Understanding PLC Program Execution and Scan Time
In an automated system, a Programmable Logic Controller (PLC) continuously executes its logic program in a repetitive cycle, known as the scan cycle, as long as the system remains powered.
How a PLC Executes a Program
Input Status Update The PLC reads signals from physical input devices (sensors, switches) and stores them in a dedicated memory area called the I/O Image Table. This acts as a snapshot of all input states.
Logic Evaluation The PLC executes the program from the first rung to the last, using the I/O Image Table rather than real-time input values. This ensures consistent logic evaluation during the scan.
Output Update Once all logic instructions are processed, the PLC updates the output states—activating actuators, relays, etc.—based on the computed results.
What Is Scan Time?
Scan time is the total time taken to complete one full input-process-output cycle.
For modern PLCs and small programs, scan time may be just a few milliseconds.
Older PLCs or complex programs may take 50–100 ms or more, potentially causing sluggish responses in fast-changing processes.
If scan time is excessive, the system may fail to detect quick events, leading to inaccurate control or missed inputs.
Role of Special-Purpose I/O Modules
To overcome scan time limitations, certain tasks are handled by dedicated I/O modules:
High-Speed Counters: Capture fast pulses from devices like shaft encoders.
Precision Timers: Measure short durations more accurately than standard scan-based logic.
Isolated I/O Units: Operate independently from the scan cycle to ensure reliable real-time performance.
These modules collect data at hardware speed, unaffected by PLC scan delays. The PLC simply reads their accumulated values for decision-making.
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