Today we will talk about a plc operation. Read all field input devices using the input interface like input module, execute the user program stored in the application memory, then on the basis of any control program programmed by the user like ladder logic or structure text, turn the field output device on or off, or whatever control is required. application This process of sequentially reading the input, executing the program in memory, and updating the output is known as scanning.scanning is the part of any plc operation

How Scanning work in PLC Operation?


  • While PLC is in progress, the scanning process consists of the following four steps, which are repeated as a continuous personal cycle of operation

1. Step 1 – Scan input status

It checks the status at the input side

2. Step 2- Logic Solutions / Program Performance

CPU starts executing the application program written in PLC programming language.

3. Step 3 – Output status scan
It writes the data into the output module so that all outputs are updated.

4. This step includes communications, Internal
Diagnostics, etc.

  • The Scan-time is how long it takes the PLC to loop around the whole process back to the
    start. When the PLC scan begins, it maps the memory matrix for input and output.
  • A discrete input can be designated by I and discrete output by O.
  • This again depends upon the PLC designer and manufacturer
  • The next number indicates byte followed by bit.
  1. I0.0 indicates 0th byte and 0th bit.
  2. I0.1 indicates 0th byte and 1st bit and so on.

The time it takes to implement a scan cycle is called scan time. The time required to scan the program scan time, a time required to control the control program, and I / O update time, or the time required to read the input and update the output. Program scanning time usually depends on the amount of memory taken by the control program and the type of 1 ms to 100 ms.

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