Glossary · Automation fundamentals, platforms and components
Logic (control)
Also known as: Control logic, Boolean logic
German: Logik
In automation, logic refers to the rules that link input signals to outputs, commonly Boolean operations such as AND, OR and NOT combined with timers, counters and sequences, implemented in hardware or in a controller program.
- Automation components
In one sentence
Control logic is the set of rules, such as AND, OR and NOT with timers and sequences, that links input signals to outputs.
Example
The conveyor may only start if the guard is closed AND no emergency stop is pressed AND the downstream machine is ready.
How it applies
- Engineering: Logic was first implemented with relays (relay logic) and is now written in PLC languages such as Ladder diagram (LD) and Function block diagram (FBD).
- Commissioning: Logic is tested against the functional specification, signal by signal.
- Documentation: Functional descriptions should state start conditions, interlocks and sequences in plain language or tables, not only in code. Cause-and-effect matrices are a common way to document logic in the process industry.
Logic vs. safety logic
Standard control logic and safety logic may look similar. Safety logic runs on safety-rated devices, such as a Safety PLC or Safety relay, and is developed and validated under functional safety standards. Standard logic alone must not be relied upon for safety functions unless it meets those requirements.