Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 149)

Network and Application Layer Protocols
Chapter 12
12Ć2
Network Model
To implement your Data Highway network layer software, use a routing
subroutine and a queue. Messages created by the application are sent to
the router for transmission over the network. Messages that are delivered
by the network are placed on an incoming message queue that is unique
for each application. Figure 12.1 illustrates this model.
Figure 12.1
Application Model
Command
Q
R
Commands
Replies
Command
Q
Replies
Commands
R
Network
Initiator
Executor
Q
R
= Routing Subroutine Entry
= Incoming Message Queue
10062-I
Reply messages are not necessarily sent in the same order that their
corresponding command messages were received. It is impossible for the
network to guarantee delivery, and in some cases it may not be possible to
provide notification of non–delivery. Therefore, the command initiator
should maintain a timer for each outstanding command message.
Non–deliverable reply messages are not returned to the command
executor.
The application task is notified via the operating system when a message
arrives on the queue. Messages do not necessarily have to be removed
from the queue in order of arrival.