Industrial Engineer · phase 3 of 16
Coil / Contact
Ladder elements that write a Boolean result or test a Boolean condition.
The Explain card
- Plain English
- A contact tests a bit and a coil writes one. A normally open contact passes power when its bit is true, a normally closed contact passes power when its bit is false, and a coil sets its bit to whatever power reaches it.
- Example
- A contact for the start button and a coil for the motor run bit. Press the button and the coil turns on for as long as the button is held.
- Why it matters
- Every ladder program is built from these two things. A plain coil is overwritten every scan, so it turns off the moment its rung goes false, which is what makes the seal in circuit necessary.
- Hook
- Contacts ask, coils answer.
Where it sits in the deck
Phase 3: PLC Program Organization and Execution
How the logic is arranged and how the scan actually runs it, which is the difference between code that works and code that works every time.
