What is a Ladder Logic Programming ??

Question

i've heard it a lot while learning PLC,but still i don't know what it means.
Would you pls help me? Thanks.

Answer 1

It is also called "relay logic".

It is a way of drawing logic diagrams, using relay contacts, in a string or ladder. AND functions would be 2 or more N.O. relay contacts in series. OR functions would be 2 or more N.O. relay contacts in parallel. NAND functions would be 2 N.C. relay contacts in parallel, and so on.

The string of contacts would be used to control a motor, while the individual relay coils are controlled by various gauges, safety switches, and process switches associated with that motor.

for example: a conveyor belt motor would only operate when:
1. a safety switch is on, AND
2. product is present to be moved at an electric "eye", OR
3. a manual start switch is pressed, AND
4. a motor current sensor doesn't detect an overload,
5. etc.

In the "old" days they used actual relays, but now it is done with computer logic in the PLCs, using switch outputs to drive motor controllers.

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Answer 2

To add to the above answer: the logic is written in "rungs" thus the reference to a ladder.