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Old January 3rd, 2018, 03:33 PM   #28
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Originally Posted by Mike Lamond View Post
This thread was referenced from another thread today, so I'll add my comment here.

About 10 years ago, on my first major Logix 5000 project, I tested a wall clock-derived one second pulse similar to Bernie's code in post #18 against a 1000 msec repeating timer. Over time, the timer lost counts against the clock pulse. Using a 500msec periodic task to toggle a bit was not an option as we were using Version 13 redundancy which allowed only continuous tasks.

One-shot vs 500ms on/ 500ms off doesn't matter to elapsed time counter inputs. However, flow totalizers use math functions which execute on every scan where the rung is true, so they require a one-shot trigger to work properly.

Assuming that one does have the ability to employ a periodic task, no pulse reference bit is required. An unconditional logic statement will execute once per second in a program running under a periodic task with a 1000 ms time base.

Why bother with a redundant trigger?
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