Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 20)

Communication Concepts
Chapter 2
2-8
Each of the physical links just described requires three layers of software
to enable communication to take place. The layers are defined as follows:
Application Layer — Controls and executes the actual tasks, or
commands, specified in the communication between stations. To
program this layer, use the commands described in Chapter 5.
Network Management Layer — Handles queueing, sequencing,
routing, and error status reporting for communication. If your physical
link contains only Allen-Bradley PCs, you do not have to program this
layer. Otherwise, refer to Chapter 5 for a description of how to
program this layer for an asynchronous link to a computer.
Data Link Layer — Controls the flow of communication over the
physical link by establishing, maintaining, and releasing the
communication channel between stations. If your physical link
contains only Allen-Bradley PCs, you do not have to program this
layer. Otherwise, refer to Chapter 4 for a description of how to
program this layer for an asynchronous link to a computer.
Application Layer
The application layer concerns the specific commands that you can
program at a given station to cause that station to communicate over the
link. This layer is the same for both asynchronous and network links.
The types of commands that a station can transmit and receive vary with
the type of processor at that station. Chapter 5 describes the commands
that each type of PC processor can transmit or receive. To program your
computer to communicate with a PC, use the appropriate command
message format shown in Chapter 5.
Message Structures
All messages on a network have the same fundamental structure,
regardless of their function or destination. If you could freeze a block
while it is in transmission, you would see two types of message bytes:
Protocol Bytes
Data Bytes
Protocol bytes are used by the network to get the message to its
destination. Data bytes are delivered to the application at the destination.
The methods by which these bytes are filled is determined by the nature
of the station from which the transmission block originates. For example,
if a transaction originates from a PC station, the station interface module
Software Layers