Rockwell Automation FactoryTalk Historian SE 3.0 UniInt Interface User Guide - Manual (Page 37)

UniInt Interface User Manual 31
Time
UniInt 3.x handles time differently depending upon whether or not UniInt is running in
extended PI API mode. A startup message in the
pipc.log file clearly indicates whether or
not UniInt is running in extended PI API mode. When the extended PI API features of UniInt
are not enabled, UniInt 3.x handles time in the same manner as UniInt 2.x. This is called
UniInt 2.x compatibility mode.
UniInt 2.x Compatibility Mode
UniInt 2.x compatibility mode means that the extended PI API features of UniInt are not
enabled. In this mode, UniInt-based interfaces typically timestamp data using one of the
following:
1. The system time on the Historian Server node.
2. A timestamp provided by the DCS.
However, the interface-specific documentation must be consulted to determine the exact
manner in which time is handled.
When the system time on the server node is used to timestamp the data, UniInt keeps track of
a time offset between the server node and the interface node and updates this offset every 30
minutes. The local system time plus the offset is equal to the Historian Server time. If
communication to the home node is lost, UniInt will apply the last offset that it calculated
until the offset can be verified when communication to the server is restored. If the time
offset between the server node and the interface node changes by more than 30 minutes,
UniInt will not adjust the time offset because UniInt assumes that a mistake was made in
adjusting the time either on the server or the interface node. An appropriate error message
will be written to the
pipc.log file.
Many interfaces that support the use of a DCS timestamp provide two methods for time
stamping the data. One option is for the interface to use the DCS time directly. In this case, it
is up to the user to make sure that the DCS system time is synchronized with the system time
on the Historian Server node. At the very least, the DCS system time cannot be more than 10
minutes ahead of the system time on the Historian Server node because Historian cannot
archive values that are more than 10 minutes in the future. A second option for the interface
is to apply an offset to the DCS timestamp, where the offset is equal to the server time minus
the DCS time. The offset is typically recalculated on a periodic basis.
Extended PI API Mode
In extended PI API Mode, UniInt interfaces typically use one of the following to timestamp
data:
1. Coordinated Universal Time (UTC).
2. A timestamp provided by the DCS.