Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 96)

Station Interfacing
Chapter 8
8-4
The reply message is an automatic function of communication adapter
module operation, transparent to the user program. To send a reply
message, a station must have mastership.
Acknowledgement
When a station receives a message addressed to it, whether it is a
command or reply message, the receiving station sends an
acknowledgement. The acknowledgement is merely a signal that confirms
that a message has been received. Transparent to the user program, the
acknowledgement is an automatic function of communication adapter
module operation.
The acknowledgement serves 2 basic functions: it indicates that the
receiving station is operating and has received the message, and it
indicates that the received command or reply message is in intelligible
format. Should the proper acknowledgment not be received, a fault in the
receiving module may be indicated. In this event, the master station
communication adapter module re-tries transmission of the message. The
communication adapter module attempts several re-tries for a command
or reply message. Re-tries are automatic, transparent to the user program.
If re-tries are unsuccessful, a LOCAL FAULT bit may be set ON at the
local (sending) station processor. (LOCAL FAULT bits are described in
chapter 6.)
Acknowledgements are sent only by a station that is in the receive mode,
not the current master station.
Message Priority
Each message transmitted over the Data Highway communication link has
one of these priority levels: normal and priority.
These priority levels determine the order in which stations obtain
mastership and the order in which messages are transmitted. Priority
messages of a given type (either command, reply, or polling) will always
be transmitted before normal messages of the same type.
The programmer designates a priority level for each command message.
The command code, an element in each command rung, specifies the
priority level of the command message. The station that receives a
command message automatically establishes the same priority level for its