PLC scan cycle: what actually happens every millisecond
Inputs, logic, outputs, watchdogs, and the timing choices that decide whether a PLC program is predictable or merely fast on a good day.
Scan time
One PLC read–execute–write cycle.
Time for one PLC scan: sample inputs, execute logic, update outputs, housekeeping. Deterministic scan (bounded max, watchdog-guarded) matters more than raw speed for interlocks.
See also: PLC · Watchdog timer
Use the concept: 4–20 mA loop scaling · Industrial automation fundamentals · Modbus RTU CRC & frame helper
In depth
PLC FAT and SAT checklist: test behavior and handover evidence
Plan factory and site acceptance around requirements, observable results, failure states, approved changes and recoverable project files.
PLC analog input scaling: raw counts, 4–20 mA and diagnostics
Map raw counts into engineering units, check the configured channel range, and keep signal-quality diagnostics separate from the displayed value.