he PLC program is concerned with connections to the outside plant,and the input and output devices need to be identified inside the program.Before we can examine how the program is written we will first discusshow various manufacturers treat the I/O.

  A medium-sized PLC system consists of severalracks, each containing CARDS, with each card interfacing generally to 8, 16or 32 devices. I/O addressing is usually based on this rack/card/bit idea.