Adjust For Bandwidth Limitations - Rockwell Automation Allen-Bradley ControlNet 1734-ACNR User Manual

5-6 Produce and Consume Tags (Interlock Controllers)
Guideline
Combine data that goes to the
same controller.
Adjust for Bandwidth
Limitations
Adjustment
Increase the requested packet interval (RPI)
of your connections – Recommended
method
Reduce your network update time (NUT).
For a ControlNet bridge module, CN2(R) or
CNB(R) in a remote chassis, select the most
efficient communication format for that
chassis:
Separate the tag into two or more smaller
tags.
Create logic to transfer the data in smaller
sections (packets).
Publication CNET-UM001C-EN-P - November 2005
Details
If you are producing several tags for the same controller:
• Group the data into one or more user-defined data types. (This uses less connections than
producing each tag separately.)
• Group the data according to similar update intervals. (To conserve network bandwidth, use a
greater RPI for less critical data.)
For example, you could create one tag for critical data and another tag for data that is not as critical.
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 the size of your system, you may not have enough bandwidth
on your ControlNet network for large tags. If a tag is too large for your
ControlNet network, make one or more of the following adjustments.
Table 5.3 Tag Adjustments
Description
At higher RPIs, connections can take turns sending data during an update period.
At a faster NUT, less connections have to share an update period.
Are most of the modules in the chassis
non-diagnostic, digital I/O modules?
Yes
No
The Rack Optimization format uses an additional 8 bytes for each slot in its chassis. Analog
modules or modules that are sending or getting diagnostic, fuse, timestamp, or schedule
data require direct connections and cannot take advantage of the rack optimized form.
Selecting "None" frees up the 8 bytes per slot for other uses, such as produced or
consumed 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.
For information on how to do this, see the Logix5000 Controllers Common Procedures
Programming Manual, publication 1756-PM001.
Then select this communication format
for the remote CN2 or CNB module:
Rack Optimization
None