What is a PLC scan cycle and its typical sequence?

The PLC scan cycle is the continuous sequence of operations the CPU performs repeatedly. Typical sequence: (1) Input scan - read all input states into memory, (2) Program execution - execute logic from first to last rung using input image, (3) Output scan - write output memory to physical outputs, (4) Housekeeping - communication, diagnostics. Scan time (typically 1-100 ms) depends on program size and CPU speed. Understanding scan cycle is essential for timing-critical applications and troubleshooting unexpected behavior.