Physical Components vs. Program Components
When learning PLC programming, one of the most difficult concepts to grasp is the difference between physical components and program components. We will be connecting physical components (switches, lights, relays, etc.) to the external terminals on a PLC. Then when we program the PLC, any physical components connected to the PLC will be represented in the program as program components. A programming component will not have the same reference designator as the physical component, but can have the same name. As an example, consider a N/O pushbutton switch S1 named START. If we connect this to input 001 of a PLC, then when we program the PLC, the START switch will become a N/O relay contact with reference designator IN001 and the name START. As another example, of we connect a RUN lamp L1 to output 003 on the PLC, then in the program, the lamp will be represented by a relay coil with reference designator OUT003 and name RUN (or, if desired, “RUN LAMP”).