Allen-Bradley 1779-KFMR User Manual page 101

Program and Message Types
The network protocol was designed on the assumption that application
programs are of two types: command initiators and command executors.
Corresponding to this division there are two message types:
Command Messages — Initiated by command initiators and carried
over the network to a command executor.
Reply Messages — The replies that command executors send to
command initiators.
You can send a write with no reply command. Otherwise, for each
command message there is normally one and only one reply. (A rare
exception occurs when the data link delivers a message but receives no
acknowledgment to verify delivery.) At the command initiator, the
network layer sends a reply message to the application layer. At the same
time, the command executor receives and executes the command packet,
and sends a single reply message to the command initiator. In any case,
the command executor generates only one reply message for each
command it receives.
If the network layer of the command initiator node cannot deliver a
command to another node, it generates a reply message with an error code
for the command initiator in its own application layer. If a reply cannot
be delivered, the network layer destroys the command.
Message Order
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.
Chapter 7
Introduction to Message Sets
7 5