This paper introduces the basic hardware and software components of a programmable logic controller (PLC). It describes the general architecture and basic instruction set common to all PLCs. Basic programming techniques and logic designed are covered. It also describes the operating features of the PLC, the advantages of the PLC over the Hard-wired control systems, practical applications, troubleshooting and maintenance of PLCs. So, in this series we can describe the major components of any PLC, interpret PLC specifications, convert conventional relay logic to a PLC language, operate and program a PLC for a given application, and apply troubleshooting techniques.