Purchase now: Allen-Bradley 1756-CN2R Redundant ControlNet Module
Purchase now: Atlantechdrives provides you the 1756-CN2R, an elite, high-availability ControlNet communication bridge module expertly engineered by Rockwell Automation for the most critical ControlLogix 1756 automation architectures. In enterprise-level continuous manufacturing—such as massive petrochemical cracking plants, pharmaceutical batching, or municipal water treatment facilities—a single network cable failure can cause millions of dollars in catastrophic damage. The 1756-CN2R completely eliminates this single point of failure by providing Redundant Media. Operating at a deterministic 5 Mbps, this module features two distinct BNC coaxial ports (Channel A and Channel B). It transmits identical CIP data packets down both physical network paths simultaneously. If a forklift accidentally severs the Channel A coaxial trunk line, the receiving devices instantly and seamlessly accept the identical packet arriving via Channel B without missing a single microsecond of logic execution. It is the ultimate networking module for achieving SIL (Safety Integrity Level) compliant communication reliability.
Technical Specifications of the 1756-CN2R
The 1756-CN2R not only provides physical cable redundancy but is also fully certified to operate within ControlLogix CPU redundancy architectures (pairing two processors with 1756-RM2 modules), offering a totally fault-tolerant system from the CPU down to the wire.
- Part Number: 1756-CN2R
- Platform: ControlLogix 1756 System
- Network Protocol: ControlNet (CTDMA token-passing)
- Media Redundancy: Dual Channels (Channel A and Channel B)
- Connection Capacity: Supports up to 100 ControlNet connections
- Media Type: RG-6 Coaxial Cable
- Connector Type: Dual BNC Connectors (plus an RJ45 Network Access Port)
- Backplane Current Draw: 1100 mA at 5.1V DC
Expert Tips: True Redundancy and Physical Routing
Having a module capable of redundant media is useless if the physical installation is flawed. Expert Tip: A shockingly common engineering mistake is routing both the Channel A and Channel B coaxial trunk cables through the exact same overhead cable tray or underground conduit. If a catastrophic event occurs—such as a crane dropping a heavy load or a severe electrical fire inside the conduit—both cables will be destroyed simultaneously, completely defeating the purpose of the 1756-CN2R. To achieve true, high-availability redundancy, you must strictly ensure that Channel A and Channel B take entirely different physical geographical routes through the factory to reach the remote I/O drops. Furthermore, you must ensure the total cable length of both paths is roughly equivalent so the high-speed data packets arrive within the acceptable synchronization window.
Maintenance: RSNetWorx and Media Status Indicators
Maintaining a redundant ControlNet network requires vigilant observation of the module’s diagnostic LEDs. The front faceplate of the 1756-CN2R features distinct status lights 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 terminating resistor). The machine will continue to run perfectly on the surviving channel, but maintenance technicians must immediately use RSNetWorx for ControlNet to diagnose the exact node where the break occurred before the second channel also fails.
Frequently Asked Questions (FAQs)
Q: Can I use the 1756-CN2R 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 cable to Channel A. However, the module will continuously report a «Channel B Media Fault» in the diagnostics, which can be annoying for maintenance crews. If you only need one channel, it is more cost-effective to buy the standard 1756-CN2.
Q: Does the 1756-CN2R support modern Studio 5000 Logix Designer software?
A: Absolutely. While ControlNet is an older, highly established protocol, the 1756-CN2R is fully supported in the latest versions of Studio 5000, allowing modern L7 and L8 processors to seamlessly control legacy remote I/O racks.
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