THE FIRST PLC - 1

The first plc offered functionality,thus replacing the original hardware relay logic which used electrically separated device to mechanically switch electrical circuits.They met the requirement of modularity, expand ability, and programmability and ease to use in an industrial environment.These controllers were easily installed  because it uses space and were reusable.The controller programming although was a bit tedious but it had recognizable plant standard of the ladder diagram format.in a short period, programmable controllers usage started to spread to other industrial areas, By 1971.PLC;s were being used to provide relay replacement which was the first step towards the application of control automation in other industries,such as food and beverages, metal manufacturing, pulp and papers (Bryan,1968).
The conceptual designed of the PLC: The first programmable controllers were more or less replacers and their primary function was to perform the segmental operations that were previously implemented with relays .These operation include ON and OFF control machines and process that required repetitive operation such as transfer lines and grinding machines, jig saw,and boring equipment and machine.However this programmable machines were a vast improvement over relays.they were easily installed and considerable less space and energy, had diagnostic indications that aided trouble shooting and unlike transfer relays, were reusable if a project was scrapped.
The programmable controller could be considered as new devices when they are compared to their predecessors in traditional control equipment technology, such as old hard wired relay systems, analog instrumentation and other types of early solid - state logic. Although PLC function such as speed of operation, types of interfaces, and data processing capabilities,have improved through out the years,their specification still holds to their designers original intension they are simple to maintain.