Major Components of PLC


As discussed earlier, A PLC is essentially a microcomputer consisting of hardware and software. The major components are

  1. Power Supply module
  2. Input module
  3. Central processing unit
  4. Output modules
  5. Software


a)Power supply module:

Usually input output modules require 24V DC and processor require 5V DC. Usually power
supply is integral part of PLC. Power supply units convert 120/230 V AC line voltage to
standard supply of 24 VDC or 5V DC using standard rectifier circuits

b)Input module

Input devices include push buttons, sensors, potentiometers, pressure switches. The function of
the input module is to covert high voltages from input devices to low level logic voltages that the
CPU uses internally for processing.

Input module can process both analog input and digital input. Digital inputs are more preferred in

c)Analog input module

These are used to convert analog signal form analog devices like temperature
sensors, pressure sensors etc. to digital signals using ADC (Analog –to digital convertor).
Analog signal is varying voltage in the range of 0-12 V or current in the range of 5-20 mA.
These values of current or voltage is converted into integer value (say16 bit word)

Digital `is used to convert signal digital input to 5V digital signals that CPU uses internally to
execute a user program.

d)Central processing unit

The central processing unit controls and processes all operations within the PLC and hence
called brain of the PLC. The CPU can perform various arithmetic and data manipulation function
with the local and remotely located Input/output sections.

Further, the processor can perform many communication functions it needs to interface with a personal computer, remote Input/Output, other PLCs and peripheral devices

Functions of CPU are :

  1. It receives input from various sensing devices and switches
  2. It executes the user program
  3. It makes various decisions to control the operation of the equipment or process
  4. It can perform various arithmetic and data manipulation functions
  5. It delivers corresponding output signals to various load control devices such as relay coils
    and solenoids

e)Output module

Output devices include contactor coils, solenoid coils, lamps, etc. Output module amplifies the
low–level logic signals generated by the CPU and pass these modified signals to the final control
elements to operate the output devices.

f) Software

PLC consists of two parts: Operating systems and user program. The PLC operating system
provides effective support ranging from the creation of project structure to the creation of user
programs. The OS system is accessed through a graphical user interface window ( also known as
Main window). The main window contains all the functions needed to set up a project , configure
the hardware , write and test programs. User program can be written in any standard PLC
programming language like ladder diagram or statement list.

While processing a PLC program, the CPU scans and executes the main program cyclically;A
program scan cycle consists of sequential operations that include input scan, program scan, and
output scan. In the input scan, the CPU updates the process image input table, in the output scan;
the CPU updates the process image output table.

After the completion of each scan cycle, the CPU returns to the beginning of the next cycle and again repeats the cycle. The time taken to scan one program is called scan –cycle time.