ProSoft Technology MVI56-AFC - Manual (Page 138)

Events MVI56-AFC ♦ ControlLogix Platform
User Manual Liquid and Gas Flow Computer
Page 138 of 316 ProSoft Technology, Inc.
February 25, 2011
9.5.8 Other Considerations
Expired Events
If the starting event number is sufficiently small relative to the events on file (in
particular, if it is 1999 or more before the number of the next event to be
recorded), then that event is no longer on file and has already been overwritten.
In this case, the event returned by a Fetch is all zero. This is not an issue for the
Active host, especially when the event log is configured to be locked, as the
Active host is interested only in non-downloaded events and those always remain
on file with a locked log. But for Passive hosts, and when the event log is
configured as unlocked, download sessions must be performed frequently
enough so that the requested events still remain on file. Because event numbers
wrap from 65535 to 0, and because events that have not yet been written do not
in fact exist and have never existed, an event number that is numerically equal to
or greater than the number of the next event to be recorded is deemed to be the
number of an event from the previous wrap cycle.
Persistence
A log-download session does not survive a reset of the module (e.g. power
cycle).