Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 93)

Chapter
8
8-1
Station Interfacing
To execute commands, a 1771-KA2 communication adapter module at
one station interacts with all other stations interface modules on the Data
Highway. This chapter describes this interaction of station interface
modules.
Because much of this interaction is transparent to user programming, the
information given here is largely for background. However, an
understanding of station interaction is useful in optimizing use of each
individual communication adapter module and of the Data Highway as a
whole. Specifically, the information aids in an understanding of the
following:
Floating master operation and polling
Message types
Sequence of messages for command execution
Acknowledgements
Re-tries of transmitted messages
Priority command designation
Automatic disconnection of faulted master
Commands that can be sent by computer
Central to the interaction of station interface modules is the concept of
shared mastership--the floating master. With this arrangement, no single
station controls the Data Highway communication link at all times.
Instead, each station vies for mastership based on its need to send
messages.
The advantage of this arrangement is that Data Highway operation can
continue even if one or more stations are unable to maintain
communication. Thus, disconnection of a station or a fault at a station
processor or communication adapter module does not disable
communication between other operating stations. This minimizes the
need for redundancy in many applications.
General
Floating Master