Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 81)

Message Packet Formats
Chapter 5
5-2
Figure 5.1
Network Model
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Command
Initiator
Command
Executor
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Commands
Commands
Network
Q Q
R R
R Q = Routing Subroutine Entry = Incoming Message Queue
11576
Messages do not necessarily arrive at their destination in the same order
that they were sent. It is impossible for the network layer to guarantee
delivery, and in some cases it may not be possible to provide notification
of non-delivery.
In particular, the network layer does not notify the command executor
about non-delivery of a reply message. Therefore, it is advisable that your
command initiator maintain a timer for each outstanding command
message. If the time limit expires before the command initiator receives
the corresponding reply to its command, it can either retransmit the same
command message or indicate an error condition.
If your network layer software cannot deliver a command message, it
should generate a reply message with an error code in it and send that
reply to the command initiator. If it cannot deliver a reply message, the
network layer software should simply destroy the reply without
notification to the command executor.
If your link layer software cannot deliver a message over the
asynchronous link, it also should return an error message to the command
initiator.