PLC Scan Time

Formula

Description

A PLC (Programmable Logic Controller) executes its program in a cyclic scan consisting of three main phases: reading all inputs, executing the user logic program, and updating outputs and handling communications. The total scan time determines the worst-case response latency of the control system. An input change occurring just after the input scan phase will not be processed until the next cycle, resulting in a maximum response time of nearly two full scan cycles. Fast processes like high-speed counting or motion control may require dedicated hardware interrupts rather than relying on the normal scan cycle.

Variables

• t_total — Total scan cycle time (s)
• t_io — Time to read inputs and write outputs (s)
• t_logic — Time to execute the user program (s)
• t_comms — Time for network communication and housekeeping (s)

Practical Notes

Typical PLC scan times range from 1-50 ms depending on program size and I/O count. Modern PLCs offer periodic tasks with fixed execution rates for time-critical control loops, event-driven tasks triggered by interrupts, and background tasks for non-critical housekeeping. Watchdog timers detect scan time overruns that indicate program errors or excessive load.