1. The PLC Scan Cycle: The operation of a PLC consists of three main repetitive steps:
• Input Scan: The PLC reads the status of all connected input sensors and stores them in an internal image table.
• Program Scan: The PLC executes the user-created logic (typically Ladder Logic) from the first rung to the last, using the data from the input image table.
• Output Scan: The results of the program logic are updated to the physical output terminals to drive actuators.
2. Defining Scan Time: The
Scan Time is the total time it takes for the PLC to complete one full cycle of these three steps. It represents how long it takes to
execute the entire user program and update the system.
3. Significance: Scan time is critical in industrial automation. If a program is very long or complex, the scan time increases. If the scan time becomes too slow, the PLC might miss a very fast signal from a sensor, potentially leading to timing errors or safety issues in the controlled process. Typical scan times are in the range of 1 to 20 milliseconds.