Allen-Bradley Logix5000 Reference Manual page 192

Chapter 4 Input/Output Instructions (MSG, GSV, SSV, IOT)
If the MSG instructions are to
Different devices
Same device
192
If a MSG instruction uses a connection, you have the option to leave the
connection open (cache) or close the connection when the message is done
transmitting.
If you
Cache the connection
Do not cache the
connection
The controller has the following limits on the number of connections that you
can cache.
If you have this software and
firmware revision
11.x or earlier
12.x or later
If several messages go to the same device, the messages may be able to share a
connection.
And they are
Enabled at the same time
NOT enabled at the same time
EXAMPLE
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
Then
The connection stays open after the MSG instruction is done. This
optimizes execution time. Opening a connection each time the
message executes increases execution time.
The connection closes after the MSG instruction is done. This frees up
that connection for other uses.
Then you can cache
· Block transfer messages for up to 16 connections.
· Other types of messages for up to 16 connections.
Up to 32 connections.
Then
Each MSG instruction uses 1 connection.
Each MSG instruction uses 1 connection.
The MSG instruction uses 1 connection and 1
cached buffer. They share the connection and the
buffer.
Share a Connection
If the controller alternates between sending a block-transfer
read message and a block-transfer write message to the same
module, then together both messages count as one connection.
Caching both messages counts as one on the cache list.