Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 29)

Publication AG-UM008C-EN-P - February 2005
Designing Communication 1-11
Designing a Polling Scheme
Each master station in a SCADA application must have a polling
scheme configured. To design a polling scheme, do the following:
•
choose the type of scheme best suited for your application
•
optimize your polling scheme to obtain the best efficiency
The master station you are using determines the type of polling
choices you have; however, Allen-Bradley master stations offer similar
choices, such as:
•
normal and priority polling lists
•
ability to poll a slave station:
– once per occurrence in the poll list (single)
– until it has no more messages to send (multiple)
Choosing Normal or Priority Polling Lists
Slave stations listed in a priority poll list are polled more frequently
than those listed in the normal poll list. Place the slave stations that
you need information from more frequently in a priority poll list.
Within each poll list, slave stations are assigned a status, which is
either active or inactive. A slave station becomes inactive when it does
not respond to a master station’s poll packet after the configured
number of retries.
If your master station is a Logix controller or PLC-5, you can use
application logic to reorder the polling lists and priority while the
application logic is executing.
Figure 1.4 and Figure 1.5 show how normal and priority lists relate to
one another.