Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 27)

1–10 Network Layers
Publication 1770Ć6.5.16 - October 1996
Message Priority
You specify the priority level for each DH command in the message
command code. The node that receives a command message must
establish the same priority level for its corresponding reply message:
This link Classifies a message as
DH
high priority or normal priority
Priority levels of messages determine the order in which nodes transmit
messages on a DH link. In the polling process, nodes with high priority
messages will always be given priority over nodes with normal priority
messages.
DH+ normal priority
DH485 normal priority
Important:
Nodes with high priority messages are given priority
over nodes with normal priority messages throughout
the command/reply message cycle. For this reason, a
command should be given a high priority designation
only when special handling of specific data is required.
Using an excessive number of high-priority commands
defeats the purpose of this feature and could delay or
inhibit the transmission of normal priority messages.
Delivery Order of Commands
The sending node, the network, and the receiving node execute
commands based on network conditions, including—but not limited
to:
•
nodes buffering commands
•
retries due to noise on the network
•
priority levels
If your application requires that commands be delivered in a specific
order, your logic must control the initiation of one command at a
time on the network and verify delivery before initiating additional
commands. This verification is completed by:
•
a done bit or an error bit in a PLC processor
•
a reply message in a computer
A done bit or a successful reply causes the next command to be
initiated. If an error bit or a reply with non-zero status is returned,
you must decide the appropriate action based on your application.
Important:
If any node on the network initiates multiple commands
(for example, the sending node sets multiple bits at any
one time), the order in which these commands get
executed at the receiving node cannot
be guaranteed.