Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 81)

6–2 Application Layer Message Packets
Publication 1770Ć6.5.16 - October 1996
There are two types of application programs:
•
command initiators
•
command executors
This application program Sends
command initiator
command messages - specify which command function
to execute at a particular remote node
Each command message requires one reply message.
The command initiator must check for error codes and,
depending on the type of error, retransmit the message or
notify the user.
The command initiator should also use a timer to be aware
of lost reply messages (due to noise or other factors).
If the time limit expires before the initiator receives the
reply, the initiator can retransmit or notify the user.
command executor
reply messages - responsible for interpreting and
executing the command message.
The executor must issue a reply message for each
command it receives. If the executor cannot execute a
command, it must send the appropriate error code.
Internally, Allen-Bradley asynchronous interface modules use a
routing subroutine and a message queue. When the module receives
a message over its asynchronous link, it puts the message in its
queue. The routing subroutine then takes the message from the
queue and transmits it over the DH, DH+, or DH485 network.
The module also queues messages received from the network.
The routing subroutine takes these messages and retransmits them
over the asynchronous link.
If this layer Cannot deliver a It should
application
command message
generate a reply message with the
appropriate error code and send the reply to
the initiator. application
reply message
destroy the reply without notification to the
command executor.
dataĆlink
message over the
asynchronous link
return an error message to the command
initiator.
How Your Application
Program Sends and
Receives Messages