Purchase now: Allen-Bradley 1756-CNBR Redundant Module
Purchase now: Atlantechdrives provides you the 1756-CNBR, a highly critical, legacy-generation high-availability ControlNet communication bridge module meticulously engineered by Rockwell Automation for the ControlLogix 1756 platform. In enterprise-level continuous manufacturing—such as sprawling petrochemical refineries, large-scale pharmaceutical batching operations, or massive continuous glass manufacturing—a single network communication failure can result in millions of dollars in catastrophic product loss or severe environmental hazards. The 1756-CNBR completely eliminates the vulnerability of a single point of failure by providing Redundant Media connectivity. Operating at a highly deterministic 5 Mbps, this specialized module features two distinct BNC coaxial ports (Channel A and Channel B). It transmits identical control data packets down both physical network paths simultaneously. If an excavator accidentally severs the Channel A coaxial trunk line in the plant, the receiving field devices instantly and seamlessly accept the identical packet arriving via Channel B without dropping a single connection or interrupting the PID loops. It is the ultimate legacy networking module for achieving highly fault-tolerant communication architectures.
Technical Specifications of the 1756-CNBR
Like its standard counterpart, the 1756-CNBR is bound by specific legacy processing constraints. It delivers physical wire redundancy while maintaining the strict 40-connection limit inherent to the first-generation ControlNet modules.
- Part Number: 1756-CNBR
- Platform: ControlLogix 1756 System
- Network Protocol: ControlNet (CTDMA token-passing)
- Media Redundancy: Dual Channels (Channel A and Channel B)
- Connection Capacity: Supports up to 40 ControlNet (CIP) connections
- Media Type: Standard RG-6 Coaxial Cable
- Connector Type: Dual BNC Connectors (plus an RJ45 Network Access Port)
- Backplane Current Draw: 970 mA at 5.1V DC; 1.7 mA at 24V DC
Expert Tips: True Redundancy and Physical Coaxial Routing
Having a module capable of redundant media is entirely useless if the physical installation inside the factory is flawed. Expert Tip: A shockingly common and dangerous engineering mistake is routing both the Channel A and Channel B coaxial trunk cables through the exact same overhead cable tray or underground PVC conduit. If a catastrophic localized event occurs—such as a crane dropping a heavy steel coil or a severe electrical fire inside the conduit—both cables will be destroyed simultaneously, completely defeating the purpose of the 1756-CNBR. To achieve true, high-availability redundancy, you must strictly ensure that Channel A and Channel B take entirely different physical geographical routes through the facility to reach the remote I/O drops. Furthermore, ensure the total cable length of both paths remains roughly equivalent to prevent signal propagation timing errors.
Maintenance: Diagnosing Media Status LED Indicators
Maintaining a redundant ControlNet network requires vigilant observation from maintenance engineers. The front faceplate of the 1756-CNBR features distinct status LEDs for «Chan A» and «Chan B». Under healthy, fully redundant operation, both lights should alternate rapidly or remain solid green. If one light turns solid red or flashing red, it indicates that one side of the redundant media is broken, disconnected, or suffering from severe noise reflections (often due to a failed 75-ohm physical terminating resistor). The factory machinery will continue to run perfectly on the surviving channel, but technicians must immediately use RSNetWorx to diagnose and repair the broken channel before a second failure crashes the entire line.
Frequently Asked Questions (FAQs)
Q: What is the primary difference between the 1756-CNBR and the 1756-CN2R?
A: The 1756-CNBR is the legacy, first-generation redundant module and is strictly limited to 40 CIP connections. The newer 1756-CN2R is the modern replacement, featuring a vastly superior internal processor capable of managing 100 simultaneous connections without network lag.
Q: Can I use the 1756-CNBR on a non-redundant network by only plugging in Channel A?
A: Yes. The module will function perfectly as a standard, single-media ControlNet bridge if you only connect a BNC cable to Channel A. However, the module will continuously report a «Channel B Media Fault» in the diagnostics, which can trigger nuisance alarms on your HMI.
Do you need more references from this brand? Explore our full catalog of Allen-Bradley here.
To ensure proper mounting and backplane connectivity for this ControlNet module, it should be installed in a 1756-A10 10-slot chassis.
This redundant media bridge is a legacy standard; for modern system upgrades, we recommend checking the 1756-CN2 bridge module.
