Inst ToolsInst ToolsInst Tools
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Search
  • Books
  • Software
  • Projects
  • Process
  • Tools
  • Basics
  • Formula
  • Power Plant
  • Root Cause Analysis
  • Electrical Basics
  • Animation
  • Standards
  • 4-20 mA Course
  • Siemens PLC Course
Reading: PLC Scan Time
Share
Font ResizerAa
Inst ToolsInst Tools
Font ResizerAa
  • Courses
  • Design
  • PLC
  • Interview
  • Control System
Search
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > PLC Tutorials > PLC Scan Time

PLC Scan Time

Last updated: March 4, 2021 12:07 pm
Editorial Staff
PLC Tutorials
4 Comments
Share
5 Min Read
SHARE

What is Scan Time?

Every control system will take some time to execute its operation. Likewise, PLCs also takes time to perform a task. In simple terms, PLCs mainly has 3 parts which are input modules, output modules, and processor (CPU).

Contents
What is Scan Time?Definition of PLC Scan TimeWhy the Scan time Important?

The input modules will read the status of switches, transmitters, and gives the data to the processor. The processor will execute the logic as per the user program. The processor will give the command to the output modules. The output modules are connected with final control elements like control valves, motor control feeders, etc.

The inputs and outputs may be in the form of NO, NC, 4-20mA, or a Bit/Pulse. So, the above all mentioned steps require some time in PLC which means for reading inputs, solve the logic, and write output, each step required some time which is nothing but the PLC scan time.

Definition of PLC Scan Time

Time taken by the PLC to read the inputs, solve the Logic, and to write the output is called a PLC Scan Time.

These functions work in the loop it may vary its time taken for execution from 0ms to 150ms to a large till 1000ms.

The variation in scan time depends on the following factors:

  • Number of inputs
  • Length of logic/Loops in program
  • Number of Outputs

Let’s get clear the concept by the following example;

In PLC, we have an analog input card, these analog signals need to be converted in digital format so that the processor can read it and perform an action.

Say If 1 channel generally needs a 3ms time so, if we used a 4 channel to PLC then the scan time will be 4 X 3=12ms.

Why the Scan time Important?

Suppose we have a PLC whose scan time is 5ms, and we need to assign an input for an encoder which generates a pulse every 1ms time. Refer below figure (a).

Why the PLC Scan time is Important

Figure: a

In the graph below, as you can see the PLC takes a 0.3ms time to read input, 0.3ms time to write output, and 4.4ms time to executive logic. I.e. total 5ms scan time is required by PLC.

PLC Scan Time

Figure: b

As we can see the encoder pulse start after 0.5ms and PLC scan time starts with 0ms. The PLC will read the first pulse of the encoder which is at 0.5ms but after the first pulse as soon as the second pulse which will be at 1ms, the PLC will not be able to read this input because the PLC input read scan time is 0.3ms only.

So at each new scan time, the PLC will read only the first pulse of the encoder and other pulses will be neglected by PLC because PLC scan time is Greater than Encoder time. These are a serious issue for the process.

What if I have already a PLC whose scan time is greater than an encoder?

In the PLC software there is Instruction, by which you can avoid the above problem they are as followed;

  • Immediate Instruction – Input Immediate, Output Immediate
  • Interrupts instruction.

These instructions are mainly seen in Siemens Old PLC’s.

The Instruction works such that the input immediate will pause the logic execution step and read the Inputs again and again and then continue the logic step, same the other Instruction also works respectively.

Author: Jadhav Amit R

If you liked this article, then please subscribe to our YouTube Channel for PLC and SCADA video tutorials.

You can also follow us on Facebook and Twitter to receive daily updates.

Read Next:

  • FB Block in Siemens PLC
  • PLC Input Output Modules
  • One-Shot Rising PLC Program
  • Midline Instruction in Siemens
  • Choose Right Fuse for a Panel
Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address
You've successfully subscribed !

Continue Reading

Top 100 PLC Objective Questions and Answers
FOR DO Statement in SCL Language
How to implement SR Flip Flop using PLC Ladder Logic
How to Use the Shift and Rotate Instructions in PLC?
How to Use Diagnostic Buffer for SIEMENS PLC?
DP Master System Configuration in PLC
Share This Article
Facebook Whatsapp Whatsapp LinkedIn Copy Link
Share
4 Comments
  • Pradeep says:
    June 8, 2020 at 2:13 pm

    Good example. Whether there are such logics in a DCS ?

    Reply
  • Amit jadhav says:
    July 22, 2020 at 4:27 pm

    Yes, usually the design of DCS scan time starts from 0.5ms to greater upto 500ms these are adjustable according to process.

    Reply
  • Sunny Madas says:
    July 22, 2020 at 4:35 pm

    Verry important information necessary for Instrumentation and plc selection

    Reply
  • Cláudio Timóteo de Souza says:
    April 6, 2021 at 1:41 pm

    Is scanning time a clock?
    Isn’t it symmetrical? duty cycle greater than 50%?
    During level 0 it do nothing?
    The scan time is generally synchronous?
    If I use a bit memory contact NC on series with a coil, I get de scan time?

    Thanks!

    Reply

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Stay Connected

128.3kFollowersLike
69.1kFollowersFollow
210kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

Siemens PLC programming: BCD, Integer, Double Integer, Real
PLC Program for Control Valve Scaling
Free SCADA Training Course
Concept of Latching in PLC
Applications of PLC Programming in Bottle Filling Process
What is Motor Jogging ?
Software Troubleshooting Errors and Faults in Siemens PLC
PLC Emergency Stop Example Program

Keep Learning

HMI configuration for Delta VFD

Modbus Communication between Delta PLC with VFD

SCADA Standards

SCADA for Substation Automation

Traffic Barrier Control

PLC Programming for Traffic Barrier Control

Wiring-Connection-from-PLC-to-Solenoid-Valves

How to Connect a Solenoid Valve with PLC?

Comparison Instructions in PLC Programming

Comparison Instructions in PLC Programming

Overview of Industrial Control Systems

Overview of Industrial Control Systems

PLC Cooking Timer Example for Kitchen Automation

PLC Cooking Timer Example for Kitchen Automation

Ladder Logic Example of Two Motors

Ladder Logic Example of Two Motors Interlinked with another Motor

Learn More

Structured Text PID control loop in PLC

How to Program PID Control in PLC with Structured Text?

Turbine Flow meter coefficient

Turbine Flow Meter Coefficient and Scaling Factor

Porpoising PID Controller

Recognizing a Porpoising PID Controller

Level Calculation with Remote Seals

Level Calculation with Remote Seals

Solid State Relay Working Principle

What is a Solid State Relay? – Working Principle, Types of Control

Power Electronics Objective Questions

Type E Chopper Objective Questions

Frequency Response MCQ

Polar Plots MCQ

Rewire Tool

What is Rewire Tool in Simatic Manager?

Menu

  • About
  • Privacy Policy
  • Copyright

Quick Links

  • Learn PLC
  • Helping Hand
  • Part Time Job

YouTube Subscribe

Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Welcome Back!

Sign in to your account

Username or Email Address
Password

Lost your password?