Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 95)

Station Interfacing
Chapter 8
8-3
The function of both command and reply messages is the transfer of data
between stations. As soon as the communication module attains
mastership, it sends its command and reply messages (up to a maximum
number--16 for the 1771-KA2).
After the module sends its command and reply messages, it transmits a
sequence of polling messages. By this sequence, it selects the next station
to be master of the Data Highway communication link.
Command Messages
A command message from the master communication adapter module
instructs another station interface module to execute a command. In
general, a command from a communication adapter module can do one of
the following:
Write data table data to another station
Write bit status to another station
Request data table data from another station through a read command
The command rung, entered in the communication zone of user program,
instructs the communication adapter module to format a command
message. Once this message is sent, execution of the command begins.
(Command types, execution, and programming are described in chapters 4
through 7.)
Each command message causes a reply message to be generated by the
receiving station.
Reply Messages
Reply messages are generated by a station in response to command
messages that it receives. The reply message indicates whether the
command message was received and whether the station interface module
has completed the sequence of events required of it for command
execution. For commands that write data, the reply message indicates
whether the write operation has been completed at the receiving station.
For commands that read data the reply message contains the data
specified by the command. If the command was not executed
successfully, the reply message contains an indication of the error in the
STS byte.