Determine Connections For Messages; Guidelines For Caching Message Connections - Rockwell Automation Allen-Bradley ControlNet 1734-ACNR User Manual

Determine Connections for
Messages
Messages transfer data to other modules, such as other controllers, I/O
modules or operator interfaces. Each message uses one connection, regardless
of how many modules are in the message path. To conserve connections, you
can configure one message to read from or write to multiple modules. Also,
you configure multiple messages for the same path and use only 1 connection
if only 1 message is active at a time; however, this requires that you write your
ladder logic correctly to make sure only 1 message is active at any time.
These connected messages can leave the connection open (cache) or close the
connection when the message is done transmitting. The following table shows
which messages use a connection and whether or not you can cache the
connection:
Table 6.2 Message Connections and Communication Methods
This Type of Message
CIP data table read or write
PLC2, PLC3, PLC5, or SLC (all types)
CIP generic
block-transfer read or write
(1)
You can connect CIP generic messages, but for most applications we recommend you leave CIP generic
messages unconnected.

Guidelines for Caching Message Connections

Follow these guidelines when you consider whether to cache a connection or
not:
Table 6.3 Caching Guidelines
If the Message Then You Should
Executes
Repeatedly Cache the connection.
This keeps the connection open and optimizes message
completion time. Opening a connection each time the message
executes increases execution time.
Infrequently Do not cache the connection.
This closes the connection upon completion of the message,
which frees up that connection for other uses.
Peer-to-Peer Messaging
Using this
Communication Method
CIP
CIP
CIP with Source ID
DH+
CIP
na
Publication CNET-UM001C-EN-P - November 2005
6-5
Uses a
Connection
yes
no
no
yes
(1)
your choice
yes