Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 230)

Data Link Layer
Appendix E
E-3
A station must have mastership of the Data Highway before it can
transmit any messages or polling sequences. As part of the data integrity
of the highway, all commands must receive a reply before a transaction is
considered complete. Since the highway treats commands and replies as
the same type of message, it takes at least one change of mastership to
complete a single transaction.
Any command has to be formatted in the application program of the local,
or transmitting, station. For a PC, the format is part of the PC user
program. For a computer, the formatting has to be done as part of the
computer program (Chapter 5).
A reply message is generated by a station in response to a command
message it receives. The reply message indicates that the command
message was received and that 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 that the write
operation has been completed at the receiving station.
When the replying station is a programmable controller, the reply message
is an automatic function of the interface module operation and is
transparent to your program. If the replying station is a computer, you
must program the computer to formulate the response and the reply
message.
To transfer mastership between stations, the station interface modules use
an orderly polling scheme. Normally, mastership passes from one station
to another in a round-robin fashion. During installation, you assign each
station a unique number between 001 and 376 (octal). Each master passes
mastership to the station with the next higher station number that is
requesting mastership. All polling arithmetic is performed modulo-256
relative to the station number of the current master. For example, when
the master is 200 (octal), Station 220 will receive mastership before
Station 177, since 220 is closer in sequence to 200 than is 177. Figure E.2
illustrates this polling scheme.
Message Transmission
Polling