National Electrical Manufacturers Association (NEMA) defines PLC as a digital electronic device that uses a programmable memory to store instructions and to implement specific functions such as logic, sequence, timing, counting, and arithmetic operations to control machines and processes.
● The PLC is an assembly of solid-state digital logic elements designed to make logical decisions and provide outputs.
● PLC is a programmed interface between input sensors & output devices.
The entire PLC system can be divided as follows.
a) Central Processing Unit
b) Input Modules
c) Output Modules
d) Power Supply
e) Bus system
a) Central Processing Unit
● The brain of the whole PLC is the CPU module. This module typically lives in the slot beside the power supply.
● Manufacturers offer different types of CPUs based on the complexity needed for the system.
● The CPU consists of a microprocessor, memory chip, and other integrated circuits to control logic, monitoring, and communications.
● The CPU has different operating modes.
● In programming mode, it accepts the downloaded logic from a PC.
● The CPU is then placed in the run mode so that it can execute the program and operate the process.
b)Input Module
These modules act as an interface between the real-time status of the process variable and the CPU.
Analog input module : Typical input to these modules is 4-20 mA, 0-10 V Ex: Pressure, Flow, Level Tx, RTD (Ohm), Thermocouple (mV).
Digital input module: Typical input to these modules is 24 V DC, 115 V AC, 230 V AC.
c)Output module
These modules act as a link between the CPU and the output devices in the field.
Analog output module: Typical output from these modules is 4-20 mA, 0-10 V
Ex: Control Valve, Speed, Vibration
Digital output module: Typical output from these modules is 24 V DC.
d)Power Supply
●The power supply gives the voltage required for the electronics module (I/O module, CPU module memory unit) of the PLC from the line supply.
●The power supply provides the isolation necessary to protect the solid-state devices from the highest voltage line spikes.
●As I/O is expanded, some PLC may require additional power supplies in order to maintain proper power levels.
e)Bus System
●It is the path for the transmission of the signal between the power supply module, CPU, and I/O modules.
●The bus consists of several single lines i.e. wires or tracks.
●Data bus
●Address bus
●Control bus
