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PLC Scan

The PLC performs a continuous scan process where it reads all inputs, executes the user program to determine outputs, and writes outputs. During each scan, it performs an I/O scan to record input statuses and energize outputs set to ON. It then executes the program instructions sequentially. The scan time indicates how fast the PLC can react to input changes, with faster scan times needed for high-speed operations to avoid missing input changes. The scan is dependent on both the program and processor clock frequency.

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2K views8 pages

PLC Scan

The PLC performs a continuous scan process where it reads all inputs, executes the user program to determine outputs, and writes outputs. During each scan, it performs an I/O scan to record input statuses and energize outputs set to ON. It then executes the program instructions sequentially. The scan time indicates how fast the PLC can react to input changes, with faster scan times needed for high-speed operations to avoid missing input changes. The scan is dependent on both the program and processor clock frequency.

Uploaded by

money_kandan2004
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PPTX, PDF, TXT or read online on Scribd
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PLC Scan

(or)
Program Scan
Program Scan
During each operating cycle, the processor reads all
inputs, takes these values, and energizes or de-energizes
the outputs according to the user program. This
process is known as a scan.
I/O scan records status data of input
devices. Energizes output devices that
have their associated status bits set to
ON (1)

Program scan instructions are


executed sequentially
Because the inputs can change at any time, the PLC
must carry on this process continuously.
Scan Process
The scan time indicates how fast the controller can react
to changes in inputs. Scan times vary with computer
model and program content, and length. If a controller
has to react to an input signal that changes states twice
during the scan time, it is is possible that the PLC will
never be able to detect this change.

Scan time may be a concern


in high speed operations
Scan Process

Read inputs

The scan is a
a continuous
and sequential
process
Adjusts Run
outputs program
Data Flow Overview

Input Output
Input data Input Output data Output
image
modules table file image
modules
table file

Examine data Return results


Program

Check/compare/examine Take some


specific conditions action
Scan Process
Input Output
Module Module
Input Output
Input Output
file device
device file
I:3/6 O:4/7
I:3/6 O:4/7

Program
When the input is The processor turns
closed, the input
O:4/7 light output O:4/7
module senses a I:3/6
ON during the next
voltage and an ON During the program I/O scan
condition (1) is scan the processor
entered into the sets instructions I:3/6
input table bit I:3/6 and O:4/7 to ON (1)
Scan Patterns
Horizontal Scanning
Order

The processor examines


input and output
instructions from the
first command, top left
in the program,
horizontally, rung by
rung.
End of ladder

In addition to the program itself, the scan time is also


dependent on the clock frequency of the processor!
Scan Patterns

Vertical Scanning
Order

The processor examines


input and output
instructions from the
first command, vertically,
column by column and
page by page. Pages are
executed in sequence.
End of ladder

Misunderstanding the way the PLC scans can cause


programming bugs!

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