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Can i use the read immediate input as high speed counter. Is it necessary to use read immendiate input in _Interrupt 1.25 mS or i can use it in a normal subroutine.

Hi Sunit,

The principle behind the  Immediate functions is to access I/O information (read/write) regardless of the PLC scan time (as you probably know - the PLC scan is a loop, starting with reading the inputs, then performing the ladder logic and finally activating the outputs).

Regarding using this feature as a high speed counter - the answer is no. that is because the normal digital input has a delay response much slower than the typical scan time (usually 10ms).

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Kind of.  As Eyal points out, regular inputs are slower than HSC inputs.


You will have your best luck using inputs on the PLC's main I/O board.  It doesn't work with expansion modules.  Set up the 1.25 ms Interrupt subroutine and put your Immediate Input blocks in it, along with any logic such as incrementing an MI.  Be sure to use a positive transition on the input you've just read.  Also don't forget that the input has to be off for at least one execution of the Interrupt routine, so your actual response time is 2.5 ms, which translates to 400 Hz.


If you can live with a 400 Hz response time it will work.


Joe T.

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