Allen-Bradley ControlNet Compact GuardLogix 1769, User Manual, Page: 76 / 100

Chapter 5 Produce and Consume Tags (interlock controllers)
Adjust for Bandwidth
Limitations
When you share a tag over a ControlNet network, the tag must fit within the
bandwidth of the network:
•
As the number of connections over a ControlNet network increases,
several connections, including produced or consumed tags, may need to
share a network update time (NUT).
•
A ControlNet node can transmit approximately 500 bytes of scheduled
data in a single NUT.
Depending on system size, your ControlNet network may lack the bandwidth for
large tags. If a tag is too large for your ControlNet network, make one or more of
these adjustments.
Table 20 - Tag Adjustments
Adjustment Description
Increase the requested packet interval (RPI) of your
connections. This is the recommended method.
At higher RPIs, connections can take turns sending data during an update period.
Reduce your network update time (NUT). At a faster NUT, fewer connections have to share an update period.
For a ControlNet 1756-CN2, 1756-CN2R, 1756-
CNB, or 1756-CNBR bridge module in a remote
chassis, choose the most efficient communication
format for that chassis.
Are most of the modules in the chassis
nondiagnostic, digital I/O modules?
Then choose this communication format for
the remote 1756-CN2 or 1756-CNB module
Yes Rack optimization
No None
The rack optimization format uses an additional eight bytes for each slot in its chassis. Analog
modules or modules that are sending or receiving diagnostic, fuse, timestamp, or schedule data
require direct connections and cannot take advantage of the rack-optimized form. Selecting None
frees up the eight bytes per slot for other uses, such as produced or consumed tags.
Separate the tag into two or more smaller tags. 1. Group the data according to similar update rates. For example, you could create one tag for data
that is critical and another tag for data that is not as critical.
2. Assign a different RPI to each tag.
Create logic to transfer the data in smaller
sections (packets).
For information on how to do this, see the Logix5000 Controllers Common Procedures Programming
Manual, publication 1756-PM001 .
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Rockwell Automation Publication CNET- UM001E-EN-P - June 2016