Allen-Bradley 1756-CN2R ControlLogix Redundant ControlNet Bridge Module

The Allen-Bradley 1756-CN2R is a redundant-media ControlNet bridge module for the Rockwell Automation ControlLogix 1756 platform. It installs in any standard ControlLogix chassis slot and bridges the backplane to a ControlNet network, while its two BNC coaxial ports—Channel A and Channel B—carry identical CIP data over two physically separate cable paths. If one trunk is severed, shorted, or terminated incorrectly, the Allen-Bradley 1756-CN2R continues to deliver every scheduled I/O update and every produced/consumed tag over the surviving channel without the controller faulting and without a single I/O point dropping out. That is the defining difference between this module and the single-port 1756-CN2, and it is why the Allen-Bradley 1756-CN2R is specified for continuous-process and high-availability machinery where a cable failure must never become a production stoppage.ControlNet itself is built on the Common Industrial Protocol (CIP) and uses Concurrent Time Domain Multiple Access (CTDMA) to combine an I/O network and a peer-to-peer network in one deterministic, repeatable medium. The Allen-Bradley 1756-CN2R runs that network at a fixed 5 Mbps and supports up to 128 Logix communication connections—three of the module’s 131 total connections are permanently reserved for redundant control, leaving 128 available for standard use. A single ControlNet segment can address up to 99 nodes, so one Allen-Bradley 1756-CN2R can serve a large distributed I/O architecture of remote 1756 racks, Flex I/O drops, drives, HMIs, and peer controllers while still leaving connection headroom for MSG instructions and third-party CIP devices.Configuration and diagnostics are handled through RSNetWorx for ControlNet and Studio 5000 Logix Designer, with a USB 1.1 port on the front face providing local access for firmware updates and on-site diagnostics without a laptop network connection. For redundancy architectures, the Allen-Bradley 1756-CN2R (Series B) is the recommended bridge for Enhanced Redundancy systems built around the 1756-RM module; it is the direct, supported successor to the now-obsolete 1756-CNBR, and it is important to note that Series A versions of the CN2 do not support redundancy at all.

Category: A-B

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Description

Technical Specifications

Parameter	Value
Product Model	1756-CN2R
Manufacturer	Rockwell Automation / Allen-Bradley
Product Type	ControlLogix ControlNet Redundant Media Bridge Module
Network Protocol	ControlNet (CIP / CTDMA)
Data Rate	5 Mbps (fixed)
ControlNet Ports	2 × BNC (Channel A and Channel B)
USB Port	1 × USB 1.1 Series B, 12 Mbps (local config/diagnostics)
Logix Communication Connections	128 (131 total; 3 reserved for redundant control)
Maximum Nodes per Segment	99
Media / Cable	1786-RG6 quad-shield RG6 coaxial
Backplane Current Draw	1.3 A @ 5.1V DC; 3 mA @ 24V DC
Power Dissipation	6.6 W (22.9 BTU/hr)
Isolation Voltage	30V DC continuous; tested at 500V AC for 60 s
Slot Width	1 (any ControlLogix chassis slot)
Chassis Compatibility	1756-A4, 1756-A7, 1756-A10, 1756-A13, 1756-A17
Compatible Power Supplies	1756-PA72, PA75, PB72, PB75, PC75, PH75; redundant PA75R, PB75R, PSCA2
Redundancy Support	Enhanced Redundancy (1756-RM); redundant media
Configuration Software	RSNetWorx for ControlNet v6; Studio 5000 / RSLogix 5000
Supported Features	I/O bridge, produced/consumed tags, MSG instructions, adapter mode, remote I/O
Indicators	Module status, network status, Channel A, Channel B
Weight	0.293 kg (0.64 lb)
Operating Temperature	0°C to 60°C (32°F to 140°F)
Storage Temperature	−40°C to 85°C
Relative Humidity	5% to 95%, non-condensing
Shock / Vibration	30 G operating; 2 G @ 10–500 Hz
Enclosure Rating	Open style (none)
Certifications	UL, CSA, CE, ATEX, IECEx, C-Tick, FM, GOST, KC, RCM, ABS, DNV, GL, LR, TÜV
Warranty	1 year

Main Features and Advantages

True redundant media, not just a second port: The two BNC ports of the Allen-Bradley 1756-CN2R are not an either/or choice—they are a live redundant pair carrying the same traffic simultaneously. A broken trunk, a loose BNC connector, or a missing 75-ohm terminator on one channel is reported as a Channel A or Channel B media fault while the process continues uninterrupted on the other. The module’s status LEDs make the failure visible at the rack, and RSNetWorx pinpoints the offending node or cable segment.Capacity that removes the bottleneck: With 128 available Logix connections, the Allen-Bradley 1756-CN2R offers roughly twice the connection headroom of the legacy 1756-CNBR and CNB family. This matters in real plants: every remote I/O rack, every MSG instruction, and every third-party CIP device consumes a connection, and the older 40–48 connection envelope is easily exceeded once a system grows beyond its original design. The higher capacity of the Allen-Bradley 1756-CN2R is why upgrading an existing CNB/CNBR installation is often the cure for persistent CIP connection-timeout faults.Deterministic control-layer performance: ControlNet’s scheduled-data mechanism gives I/O updates and controller-to-controller interlocking priority over program uploads, downloads, and messaging. The Allen-Bradley 1756-CN2R therefore delivers repeatable scan behavior even while an HMI is being serviced or a program is being transferred—a characteristic that makes it suitable for high-speed interlocking and motion-coordination tasks where jitter cannot be tolerated.One module fits both roles: A single Allen-Bradley 1756-CN2R can operate as a standard single-media bridge if only Channel A is connected, which simplifies spares holding: the plant keeps one part number that covers both redundant and non-redundant applications. It also supports adapter functionality for remote ControlLogix I/O, so the same module can serve as the communication engine for a remote rack.Physical-layer flexibility: ControlNet supports RG-6 coaxial and 200/230 micron hard-clad silica fiber, and the Allen-Bradley 1756-CN2R can use coax and fiber repeaters to extend distance and provide electrical isolation. The 1786-RG6 quad-shield cable is the standard medium, and no routing tables are required—the network is self-configuring once the node addresses and the maximum unscheduled node address are set.