[PLC Sample Code] - LONG DURATION PRECISION TIMER.pdf

[TConnolly]

LONG DURATION PRECISION TIMER.pdf

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From time to time a long time base precision timer may be required. The SLC .01 second timebase timer is good for only 327 seconds. Cascading a timer and a counter together is undesirable as it will introduce a cumulative error into the time because of differences between when the time period

actually expires and when the timer is next scanned.

By running two timers in parallel we can maintain an accurate time reference from which to compute a floating point time with .01 second precision up to 32767 seconds. The first timer has a one second time base and it is the primary timer. The second timer has a .01 second time base and it is

used solely to compute the fractional seconds. By using a comparison to subtract one second from the .01 second time base ACC when ever it exceeds a full second we can maintain an accurate time independent of the processor scan time so long as the rung is evaluated more than once a

second.

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• Submitter
  TConnolly
• Submitted
  06/02/2006
• Category
  PLC Sample Code