Instruction Manual

Rockwell Automation Publication 1756-UM514C-EN-P - June 2014 163
Appendix B
Application Guidelines and Tips
Use this appendix to better understand how to use your 1756-DHRIO and
1756-DHRIOXT modules in DH+ and remote I/O applications.
Cached and Uncached
Connections
A connection is defined as a message from point A to point B. A ControlLogix
controller can make a total of 250 connections. With RSLogix 5000 version 10
or later, the controller can cache a total of16 DH+ connections and 16 remote
I/O connections. In RSLogix 5000 versions 9 and earlier, the controller can cache
only remote I/O connections.
Up to five messages can be sent between points and be counted as one
connection. When you read and write to the same module on remote I/O,
however, you must interlock messages.
The two types of connections in DH+ and remote I/O applications are as
follows:
• Cached Connections
• Uncached Connections
Topic Page
Cached and Uncached Connections 163
ControlLogix Controller Constraints 164
Message Manager 165
Messages Between a ControlLogix Controller and PLC
Devices
165
RPI Configuration Settings 165
RPI Formula without Block Transfer Modules 166
Increase the Unconnected Message Buffer Limit 166
Remote I/O Performance: ControlLogix Controllers versus
PLC-5 Controllers
168
DH+ Baud Rate Comparison 169