PLC (programmable logic controller) block diagram. CPU, INPUT, and OUTPUT are the three primary units in a PLC. CPU:- A processor is found in a CPU. The CPU reads and executes the programming instructions that the programmer has written. All activity is controlled by the CPU, which receives input and regulates all output according to the programme.
First and foremost, when reading conventional wiring diagrams, you should read them from left to right and top to bottom. It's as if you're reading a novel! However, in some cases, such as this page, designers make exclusions in order to get a better layout.