Allen-Bradley 1756-CNBR/E ControlNet Bridge Modules

Technical Dossier

Product Details And Specifications

Allen-Bradley 1756-CNBR Series Technical Notes

The Allen-Bradley 1756-CNBR series is a family of ControlNet bridge communication modules designed for use within the Rockwell Automation ControlLogix platform. These modules serve as the primary interface between the ControlLogix backplane and ControlNet networks — a deterministic, repeatable media protocol widely deployed in process-critical environments including petroleum refineries, chemical processing plants, nuclear power generation facilities, and large-scale water treatment infrastructure.

The Evolution of 1756-CNBR Architecture

The 1756-CNBR series was introduced as part of Rockwell Automation's ControlLogix platform expansion in the late 1990s, when ControlNet was positioned as the preferred deterministic network for I/O and controller-to-controller communication. Early revisions (Series A, B) established the foundational backplane interface and dual-port BNC coaxial connectivity standard.

Series C and D iterations introduced firmware improvements addressing scheduled bandwidth allocation and NUT (Network Update Time) stability under high node-count topologies. Series E — represented by the 1756-CNBR/E — reflects the mature, field-stabilized revision with enhanced diagnostics, improved EMI tolerance, and compatibility with RSNetWorx for ControlNet scheduling software versions 11.x and above.

With Rockwell Automation's strategic shift toward EtherNet/IP as the primary industrial network, the ControlNet platform has entered a maintenance lifecycle phase. New system designs increasingly specify 1756-EN2T or 1756-EN2TR EtherNet/IP modules. However, the 1756-CNBR series remains in active service across legacy installations where ControlNet infrastructure replacement is not economically or operationally feasible. Long-term maintenance support for this series is a defined requirement for facilities operating under 15–25 year asset lifecycle plans.

1756-CNBR Full Catalog & Functionalities (SKU List)

The following SKUs represent the verified product range within the Allen-Bradley 1756-CNBR and associated ControlNet module family for ControlLogix. Each entry reflects a distinct hardware revision or functional variant.

1756-CNBR/A: ControlNet bridge module, Series A, dual BNC coax ports, original platform release.
1756-CNBR/B: Series B revision; firmware update for improved NUT scheduling stability.
1756-CNBR/C: Series C; enhanced scheduled bandwidth management, expanded node support.
1756-CNBR/D: Series D; improved backplane communication throughput and diagnostic LED accuracy.
1756-CNBR/E: Series E (current); mature revision with EMI hardening and RSNetWorx 11.x+ compatibility.
1756-CNB/A: ControlNet bridge (non-redundant media), single BNC port, Series A.
1756-CNB/B: Series B single-port ControlNet bridge; cost-optimized for non-redundant topologies.
1756-CNB/C: Series C single-port; firmware parity with 1756-CNBR/C scheduled messaging.
1756-CNB/D: Series D single-port ControlNet bridge module.
1756-CNB/E: Series E single-port; final production revision for non-redundant ControlNet segments.
1756-CNBR: Base catalog number (revision unspecified); dual-port redundant media ControlNet bridge.
1756-CNB: Base catalog number (revision unspecified); single-port ControlNet bridge for ControlLogix.
1756-RN: ControlNet repeater module; extends ControlNet segment length and node count.
1756-ENET/A: Early EtherNet/IP bridge module; frequently co-deployed with 1756-CNBR in hybrid network architectures.
1784-PCICS: ControlNet interface card for PC-based programming terminals; used with RSNetWorx for network scheduling.
1786-RPA/B: ControlNet coax repeater adapter; passive infrastructure component for 1756-CNBR network segments.
1786-RPFM: ControlNet fiber media repeater; extends ControlNet over fiber in electrically noisy environments.

Sourcing Hard-to-Find & Obsolete 1756-CNBR Parts

The 1756-CNBR series, while no longer in active production expansion, remains a critical spare parts requirement for facilities that cannot migrate to EtherNet/IP within current budget or maintenance cycles. Rockwell Automation's official last-time-buy windows for several CNBR revisions have closed, making independent distributor inventory the primary sourcing channel for replacement units.

Quality Control for the 1756-CNBR Range

The 1756-CNBR module presents specific test challenges due to its dual-function role as both a backplane communication device and an active ControlNet network node. DriveKNMS applies a structured inspection and functional verification protocol to all 1756-CNBR units prior to shipment.

Physical inspection covers BNC connector integrity (center pin condition, dielectric condition, shell continuity), backplane edge connector pin alignment and gold contact condition, and housing integrity for all retention clips and module locking mechanisms. Firmware revision is read and documented via RSLogix 5000 or Studio 5000 module properties prior to any functional test.

Functional verification requires live ControlLogix chassis testing: the module is installed in a verified 1756 chassis with a compatible controller (1756-L6x or 1756-L7x series), powered, and confirmed to achieve LINK status on both ControlNet ports. Scheduled messaging is verified using RSNetWorx for ControlNet with a minimum two-node test topology. Unscheduled messaging (MSG instruction) is tested via controller ladder logic. All diagnostic LED states (NET A, NET B, OK) are confirmed against Rockwell Automation publication 1756-IN572.