Rockwell Automation 1757-SWKIT4000 ProcessLogix R400.0 Installation and Upgrade Guide - Manual (Page 267)

Publication 1757-IN040B-EN-P - March 2002
Performance and Capacity Specifications 11-15
System Communications Limits
Communications limits are derived from various capacities. Common
constraints are:
Node Address Range Limits
Arbitrarily high limits on a network reduce effective bandwidth and
effect recovery time. The maximum number of nodes permitted on
the Supervisory ControlNet (UMAX) is 24. This permits 10 redundant
controllers, 1 redundant pair of servers, plus 2 spare nodes. The
maximum number of nodes permitted on the Supervisory Ethernet is
10. The maximum number of nodes permitted on the I/O ControlNet
(UMAX) is 20. With the exception of planning for the future upgrade
of non-redundant controllers to redundant controllers, unused node
addresses may be utilized by Stations, Application Nodes,
1757-FIM-Only Chassis, or other appropriate nodes.
Connection Limits
Processing power and memory are finite. CNB modules, for example,
are limited to 64 connections.
Node Processing Load Limits
Processing power of nodes of the network is finite. Usually the
“highest” node in the topology processes the most. A downlink CNB
module in a Controller Chassis can process I/O from 24 unscheduled
or 64 scheduled IOMs, requiring multiple CNBs when communicating
with more than 24 unscheduled. The multiple CNBs can be
configured in parallel to a single I/O Net or to physically separate I/O
Nets.
Communications Load Limits
Bandwidth of the network is finite. Some of this bandwidth must be
used for “housekeeping” functions. Message overhead also consumes
bandwidth.
Limits expressed herein are largely derived from the above types of
constraints.