Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 94)

Station Interfacing
Chapter 8
8-2
Mastership
A Data Highway cable links as many as 64 stations. Because this cable
has a single pair of wires, only one station can transmit at a time. When a
station interface module gains control of this data link to transmit
messages, that station has mastership.
When one station interface module is master, all other station interface
modules assume a “slave” or receive mode. This enables these stations to
receive and acknowledge messages sent to them. The relationship
between the master station interface module and all other stations is
shown in Figure 8.1.
Figure 8.1
Master Station Functions
Master
Station
All
Other
Stations
1. Send Command/Reply Messages
Acknowledgment
2. Send Polling Message Sequence
Signal if Ready
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As master, a communication adapter module can send the following types
of messages:
Command
Reply
Polling