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PLC INPUT Interrupt Behavior

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Lorne Van Dusen:
Gary
I think you have the right idea but you are over doing it.
Leave the Interrupt as a Rising Edge trigger then in the first line of the function disable the interrupt then do a REFRESH of the IO then do a TESTBIT of the input you want then you can set any relay you want or reset any counters etc.
Then before you exit turn the interrupt back on.
Then in your ladder logic you can reset the relay you set in the function any way you want

garysdickinson:
Lorne,

I think your approach is reasonable. 

The feedback that I got from TRI is that the behaviors vary by PLC family. 

1. The Wx100 is based on a very different processor and the state of the physical INPUTs may continued to be visible to ladder logic while being used as an interrupt source. This is not the case with Fx based PLCs or the older FMD based PLCs.

2. There is some sort of debounce logic on INPUTs to filter out bounces that show up with mechanical contacts.  This mechanism is turned off for those INPUTs that are being used as INPUTs on some PLC  families.  The debounce mechanism is be based on ladder logic scan boundaries and may require more than a single scan to get settled.

The bottom line is that it can take one or more full scans of the ladder logic on most PLC families from when an INPUT interrupt is disabled until that INPUT's true state is visible to ladder logic.

Thanks for helping me think through this stuff.

Gary d

Lorne Van Dusen:
Gary I had had a similar issue and when I did some detailed testing I discovered that there was a 24VDC relay that was connected to a PLC output and the manufacture of the equipment forgot to install a diode across the coil. Be-leave it or not that spike created cause the PLC to call my interrupt that I used to reset the HSC value. 
I found out with some help from tech support that within the interrupt I could disable the interrupt then I could do a quick Refresh and read the input that was tied to the interrupt which now is at level logic then reset my HSC value then re-enable the interrupt . All this added less than 1ms which actually prevented the spike from resetting the HSC and was still more than fast enough to reset the HSC value instantly. 

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