Rockwell Automation 1784-KTS User Manual download pdf (Page 82)

Publication CNET-UM001C-EN-P - November 2005

5-6 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 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

Combine data that goes to the

same controller.

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.

Guideline Details

Adjustment Description

Increase the requested packet interval (RPI)

of your connections – 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, less connections have to share an update period.

For a ControlNet bridge module, CN2(R) or

CNB(R) in a remote chassis, select the most

efficient communication format for that

chassis:

Are most of the modules in the chassis

non-diagnostic, digital I/O modules?

Then select this communication format

for the remote CN2 or CNB module:

Yes Rack Optimization

No None

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.

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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