Programmable Logic Controller, PLC, a digitally operated electronic device with internal storage for a programmable memory to give instructions to the hardware implementing specific functions such as logic, sequencing, timing, counting and so on, through digital or analog input or output modules. The following figures show the appearance of the Mitsubishi PLC module FX2N.
PLCs are widely used in many industrial applications. In essence a PLC is a unit containing a microprocessor in which various sequence of operation can be stored and implemented according to a program. PLCs replace relay panels in old days which rely on mechanical movements, slow and highly possible of technical problems such as bouncing and short contact life.
One of the reasons of using PLC as the system control is its ease of programming. The PLC program uses some simple ladder diagrams (LAD), as shown in Fig. 2, as the program language, in which they are easy-to-understand graphical representation of real life contacts and coils and they are much simpler than the high level language used in the computer. Therefore it is very easy for the programmers to modify or change the sequences of the programs. For example, when something has been changed (e.g. when 1 more sensor is needed to be added to the system), the PLC program need to be changed accordingly. With the use of this simple ladder diagram, we just need to modify something (e.g. add 1 more network or input) in the ladder diagram and then download the modified program to the PLC and the system can be run with the new program. In other words, PLC provides the flexibility that sequences can be very easily changed as desired.