Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 97)

Station Interfacing
Chapter 8
8-5
corresponding reply message. (The command code is described in
chapter 5.)
Priority commands are executed ahead of normal commands throughout
the command/reply message cycle. For this reason, a command should be
given priority designation only when special handling of specific data is
required. Using an excessive number of priority commands defeats the
purpose of this feature and could delay or inhibit the transmission of
normal messages.
When a station communication adapter module has a priority command or
reply message ready for transmission, the module responds to the next
priority poll to gain mastership.
Polling
No single station is permanent master of the Data Highway
communication link. Each station bids for mastership when it needs to
send a message. The station interface module at the current master station
conducts a poll to select the next master station. A poll is an orderly,
systematic method to determine which stations have messages to send,
and to select one of these stations as master.
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 Data Highway performance:
Number your stations sequentially whenever possible, and
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.
A communication adapter module can execute commands from a
computer connected through a communication controller module (cat. no.
1771-KE or -KF). The command set of the computer includes the
command set of the 1771-KA2 module:
Protected write
Protected bit write
Unprotected read
Unprotected write
Commands From A Computer