Rockwell Automation 1756-ENET/B ControlLogix Ethernet Communication Interface Module User Manual - Manual (Page 21)

Publication 1756-UM051B-EN-P - November 2000
About the 1756-ENET/B Module 1-3
Rack optimized connections reduce the total number of connections
needed to transfer data when using many I/O modules in a system.
The following example illustrates the benefit of rack optimized
connections.
Assume you have set up a system that contains 10 discrete I/O
modules in a remote ControlLogix chassis. If you use direct
connections to transfer data to each of the these I/O modules, you
need 10 connections to transfer all of the data, one to each of the ten
I/O modules. If you use a rack-optimized connection to transfer the
data, you only need a single connection – the connection to the
Ethernet module.
See the Ethernet Performance and Application Guide, publication
number ENET-AP001A-EN-P, for more information on connections.
Mixing Rack Optimized and Direct Connections
You can mix communication formats for different I/O modules in the
same chassis. I/O modules set up to use Rack Optimization will
communicate at the rate of the RPI (requested packet interval)
configured for the 1756-ENET/B module. I/O modules configured for
direct communication will communicate at their set RPI and ignore the
Ethernet module’s RPI.
IMPORTANT
Although rack optimized connections offer an
efficient way to use resources, there are a few
limitations on their use:
•
You can only use rack optimized connections to
send data to and from discrete digital I/O
modules. Analog I/O requires direct connections.
•
Rack optimized connections can contain I/O data
and status information only. Additional module
information, such as diagnostics, is not available
through a rack-optimized connection.
•
All data is sent at the same time at the RPI rate of
the Ethernet module, regardless of a COS on a
particular module.