A programmable logic controller (PLC) performs two core jobs: converting input/output information and reliably realising the physical control. The conversion is handled by the program stored in the PLC memory, which includes a system program (also called the monitoring program or CNC operating system) and a user program. The system program provides the platform for editing and running the user program and handles common tasks such as self-checks, I/O refresh and communication with peripherals, host computers or other PLCs. The user program is designed by the user according to the required control logic.
Reliable physical realisation is achieved through input (I) and output (O) circuits. Each input or output point has its own circuit, and several circuits are integrated into a module (or box); several modules then form the complete PLC I/O system.
The input circuit continuously monitors the on/off state of each input point and stores it in a corresponding input register. The output circuit has an output latch that holds a high or low state and drives the output point through its physical circuit.
Reading the input registers into PLC memory is called input refresh. The PLC memory has a dedicated mapping area for input information, where each bit is called an input relay (soft contact) or process-image input register. A “1” means the contact is closed; “0” means open. The output latch corresponds to the output mapping area, and each output latch has a memory bit called an output relay or process-image output register; writing it to the output latch is called output refresh.
Besides switch signals, the PLC can also receive analogue input signals (after analogue-to-digital conversion) and produce analogue output through analogue output modules. In this way, the user program only needs to describe the transformation from the input memory area to the output memory area, which is purely a data and logic processing task made possible by the PLC’s powerful instruction set.