Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 30)

Publication AG-UM008C-EN-P - February 2005
1-12 Designing Communication
Figure 1.4 The master station scans slave stations in a set sequence.
Figure 1.5 Here is how the polling sequence applies to an application.
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1. Scans all stations in the active priority
poll file.
2. Scans one station in the inactive priority
poll file.
3. Scans stations in the active normal poll file
based on the normal poll group size, which you
specify during configuration. For example, if
the group size were 3, then three stations
would be polled in the normal file before the
master continues to the next step in the
sequence.
4. Scans one station in the inactive normal poll file
after all stations in the active normal list have
been polled.
Active Priority
Poll List
Inactive Priority
Poll List
Active Normal
Poll List
aa
bb
cc
dd
Inactive Normal
Poll List
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Active Priority
Inactive Priority
Active Normal
Inactive Normal
Master
Station
Modem
Modem Modem Modem Modem Modem Modem Modem
Group size = 1
Poll List
STN1
STN7
Poll List
STN2
STN6
Poll List
STN3
STN4
Poll List
STN5
1 2 3 4 5 6 7 Polling Sequence:
STN1
STN7
STN2
STN3
STN1
STN7
STN6
STN4
STN5
Beginning of new scan