Appendix B Scanning; Scanning Philosophy; Configured Foreground Scan Items; Configured Background Scan Items - Honeywell CIU 888 Migration Manual

APPENDIX B SCANNING

Part No.: 4417593_Rev09
The main task of the CIU 888 is scanning the connected tank gauging
instruments continuously for the measured values of the connected
devices.
B.1 Scanning Philosophy
CIU 888 incorporates time-based scanning for better scanning
performance. CIU 888 maintains a scan items pool per field port /
sequenced field ports that has the list of scan items to be scanned.
Each scan item is allocated a minimum scan period and a maximum
scan period. Minimum scan period is the minimum expected time
interval at which the scan item must be scheduled in milliseconds and
maximum scan period is maximum time interval within which the scan
item must be scheduled in milliseconds.
Based on the scan periods and current scanning pattern of the scan
items, CIU 888's scanning algorithm picks up the most useful item
among all the scan items configured for a field port and sends it to the
field. In case of sequenced field ports (channel sequencing), CIU 888's
scanning algorithm picks up the most useful item among all the scan
items configured for the field ports that are sequenced.
The following are the various scan items processed by CIU 888.
B.2 Configured Foreground scan items
The CIU 888 scans measurable data inputs (e.g. product level) that are
configured in the foreground scan more often than data inputs
configured in the background scan.
In tank gauging, the most important data to retrieve from gauging
instruments on a tank is the level. The core competency of the scanner
is to update each value on a regular basis. If tank calculation is part of
the solution, a change of each new level value will trigger the
calculations module to recalculate the tank record.
B.3 Configured Background scan items
Measured data inputs that do not change frequently, e.g. the average
temperature of a tank, do not need to be scanned with the same scan
rate as the values that are scanned in the foreground scan.
The CIU 888 scans all measurable data inputs that are configured in the
foreground with a higher priority ((i.e) less minimum and maximum scan
period) than the measurable data inputs that are configured in the
background scan.
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