User manual

Computer Gateway User Manual 4-102 5/96
4.8.2
4.8.2.2 Value-Type Meanings for Averages
Averages are of analog data and are the total of the summed good averages for the period
divided by the number of good samples.
0 = Normal: 90% or more good samples
1 = Nonstandard: Less than 90% good samples
2 = Digital Value: not applicable (if an average is requested for a parameter of
type digital, the value type returned is "data not in history").
3-4 = not used
5 = Time Change: A time change occurred during the averaging period, but
there are 90% or more good samples.
6 = Outage: History module was not in service for the entire period; value
field contains NaN.
7 = No Data: No values were available from the Data Owner for entire period;
value field contains NaN.
8-10 = not used
11 = Collection Inhibited: not applicable
12 = Not in History: Requested data was outside span of the History file; value
field contains NaN.
13 = Time Change nonstandard: The average calculation was performed
according to the new time. Samples already collected are rolled into the
new average. There are fewer than 90% good samples.
4.8.2.3 Effect of Time Change on History Gathering
Absolute History Requests—If a time change occurs during an Absolute History interval,
the number of samples returned can differ from the expected number of samples. For
example, if it is desired to obtain a day's worth of hourly averages (24) and a forward time
change occurred, 23 samples are returned. If the time change is in the backward direction,
25 samples are returned.
Relative History Requests—The number of samples returned by a Relative History request
is immune to time changes. The number of samples returned is always the absolute
difference between the start and end offsets plus one.
Snapshot History Requests—The 60-second history collection task executes at the
beginning of each minute (:00 seconds). When a time change is detected, the operating
system resynchronizes all cyclic tasks beginning with the first interval following the time
change.
For example, assume that current time is changed from 13:37:23 to 14:37:00. In this
instance, the 60-second history collection will have collected data at 13:37:00, and will be
reactivated at 14:38:00 (the first interval following the time change). At this time, a time
change record is stored into the history collection files in lieu of a collected value. Starting
at the next activation, at 14:39:00, history data is again collected. Thus, one 60-second
history collection is skipped.
The 5-, 10-, and 20-second history data collections—which are synchronized with the 60-
second collection—also replace their first possible data collections following a time change
with time change records.