Rockwell Automation 1756-DHRIO_DHRIOXT ControlLogix Data Highway Plus-Remote I/O Communication Interface Module - Manual (Page 112)

112 Rockwell Automation Publication 1756-UM514C-EN-P - June 2014
Chapter 9 Using the 1756-DHRIO and 1756-DHRIOXT Modules in Remote I/O Applications
Increased Remote I/O System
Throughput Because of the unique design of the 1756-DHRIO or 1756-DHRIOXT module
system, performance can be greatly enhanced by splitting the remote I/O
adapters across the channels. An example of a simple system has the following
devices:
• Rack 1 - Starting Quarter 0 - Size Full
• Rack 2 - Starting Quarter 0 - Size Full
If the racks are placed on the same channel at a baud rate of 230.4Kbaud, the
minimum RPI between the 1756-DHRIO or 1756-DHRIOXT module and the
remote I/O adapters is 6 ms. If the racks are split between channel A and channel
B, the update rate can be decreased to 4.5 ms.
The following algorithms are used to calculate various update rates:
@ 230.4Kbaud Update Rate = 3 ms*(number of racks [channel A or B]) +
1/2*3 ms*(number of racks [channel A or B])
@ 115.2Kbaud Update Rate = 5 ms*(number of racks [channel A or B]) +
1/2*5 ms*(number of racks [channel A or B])
@ 57.6Kbaud Update Rate = 8 ms*(number of racks [channel A or B]) +
1/2*8 ms*(number of racks [channel A or B])