seal. - XIO
StopPBis in series after both branches, so it can break the rung whichever branch is holding it.
Common mistakes
- Putting Stop inside a branchStop must be in series with both branches. Inside one branch, the other branch keeps the rung true.
- Sealing on the buttonThe seal contact must read the output, Motor, not StartPB.
- Using an OTL insteadAn OTL stays on until an OTU clears it, so a separate rung is needed to stop it. A seal-in turns off as soon as any series condition opens.
- A stop button wired normally closedA real Stop button is often wired so its input is on until it is pressed. Then the rung reads it with XIC, not XIO, and a broken wire stops the motor.