Rockwell Automation FactoryTalk Historian SE 3.0 H2H Interface User Guide - Manual (Page 23)

FactoryTalk Historian To Historian Interface User Guide 17
recovery nearly all data is obtained from disk reads. When the interface transitions to
scanning for updates, disk reads should decrease as recent data is obtained from the archive
memory cache. The percentage of data obtained via disk reads versus memory can be
managed through the scan frequency. Increasing the scan rate should increase the percentage
read from memory. Vice versa, decreasing the scan rate increases the percentages of data
obtained from disk reads. With some trial and error users can determine a scan rate that
maximizes the time between scans while minimizing archive disk reads. For Historian 3
source servers, use the
Historian Performance Monitor interface to access archive
cache versus disk read data counters. Historian 2 source servers will want to monitor disk
versus cache read ratios.
Data Timestamps
The source Historian Server provides a timestamp for each data event. These timestamps may
be adjusted to account for time zone differences and clock drift between Historian Servers. It
is required that each Historian Server have the correct system time for the configured time
zone.
Timestamps are automatically adjusted for time zone differences when both source and
receiving servers are PI3. PI3 uses Universal Coordinated Time (UTC) to timestamp data.
UTC is based on seconds since Jan.1, 1970 in Greenwich Mean Time. Each UTC time stamp
contains a time offset from Greenwich Mean Time that represents the local time zone setting.
If source and/or receiving Historian Server is PI2, users have the option of adjusting for time
zone differences. PI2 uses local time. Local time is seconds since Jan. 1, 1970 in the local
time zone. PI2 data timestamps do not include a time zone offset. For example, viewing data
from a PI2 server in a time zone two hours behind your local time will appear to be two hours
old. PI3 data includes time zone information that allows for automatic adjustment to local
time.
The interface can also adjust for clock drift between Historian Servers. Clock drift is the time
offset between Historian Servers after accounting for time zone differences. An offset of 30
minutes or less is considered clock drift. When configured to do so, the time offset is added to
the timestamp provided by the source Historian Server adjusting it to receiving Historian
Server time.
Timestamp adjustment is configured on a tag-by-tag basis through the Location2 tag
attribute. Note that all computers (interface node, source and receiving Historian Server) must
have the correct system time for the configured time zone.
Data Type Conversions
The following table displays supported data type conversions between source and receiving
Historian tags.
Receiving Tag Point Type
Sou
rce
Ta
g
Poi
nt
Ty
pe
Float
16/32/64
Int16/32 Digital String
Float 16/32/64 Yes Yes No