Question

In a PLC scan cycle, at what exact phase are the physical output terminals updated to match the states stored in the output image table?

Accepted Answer

In a standard programmable logic controller scan cycle, the physical output terminals are updated during the Output Scan phase. The PLC scan cycle consists of three primary stages: the Input Scan, the Program Execution, and the Output Scan. During the Input Scan, the processor reads the status of all physical input terminals and records these states into a temporary memory area called the Input Image Table. During the Program Execution phase, the processor runs the logic of the user program using the data stored in the Input Image Table and updates a separate temporary memory area called the Output Image Table. The Output Image Table acts as a buffer that holds the desired status of all output devices, but it does not send electricity to the physical terminals yet. Finally, during the Output Scan phase, the PLC copies the data from the Output Image Table to the physical output terminals. At this exact moment, the electrical voltage or state of the physical output modules is changed to match the values currently held in the Output Image Table, thereby triggering the real-world devices such as motors, lights, or valves.

Home → All Courses → Engineering and Technology Courses → Industrial Automation Engineering → Flashcard

In a PLC scan cycle, at what exact phase are the physical output terminals updated to match the states stored in the output image table?