Understanding The Network Keeper; Network Update Time (Nut); Requested Packet Interval (Rpi) - Allen-Bradley ControlLogix User Manual

Understanding the Network
Keeper
Every ControlNet network requires at least one module that is able to store
programmed parameters for the network and configure the network with
those parameters upon start-up. This module is called a "keeper" since it
keeps the network configuration. The keeper is configured by running
RSNetWorx for ControlNet software.
Series B 1756-CNB modules (firmware version 2.10) are multi-keepers. On
a multi-keeper network, any CNB module can keep the network at any legal
node address (01 to 99). In a multi-keeper network, the multi-keeper
capable node with the lowest node address becomes the active keeper.
If the active keeper is taken off the network, an alternative keeper can take
over for it and continue to act as keeper. As long as at least one valid
multi-keeper device is present on the network, new scheduled connections
can be established.

Network Update Time (NUT)

The network update time (NUT) is the smallest repetitive time interval in
which data can be sent on the ControlNet network. It represents the fastest
possible update rate for scheduled data transfers on that network. For
example, a network that runs with a 5ms NUT cannot send scheduled data
at a rate faster than 5ms. It can, however, send data at a slower rate.

Requested Packet Interval (RPI)

The RPI is the update rate specified for a particular piece of data on the
network. When you add a module to the I/O configuration of a controller,
you must enter the RPI as a parameter. This value specifies how often to
produce the data for that module. For example, if you specify an RPI of
50ms, it means that every 50ms the I/O module should send its data to the
controller or that the controller should send its data to the I/O module.
When you run RSNetWorx for ControlNet an Actual Packet Interval (API)
will be calculated. The API will be equal to or faster than the RPI.
RPIs are only used for modules that produce data. For example a local CNB
module does not require an RPI because it is not a data-producing member
of the system; it is used only as a bridge to remote racks.
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 NUT.
ControlNet Communication Basics
1756-6.5.3 - December 1999
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