PLC Programming Types And How PLC Works

PLC Programming Types And How PLC Works

PLC Programming: Most of the PLC programmer works on the ladder diagram programming language. It is pretty easy as compared to other PLC programming languages.

  • Ladder Diagram
  • Instruction List
  • Structured Text
  • Function Block Diagram
  • Sequential Function Charts

How Programmable Logic Controller Works

The most important working principle is the PLC is operated by continuously scanning programs. Scanning happens every time per millisecond. So, it is called as the Scan Cycle.For this scan cycle, PLC required a little amount of time in the range of milliseconds .

The scan cycle consists of the following three steps.

  1. Scan the inputs
  2. Execute the program by the CPU
  3. Activate the output
Image result for scan cycle of plc
PLC Scan Cycle

Step 1: Sense the input

Firstly, PLC sense the on/off status of the external input signals. After scanning the input, it gets stored in the input memory.This input included switches, pushbuttons, proximity sensors, limit switches, pressure switches, etc.

Step 2: Execute the program by the processor

This scanned input gets transferred to the CPU for processing from input memory.  The processor executes the programming instructions based on the input.After the execution, the result (on/off) will be stored in the device memory. 

Step 3: Activate the output:

When the program executes the last instruction,  it will send the on/off status to the output device memory.The outputs include solenoids, valves, motors, actuators, and pumps.All three steps get completed under the scan time.

What is Scan Time?

The amount of time is taken by the processor to read/sense the first input and execute the last output called the Scan time.PLC is so fast as it can easily scan and execute the program in few milliseconds 10-15 milliseconds.

Types of Programmable Logic Controller [PLC]

Two types of PLCs are used for commercial or industrial purposes.

  1. Compact PLC
  2. Modular PLC

Knowing these two types is the part of PLC basics knowledge.

What is a Compact PLC?

It is also called as Integrated PLC or Fixed PLC.The compact PLC has a fixed number of input/output modules along with power supply and CPU.

Image result for compact plc
Compact PLC Or Fixed PLC

What is a Modular PLC?

It consists of a variable number of inputs and outputs. Inputs and outputs can be added to the modular PLC systems by the user.If you look at the below PLC designing structure, it looks more like a rack. So, it is also called as rack-mounted PLC.

Image result for modular plc
Modular PLC

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