PLC scan cycle
Read inputs, execute, write outputs: see why a press shorter than one scan is missed.
- Read inputs. The CPU copies every input terminal into the process-image input (I). The program sees this snapshot, not the live terminals.
- Execute the program. Networks 1, 2, 3… from top to bottom, each from left to right, working on the images.
- Communication and self-test. Serve Micro/WIN, the status chart, and check the CPU.
- Write outputs. The process-image output (Q) is copied to the output terminals all at once.
push button (terminal) input image I0.0 lamp Q0.0
- presses seen
- 0
- presses missed
- 0
Make the press shorter than the scan time and press a few times: some presses fall between two input reads and the program never sees them. That is why fast signals (encoders) go to high-speed counters, not to ordinary inputs.