The first PLC was developed by group of engineers at general motors 1968, when searching for an alternative to replace the relay control systems.
- Simple programming
- program changing without system intervention
- smaller, cheaper & more reliable
- simple & low cost maintenance
For instance, even if program optimization and thus a reduction of required memory capacity initially still represented an important key task for the programmer, nowadays this is hardly of any significance.
Subsequent development resulted in a system which enabled the simple connection of binary signals. The requirements as to how these signals were to be connected was specified in the control program.
Since then, three decades have passed, during which the enormous progress made in the development of micro electronics did not stop short of programmable logic controllers.
Basic design of a PLC:
The function of an input module is to convert incoming signals into signals which can be processed by the PLC and to pass these to the central control unit. The Reverse task is performed by an output module. This Converts the PLC Signal into signals suitable for the
actuators.
Depending on how the central control unit is connected to the input and output modules, differentiation can be made between compact PLCs . A wide range of variants exists, particularly in the case of more recent PLCs. These include both modular as well as compact characteristics and important features such as
spacing saving, flexibility and scope for expansion.
The hardware design for a programmable logic controller is such that it is able to withstand typical industrial environments as regard signal levels, heat, humidity, fluctuations in current supply and mechanical impact.
