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Explanation of how the IO scanner health bit works on the NOE771xx.

Goals and Symptoms

Explanation of how the IO scanner health bit works

Facts and Changes

140NOE771xx

Causes and Fixes

At the end of every scan, the NOE performs a check. The NOE keeps track of the time that the query
went out until the accumulated scan times exceeds the health timeout. If the time difference between the
time that the last query went out and the end of the current scan is greater than the health timeout, the
health bit is turned off.

The following example illustrates how the process would take place:

Initial conditions:

  • Health bit is on
  • The health timeout is set a 50 msec.
  • The actual PLC scan time is 15mecs
  • 10 msec into the scan, a query is sent out.

Health bit sequence of operation.
  • Because no response has been received at the end of the scan the NOE will wait until the
end of the following scan. The accumulated time is set at 20msec.
  • Because no response has been received at the end scan, the NOE will wait until the
end of the following scan. The accumulated time is set at 35msec.
  • Because no response has been received at the end scan, the NOE will wait until the
end of the following scan. The accumulated time is set at 50msec. Since the accumulated
time has not exceeded the health timeout, the NOE will wait until the end of the next scan.
  • Because no response has been received at the end scan and the accumlater time of 65msec
has exceeded the health timeout of 50 msec, the health bit is turned off.

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