Specifications

Main Properties
PlantScape Server and Client Configuration Guide 147
Scan Point Special
A Scan Point Special is a forced scan of point parameters. Whenever scanning of
a point is disabled and then re-enabled (for example using the Point Detail
display), a Scan Point Special is performed. Application programs and free format
reports can request a Scan Point Special demand scan of a point. See
“Configuring Reports” on page 239 and the Application Development Guide for
more information.
Control Confirmation
After an OP, SP, MD, or a parameter control is issued, a demand scan is issued on
the source address. This is to confirm that the control took place correctly. If the
scanned value does not match the controlled value, a control fail alarm may be
generated.
Refer to “Alarm Properties for Analog Points” on page 158 for more information
on control fail alarms for analog points.
Refer to “Alarm Properties for Status Points” on page 156 for more information on
control fail alarms for status points.
Scan Packets
The basic unit of scanning used by the server to acquire data from a controller is
referred to as a scan packet. One scan packet represents a single input/output
(I/O) or transaction with a controller; that is, every time the server sends a
request to a controller for data, one scan packet is used.
A scan packet can access several addresses within a controller to provide values
for several server points. Because the number of requests per second is generally
a limiting factor, the scanning strategy should attempt to obtain the maximum
number of point values in the minimum number of scan packets.
The basic requirements for points to be in the same scan packet are as follows:
• Points should be at the same scan period.
• Points should have source addresses that reference the same controller and
are contiguous.
• The number of values to be acquired does not exceed the scan packet size
for the particular controller.
Most controllers also require that the addresses to be scanned are contiguous.
plscpcg.book Page 147 Monday, May 28, 2001 10:11 AM