The document provides an overview of programmable logic controller (PLC) hardware design. It describes the basic processor architecture of a PLC including the central processing unit, memory unit, input/output unit, display and indicators. It also discusses the functions of each component as well as common input and output devices used. The document concludes by listing additional components that comprise a full PLC system such as the housing unit, programming unit, secondary storage unit, power supply unit, and printing unit.