PLC programs are made up of a combination of the above “gates” together with inputs, outputs, timers, counters, internal memory bits, analog inputs, analog outputs, mathematical calculations, comparators etc. We will now look closer at some of these and the operation of the PLC.


These are the physical connections from the real world to the PLC. They can be limit switches, push buttons, sensors, anything that can “switch” a signal on or off. The voltage of these devices are usually, but not always,  24 Volt DC. Manufacturers make inputs that can accept a wide range of voltages both ac and dc. It should be remembered that an input will be ON, “status 1”, when the voltage is present at the input connection and OFF, “status 0”, when the voltage is no longer present at the input connection.


These are the connections from the PLC to the real world. They are used to switch solenoids, lamps, contactors etc on and off.
Again they are usually 24 Volt DC, either relay or transistor, but can also be 115/220 Volt AC.


These so called relays only exist in the PLCs memory, they are used to remember certain “states” or functions that have been performed. They react exactly like ordinary relays and can be on, off or latching. Manufacturers of PLCs also pre-program certain internal relays with special functions such as, always on; always off; on whilst the PLC is powering up; on for the first scan of the program; on if there is an error. For details of other pre-programmed relays you will have to check the manual for the PLC you are going to use.