Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 80)

Chapter
5
5-1
Message Packet Formats
This chapter describes how your computer application programs should
format command and reply messages for transmission over the
asynchronous link. It covers both application level and network level
functions. Basically, the application layer specifies the contents of
messages and initiates their transmissions. The network layer takes the
information from the application layer and formats it in a way that is
acceptable to the data link layer (Chapter 4).
There are two types of application programs: command initiators and
command executors. These names correspond with the two message
types:
Command Messages — Sent by command initiators to command
executors.
Reply Messages — Sent by command executors in response to
command messages received from command initiators.
Each command message requires one reply message.
Command initiators specify which command function to execute at a
particular destination station. The command executor at that destination
station is responsible for interpreting the command message and
executing the specified command function. The command executor also
issues a reply message for each command it receives. If it cannot execute
the received command, the command executor must generate the
appropriate error message.
Internally, the KF2 module uses a routing subroutine and a message queue
to implement the network layer. When the module receives a message
over its asynchronous port, it puts that message in the queue. The routing
subroutine then takes the message from the queue and transmits it over the
Data Highway link. The module also queues messages received from the
Data Highway, and the routing subroutine retransmits those messages
over the asynchronous link. Figure 5.1 illustrates this model.
General
Application Layer
Network Layer