Product Overview
The Allen-Bradley 1756-CNBR/E is a ControlNet Redundant Bridge Module for the ControlLogix platform, designed to provide communication between ControlLogix chassis over ControlNet networks while supporting redundant media for enhanced network reliability. As the redundant-media variant of the standard 1756-CNB bridge, the 1756-CNBR/E features two BNC connectors that allow connection to a second, identical ControlNet link, ensuring that communication continues uninterrupted if one trunkline or tap is severed or disconnected . This module is a critical component in ControlLogix redundancy systems, where it pairs with a matching 1756-CNBR/E in the secondary chassis to maintain seamless communication during controller switchover.
The 1756-CNBR/E belongs to Series E of the ControlNet bridge module family, with earlier Series D variants also available in the field . It operates at the standard ControlNet communication rate of 5 Mbps and supports up to 64 CIP connections, though Rockwell Automation recommends configuring no more than 48 connections per module to maintain optimal performance . Rockwell Automation officially discontinued the 1756-CNBR as of December 31, 2024, with the 1756-CN2R designated as the engineering replacement, making the 1756-CNBR/E a legacy component that remains essential for maintaining existing ControlLogix redundancy installations .
Technical Specifications
| Parameter Name | Parameter Value |
|---|---|
| Product Model | 1756-CNBR/E |
| Manufacturer | Allen-Bradley (Rockwell Automation) |
| Product Type | ControlNet Redundant Bridge Module |
| Communication Rate | 5 Mbps |
| Connectors | 2 × BNC (redundant media), 1 × NAP (RJ-45) |
| Cable Type | Quad shield RG-6 coaxial |
| Maximum Nodes | 99 |
| Maximum CIP Connections | 64 (48 recommended for optimal performance) |
| Backplane Current (5.1V DC) | 970 mA |
| Backplane Current (24V DC) | 1.7 mA |
| Power Dissipation | 5.14 W |
| Thermal Dissipation | 17.5 BTU/hr |
| Isolation Voltage | 50 V continuous, tested to 500 VAC for 60 s |
| Operating Temperature | 0°C to +60°C |
| Status | Discontinued (December 31, 2024) |
Main Features and Advantages
The 1756-CNBR/E‘s primary value lies in its redundant media capability, which eliminates single points of failure in ControlNet network cabling. By connecting both BNC ports to two physically separate ControlNet links, the module ensures that a cable cut, disconnected tap, or damaged trunkline does not interrupt communication between the controller and remote I/O . This redundancy is particularly important in process control applications where a communication loss could trigger unintended shutdowns or compromise safety interlocks.
Within a ControlLogix redundancy system, the 1756-CNBR/E operates in pairs—one module in the primary chassis and one in the secondary chassis. Proper configuration requires that each pair of redundant modules be assigned consecutive ControlNet node addresses, with no other device on the network using those addresses . The module supports the full range of ControlNet functions, including scheduled I/O transfers and unscheduled messaging, with CIP protocol ensuring guaranteed data delivery . For SIL 2 applications, redundant media is not strictly required for the module’s operation, but the 1756-CNBR/E‘s dual-link design provides an additional layer of availability that many process operators specify for critical control loops.
The Series E revision of the 1756-CNBR incorporates firmware enhancements and anomaly fixes accumulated across earlier revisions. Known issues addressed in the Series D firmware lineage include network recovery behavior after major disruptions and media redundancy state handling after RSNetWorx project downloads . The 1756-CNBR/E represents the final and most refined iteration of this module family before discontinuation.
