Rockwell Automation 1756-DHRIO_DHRIOXT ControlLogix Data Highway Plus-Remote I/O Communication Interface Module - Manual (Page 164)

164 Rockwell Automation Publication 1756-UM514C-EN-P - June 2014
Appendix B Application Guidelines and Tips
Cached Connections
A cached connection is connection between two points that remains open at all
times. Remember the following:
• When using cached connections, multiple messages from identical points
consume only one connection. In this case, the identical point messages
must be cached.
• Messages are completed much faster because the connection is open.
• Can have a total of 16 cached connections on remote I/O.
• With Studio 5000 environment version 10 and later, you can have a total
of 16 connections cached on DH+.
• If more than 16 connections are cached the messages perform like they
were uncached. If you want all connections in your application to perform
like cached connections, we recommend you cache only 16 connections.
Uncached Connections
An uncached connection is a connection between two points that opens only
when a message is enabled and closes when the connection is completed.
Uncached connections are not as fast as cached connections.
ControlLogix Controller
Constraints When using the ControlLogix controllers, remember the following constraints:
• The controller is limited by its unconnected buffers, including a default
Unconnected Buffer (UCB) limit of 10.
• In RSLogix 5000 version 8 and later, you can increase the UCB limit to
40. However, for each UCB over 10, your application receives a penalty of
1K of I/O memory. For example, if you change the UCB limit to 25, your
application loses 15K of I/O memory.
For more information on increasing the unconnected message buffer limit,
see page
166 .
• A DH+ or remote I/O message can take a total of two connections in the
UCB. One connection is used for the forward open and one connection is
used for the reply.
• Cached connections are separate from the UCB number.