Adjust for bandwidth limitations – Rockwell Automation Logix5000 Controllers Produced and Consumed Tags Programming Manual User Manual
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Produce and Consume a Tag Chapter 1
Guideline
Details
Use the same data type for the produced tag and corresponding consumed tag or tags.
To share tags with a PLC-5C controller, use a
user-defined data type.
To
This
Then
Produce
Integers
Create a user-defined data type that contains an array of INTs with an even number of
elements, such as INT[2]. (When you produce INTs, you must produce two or more.)
Only one REAL
value
Use the REAL data type.
More than one
REAL value
Create a user-defined data type that contains an array of REALs.
Consume
Integers
Create a user-defined data type that contains these members.
Data Type
Description
DINT
Status
INT[x], where x is the output size of the data from the PLC-5C
controller. (If you are consuming only one INT, omit x.)
Data produced by a PLC-5C
controller
Limit the size of the tag to 500 bytes.
If you must transfer more than 500 bytes, create logic to transfer the data in packets.
If you produce the tag over a ControlNet network, the tag may need to be less than 500 bytes. See
Use the highest permissible RPI for your
application.
If the controller consumes the tag over a ControlNet network, use a binary multiple of the ControlNet network update time (NUT).
For example, if the NUT is 5 ms, use an RPI of 5, 10, 20, 40 ms, and so forth.
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.)
•
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 data that is critical and another tag for data that is not as critical.
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).
•
Since a ControlNet network can pass only 500 bytes in one NUT, the data
of each connection must be less than 500 bytes to fit into the NUT.
Depending on the size of your system, you may not have enough bandwidth on
your ControlNet network for a tag of 500 bytes. If a tag is too large for your
ControlNet network, make one or more of these adjustments.
Rockwell Automation Publication 1756-PM011F-EN-P - October 2014
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