Evolution of the PLC 

 Before the advent of microprocessors, industrial logic and sequence control used to be performed using elaborate control panels containing electromechanical or solid-state relays, contactors and switches, indicator lamps, mechanical or electronic timers and counters etc., all hardwired by complex and elaborate wiring. In fact, for many applications such control panels are used even today. However, the development of microprocessors in the early 1980’s quickly led to the development of the PLCs, which had significant advantages over conventional control panels. Some of these are: 

 • Programming the PLC is easier than wiring physical components; the only wiring required is that of connecting the I/O terminals. 

 • The PLC can be reprogrammed using user-friendly programming devices. Controls must be physically rewired. 

 • PLCs take up much less space. 

 • Installation and maintenance of PLCs is easier, and with present day solid-state technology, reliability is grater. 

 • The PLC can be connected to a distributed plant automation system, supervised and monitored. 

 • Beyond a certain size and complexity of the process, a PLC-based system compare favorably with control panels. 

 • Ability of PLCs to accept digital data in serial, parallel and network modes imply a drastic reduction in plant sensor and actuator wirings, since single cable runs to remote terminal I/O units can be made. Wiring only need to be made locally from that point. 

 • Special diagnostic and maintenance modes for quick troubleshooting and servicing, without disrupting plant operations. However, since it evolved out of relay control panels the PLCs adopted legacy concepts, which were applicable to such panels. To facilitate maintenance and modification of the physically wired control logic, the control panel was systematically organized so that each control formed a rung much like a rung on a ladder. The development of PLCs retained the ladder logic concept where control circuits are defined like rungs on a ladder where each rung begins with one or more inputs and each rung usually ends with only one output.