A PLC does not run its program once - it runs it over and over, thousands of times a minute, in a tight repeating loop called the scan cycle. The time one full pass takes is the scan time, and it sets how fast the controller can react to the world. This guide explains what happens in each scan, why scan time matters, and how it differs from the poll rate a SCADA system uses.
Scan Time in one line: Scan time is how long a PLC takes to complete one scan cycle - reading all its inputs, executing the entire control program, and updating all its outputs - typically a few milliseconds, repeated continuously.
A classic PLC scan cycle has three main steps that repeat endlessly. First, the input scan: the PLC reads the state of every input and copies it into an internal image table, so the whole program works from a consistent snapshot. Second, the program (logic) scan: the PLC executes its control logic top to bottom, using the input image to compute new output values. Third, the output scan: the computed results are written out to the physical outputs. Housekeeping tasks - communications, diagnostics, self-checks - run around these steps.
Then it starts over. The scan time is the duration of one complete pass. For a small program it may be one to a few milliseconds; a large program with many instructions or heavy communications takes longer. Because outputs only update once per scan, an input that changes has to wait, on average, part of a scan before the program even sees it.
Scan time sets the controller's responsiveness. If a shutdown input must be caught reliably, its signal has to persist longer than one scan, and the total response time includes the scan plus input filtering and output actuation. For fast events, engineers keep programs efficient, use interrupts or high-speed inputs, and avoid bloated logic that stretches the scan. A consistent, short scan time is a sign of a healthy controller; a scan time that grows unexpectedly can signal a problem.
Scan time is often confused with SCADA poll rate, but they are different layers. Scan time is how fast the local controller runs its own logic - milliseconds. Poll rate is how often the SCADA host requests data from that controller over the network - often seconds or longer, especially over cellular or radio to remote oil-and-gas sites. Control decisions happen at scan speed in the field; SCADA sees a slower sampled view of the results, which is exactly why time-critical control lives in the controller and not in the SCADA host.
Read all inputs into an image table, execute the control program top to bottom using that image, then write the computed results to the outputs - plus housekeeping like communications - repeating continuously.
Often a few milliseconds for a modest program. It grows with program size, instruction count, and communication load, and a scan time that increases unexpectedly can indicate a problem.
Scan time is how fast the local controller runs its own logic, usually milliseconds. Poll rate is how often the SCADA host requests data from that controller over the network, often seconds - so time-critical control stays in the controller.
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