Allen-Bradley ControlNet Compact GuardLogix 1769, User Manual, Page: 17 / 100

ControlNet Overview Chapter 1
Requested Packet Interval (RPI)
The RPI is the update rate specified for a particular piece of data on the network.
By using a rack-optimized connection, the RPI can specify the rate for an entire
rack of I/O. With a direct connection, the RPI can specify the rate for a
particular module or peer-to-peer data. When you add a module to the I/O
configuration of a controller, you must configure the RPI, specifying how often
to produce data for that module. For example, if you specify an RPI of 50 ms,
every 50 ms the I/O module sends its data to the controller or the controller
sends its data to the I/O module.
Set the RPI only as fast as needed by the application. The RPI also determines the
number of packets per second that the module will handle on a connection. Each
module has a limit of how many packets it can handle per second. If you exceed
this limit, the module cannot open any more connections.
A faster RPI consumes more network bandwidth. So, to avoid wasting network
bandwidth, set the RPI only as fast as is necessary. For example, if your
application uses a thermocouple module that has data change every 100 ms, do
not set the RPI for that node at 5 ms because the network bandwidth is used to
transmit mostly old data.
IMPORTANT
You cannot set the RPI to a rate faster than the NUT. The network cannot
send data at a rate that is faster than the NUT.
When you run RSNetWorx for ControlNet software, an Actual Packet Interval
(API) is calculated. The API is equal to or faster than the RPI.
Actual Packet Interval (API)
The API is the actual update rate for a particular piece of data on the network. A
ControlNet network sets this rate equal to or faster than the RPI, based on the
binary multiple of the NUT, which is the next fastest rate at which a module can
send data. If this cannot be done, a ControlNet network provides reports that it
cannot support the configuration.
Rockwell Automation Publication CNET- UM001E-EN-P - June 2016 17