Rockwell Automation 9301 Series RSView32 Users Guide - Manual (Page 58)

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RSView32 User’s Guide
RSView32 scan classes
Any tag that communicates with Allen–Bradley or SoftLogix 5
programmable controllers through direct drivers must be assigned to
an RSView32 scan class. Each tag can then be assigned different
foreground and background scanning periods. In this way, you
determine how often a tag value is updated.
A scan class has two scan periods: the foreground period and the
background period. The foreground period applies to graphic displays
and the tag monitor. The background period applies to any
component that performs a continual background activity, such as
derived tags, events, alarms, and data log.
If the same tag is used by both a foreground and background
component, the tag is scanned at the faster period. If a value is not
required by a foreground or background component, its address is not
scanned, greatly reducing system overhead.
Guidelines for configuring scan classes
Set foreground scan periods to be shorter than or equal to
background scan periods.
A scan period of zero means scan as quickly as possible. You can
set foreground scan periods to zero, but never set background scan
periods to zero. For foreground periods, using zero is not
recommended because this setting can result in so much traffic
that overall system communication slows down.
Set scan periods for optimal efficiency. Specify a frequent scan rate
for tags representing fast–moving devices and a less frequent scan
rate for tags representing devices that change less often.
This approach optimizes system performance, providing high–
speed scanning where required and using less frequent scanning,
which saves system resources, where acceptable.