PLC : The memory unit of a PLC is the registry where the programs are stored. The fundamental unit of memory is the word. Words are made up of bits. A bit is a single piece of data. It contains information on only two states (ON/OFF or YES/NO). Longer words contain more information within. Programs are combination of words that produce control logic. To operate the PLC system there is a need for it to access the data to be processed and instructions, that is, the program, which informs it how the data is to be processed. Both are stored in the PLC memory for access during processing.