Rockwell Automation Logix5000 Controllers Messages Programming Manual - Manual (Page 12)

Chapter 1 Controller messages
The controller has the following limits on the number of connections that you can
cache.
If you have this software version and
firmware revision
Then you can cache
11.x or earlier • Block transfer messages for up to 16 connections.
• Other types of messages for up to 16 connections.
12.x or later Up to 32 connections.
If several messages go to the same device, the messages may be able to share a
connection.
If the MSG instructions are to And they are Then
Different devices — Each MSG instruction uses 1 connection.
The same device, cached, and not a
large connection Enabled simultaneously (same scan) Each MSG instruction uses 1 connection
and 1 cached buffer.
Not enabled simultaneously All MSG instructions use 1 connection
and 1 cached buffer. They share the
connection and the buffer.
The same device, cached, and a large
connection
Enabled simultaneously (same scan) Each MSG instruction uses 1 connection
and 1 cached buffer.
Not enabled simultaneously All MSG instructions use 1 connection
and 1 cached buffer. They share the
connection and the buffer.
Example: 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 the messages count as one connection. Caching both
messages counts as one on the cached buffer.
• If the controller sends 10 cached connected messages to the same bridge module (for example, 1756-
EN2T) where 7 utilize a standard connection (large connection unchecked) and 3 utilize a large connection,
then the 7 standard connection messages all utilize one cached connection. The 3 large connection
messages all utilize another cached connection. In total, the 10 messages use 2 cached connections.
To establish a connection or process unconnected messages, the controller uses an
unconnected buffer.
Term Definition
Unconnected buffer An allocation of memory that the controller uses to process unconnected communication. The
controller performs unconnected communication when it:
•
Establishes a connection with a device, including an I/O module.
• Executes a MSG instruction that does not use a connection.
Unconnected buffers
12 Rockwell Automation Publication 1756-PM012F-EN-P - October 2014