Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 21)

Communication Concepts
Chapter 2
2-9
automatically fills the protocol bytes. If the transaction originates from a
computer station, your computer software must supply the necessary
protocol. In both cases, the data bytes contain information supplied by
application programs.
Command/Reply Cycle
Any network transaction consists of two messages: a command and a
reply.
The two parts provide extra data integrity by ensuring that a required
action always returns some sort of status, whether an error code or data.
As a frame of reference, the command initiator is always referred to as a
local station, and a reply initiator is always referred to as a remote station.
Unless noted otherwise, whether in a network link or an asynchronous
link, our reference will be limited to a single local station and a single
remote station.
The network layer protocol distinguishes a command from a reply.
Obviously, the data area of a command and its corresponding reply
depend on the type of command.
Priority
Each message on a Data Highway link is classified as either high priority
or normal priority.
Each message on a PCL is classified as normal priority.
Priority levels of messages determine the order in which stations are
polled and allowed to transmit messages. In the polling process, stations
with high priority messages will always be given priority over stations
with normal priority messages.
You specify the priority level for each command in the message command
code. The station that receives a command message must establish the
same priority level for its corresponding reply message.
Important: Stations with high priority messages are given priority over
stations with normal priority messages throughout the command/reply
message cycle. For this reason, a command should be given a high
priority designation only when special handling of specific data is