Fail-safe stop
A real Stop button is wired normally closed. Its input is on until the button is pressed, and the rung reads it with XIC.
Step by step
Pick a step. The rungs are drawn by the editor after Core scans the program up to that step.
1. Press StartPB. StopOK is 1 because the Stop circuit is closed. XIC StopOK passes power and Motor turns on.
2. Release StartPB. The XIC Motor branch holds the rung, as in a seal-in.
3. Press Stop. Pressing the button opens the circuit, so StopOK goes to 0. XIC StopOK blocks power and Motor turns off.
4. Release Stop, press StartPB. The circuit is closed again and Motor starts.
5. Wire breaks. A broken wire also opens the circuit. StopOK goes to 0 and Motor stops, just as if Stop was pressed.
6. Press StartPB. Motor cannot start until the wire is fixed and StopOK is 1 again.
| Step | StartPB | StopOK | Motor |
|---|---|---|---|
| 1. Press StartPB | 1 | 1 | 1 |
| 2. Release StartPB | 0 | 1 | 1 |
| 3. Press Stop | 0 | 0 | 0 |
| 4. Release Stop, press StartPB | 1 | 1 | 1 |
| 5. Wire breaks | 0 | 0 | 0 |
| 6. Press StartPB | 1 | 0 | 0 |
Why it works
- The input is on in the healthy state. Anything that breaks the circuit, the button, a loose terminal or a cut cable, turns it off.
- XIC passes power only while its tag is 1, so it passes only while the Stop circuit is healthy.
- Name the tag for the healthy state, such as
StopOK, so XIC reads naturally.
Common mistakes
- A normally open Stop read with XIOIt works while the wiring is good, but a broken wire means Stop can never be seen. The machine keeps running.
- Using this for an emergency stopAn emergency stop must cut power through a safety circuit, not only through the PLC program. These lessons are not a safety system.