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Subject: [Bug 1398215] Re: Make output records only write on change
From: Joao Paulo Martins via Core-talk <core-talk at aps.anl.gov>
To: core-talk at aps.anl.gov
Date: Fri, 14 Feb 2020 09:49:54 -0000
First implementation of this feature will be done in the longout record.
The OOPT field will be added with the same options as the calcout
record, plus an additional one: "Write Once Then on Change". This will
be used to ensure that the output is executed after there is a change on
the "OUT" field, even when OOPT = On Change.

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https://bugs.launchpad.net/bugs/1398215

Title:
  Make output records only write on change

Status in EPICS Base:
  Triaged

Bug description:
  Add a field to the output record types to allow them to only write a
  new value to the hardware when it differs from the previous value.
  This saves adding another record to implement the only-on-change
  behavior, and can reduce the data communication volume for slow I/O.

  Can be made backwards-compatible if the default value of this field is
  output always.

  Gotcha's to think about: For extended device support, if a user
  changes the hardware address of a record the newly addressed device
  might need to reset the "previous value" somehow to ensure it gets
  written to the next time the record processes, even though the value
  hasn't changed. Could be tricky, discuss...

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