Allen-Bradley ControlNet Compact GuardLogix 1769, User Manual, Page: 86 / 100

Chapter 6 Messaging
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 can configure
multiple messages for the same path and use only one connection if only one
message is active at a time; however, this requires that you write your ladder logic
correctly to make sure that only one message is active at any time.
These connected messages can leave the connection open (cache) or close the
connection when the message has finished transmitting.
Table 22 - Message Connections and Communication Methods
Message Type Communication Method Connection
Required
CIP data table read or write CIP Yes
CIP generic CIP Optional
(1)
Block-transfer read or write Not applicable Yes
(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 to determine whether to cache a connection.
Message Execution Appropriate Action
Repeated Cache the connection.
This keeps the connection open and optimizes message completion time.
Opening a connection during the execution of each message increases
execution time.
Infrequent Do not cache the connection.
This closes the connection upon completion of the message, freeing up
that connection for other uses.
Enter Message Logic
To send or receive data from a ControlNet module via a message, you must
program a MSG instruction in the local controller’s logic. If the target module is
configured in the I/O Configuration folder of the controller, browse to choose
the module or manually type the message path in the MSG instruction.
86 Rockwell Automation Publication CNET- UM001E-EN-P - June 2016