Rockwell Automation 1756-L6x_L7x ControlLogix System User Manual - Manual (Page 92)

92 Rockwell Automation Publication 1756-UM001O-EN-P - October 2014
Chapter 5 Communication Networks
Connections Over a ControlNet Network
You indirectly determine the number of connections the controller uses by
configuring the controller to communicate with other devices in the system.
Connections are allocations of resources that provide communication between
devices as compared to unconnected messages.
ControlNet Module Connections
The 1756-CNB and 1756-CNBR communication modules support 64 CIP
connections over a ControlNet network. However, for optimal performance,
configure a maximum of 48 connections for each module.
The 1756-CN2, 1756-CN2R, and 1756-CN2RXT communication modules
support 128 connections over a ControlNet network, all of which can be
configured without risk of performance degradation.
DeviceNet Network
Communication The DeviceNet network uses the Common Industrial Protocol (CIP) to provide
the control, configuration, and data collection capabilities for industrial devices.
The DeviceNet network uses the proven Controller Area Network (CAN)
technology, which lowers installation costs and decreases installation time and
costly downtime.
A DeviceNet network provides access to the intelligence present in your devices
by letting you connect devices directly to plant-floor controllers without having
to hard-wire each device into an I/O module.
With a ControlLogix system, DeviceNet communication requires the use of a
1756-DNB DeviceNet communication module.
Table 17 - ControlNet Connections
Connection Definition
Scheduled
(unique to a
ControlNet
network) A scheduled connection is unique to ControlNet communication. A scheduled connection lets you
send and receive data repeatedly at a predetermined interval, which is the requested packet
interval (RPI). For example, a connection to an I/O module is a scheduled connection because you
repeatedly receive data from the module at a specified interval.
Other scheduled connections include connections to the following:
• Communication devices
• Produced/consumed tags
On a ControlNet network, you must use RSNetWorx for ControlNet software to enable all scheduled
connections and establish a network update time (NUT). A scheduled connection reserves network
bandwidth specifically to handle the connection.
Unscheduled An unscheduled connection is a message transfer between devices that the requested packet
interval (RPI) or the program, such as a MSG instruction, triggers. Unscheduled messaging allows
you to send and receive data when you must:
Unscheduled connections use the remainder of network bandwidth after scheduled connections
are allocated.