Programmable logic controllers for continuous process industries

A PLC consists of a central processing unit, a bank of memories and input/output modules. The PLC type is generally specified by the number of input/output modules it can handle. Normally it has fixed memory which is also expandable. Input/output modules accept inputs from push buttons and process elements and give outputs to solenoids, contactor coils, motors, etc. The central processing unit (CPU) performs the logic functions and the sequencing operations as per the programme residing in the memory. The status of all the inputs is scanned at a fast rate cyclically, and the CPU sets the outputs as per the programme contents in the memory. The PLCs are provided with a programmer which is used to load the programme in the memory, alter it either offline or online, monitor the programme and debug. Design features and equipment applications are presented.