Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 231)

Data Link Layer
Appendix E
E-4
Figure E.2
Polling Scheme
• • •
074
075
076
077
110
111
112
113
•
•
•
174
175
176
177
200
201
202
203 274
275
276
277
300
301
302
303
373
374
375
376
010
011
012
013
• • •
•
•
•
11537
The mechanics of the polling algorithm are essentially transparent to you.
This is because the communication modules handle this automatically.
However, the polling algorithm does lead to the following basic rules that
you should follow to optimize your Data Highway performance:
Number stations sequentially whenever possible.
Keep the number of high priority messages as low as possible.
Large numbers of high priority messages slow all traffic on the network.
In general, you should limit the number of high priority messages to less
than 1% of the total traffic on the Data Highway.
There are two checks used in Data Highway network message
transmission:
A 16-Bit Cyclic Redundancy Check (CRC) on a Data Highway Link
An 8-Bit Block Check Character (BCC) or a 16-Bit CRC on an
Asynchronous Link
Additionally, you can select a parity check (even parity only) on the
asynchronous link.
Data Security