Thursday, February 18, 2010

Scan Time of PLC





Repeatedly executes a ladder based on the sequence circuit as follows.
Scan Time of PLC
1.Input processing
Writes the ON/OFF status of all input terminals to the input memory before executing the program.

2.Program execution
Reads the ON/OFF status of each device (internal utility relay,timer, counter, etc.) based on the instructions, and performs arithmetic operations.
Writes the arithmetic operation result to the output memory.

3.Output processing
Outputs the contents of the output memory to output terminals

The duration of time required to perform one cycle is called the scan time (or cycle time ). The scan time varies based on program size and the instructions used in the program. And units usually scan time is μs ( micro second ).


Input response time delay
In addition to the I/O processing time, there is also an input time delay caused by the scan time. The input time delay is generated because the input status can only be read during the input processing time. If the input status is changed after input processing, the changed contents can only be read during the next scan time.
In the figure below :
Input response time delay Inputs that can be read is the number 1 and 2, but the input can not read the number 3.

Standard response time input values :
The maximum and minimum values of the input response time are as follows.
Maximum value: Input time constant + Scan time x 2 + Output response delay time
Minimum value: Input time constant + Scan time + Output response delay time
And values can be changed according to the type and brand of PLC.


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