Specifications

8 – Configuring Points
146 R400
• The rate of change of the value. If a value only changes once an hour, it is
inefficient to scan that value every five seconds.
• The rate at which history needs to be collected for the point (in the case of
the PV parameter). A point requiring one minute snapshots to be recorded
would have a scan period of no greater than 60 seconds. (For details of
point history collection, see “History Collection and Archiving” on
page 167.)
• How quickly field changes need to be available on a Station display.
Dynamic values on a display are updated from the database at the
configured update rate of the Station.
• The number of values that can be scanned from a controller at a particular
scan rate. For example, it is unlikely that 2000 analog values could be
scanned from a controller connected to server via a serial line operating at
1200 baud. Many factors influence this value; see “Scan Packets” on
page 147 for more details.
Some experimentation might be required to arrive at optimum scanning periods.
See “Analyzing the Scanning Load with lisscn” on page 148 for information on
analyzing scanning performance.
Exception Scanning
Exception scanning is only available for those controllers that support the
reporting of significant events to the server.
Significant events include:
• Changes in a status point
• Significant changes in an analog point
• Analog alarms
For more information about configuring this type of scanning, see the
controller-specific online reference information, accessible via the Quick Builder
online help.
Understanding Demand Scanning
Demand scanning provides a one-shot scan of a point parameter in response to a
request. Two main types of demand scanning are used:
• Scan Point Special
• Control Confirmation
plscpcg.book Page 146 Monday, May 28, 2001 10:11 AM