Glossary · Automation fundamentals, platforms and components
Set (bistable)
Also known as: Set coil, Latch
German: Setzen
In PLC programming, to set means to write a Boolean variable to TRUE and keep it TRUE until it is explicitly reset. IEC 61131-3 provides set coils in ladder diagram and the bistable function blocks SR (set-dominant) and RS (reset-dominant).
- Automation components
In one sentence
Setting a PLC variable latches it to TRUE until it is reset; IEC 61131-3 offers set coils and SR/RS bistable function blocks.
Example
A fault bit is set when the motor protection trips and stays set until the operator acknowledges the fault with the reset button.
How it applies
- Engineering: Set/reset logic stores events and states: faults, requests, sequence steps. Decide deliberately which input dominates when set and reset are TRUE at the same time (SR or RS).
- Safety: A latched run command must not restart a machine unexpectedly after a stop or power loss. Restart behavior belongs in the design, and safety-related Reset function logic belongs in the safety controller with a Manual reset where required.
- Documentation: Describe for each latched signal what sets it, what resets it and whether it is retained through power loss. Operators need to know which faults need acknowledgment.
Set vs. assignment
A normal assignment follows its condition every cycle. A set only writes TRUE when its condition is TRUE and leaves the value unchanged otherwise, so the value persists after the condition disappears.