Allen-Bradley 1756-CN2RXT ControlNet Redundant Bridge Module Series B

Technical Dossier

Product Details And Specifications

Allen-Bradley 1756 ControlLogix Series Technical Notes

The Allen-Bradley 1756 ControlLogix platform is Rockwell Automation's flagship programmable automation controller (PAC) architecture, deployed across the full spectrum of heavy industrial operations — petrochemical refineries, nuclear power generation facilities, offshore oil and gas platforms, pulp and paper mills, and large-scale water treatment infrastructure. The 1756 backplane supports a modular, scalable chassis design that allows engineers to configure systems from single-rack installations to multi-chassis, redundant control architectures spanning thousands of I/O points. The 1756-CN2RXT is a ControlNet communication bridge module with extended temperature rating and built-in redundancy support, purpose-built for mission-critical environments where standard operating temperature ranges and single-path communication are insufficient. Its deployment is common in redundant controller configurations (1756-RM/A, 1756-RM2/A) where deterministic, scheduled ControlNet communication must be maintained across primary and secondary chassis simultaneously.

The Evolution of 1756 ControlLogix Architecture

The ControlLogix platform was introduced in 1997 as a successor to the PLC-5 and SLC 500 families, establishing a common control engine across discrete, process, motion, and safety applications. The original 1756-A4, 1756-A7, 1756-A10, 1756-A13, and 1756-A17 chassis defined the physical backplane standard that remains backward-compatible through the current generation.

Communication architecture within the 1756 ecosystem has evolved through three distinct phases. The first phase (1997–2004) relied on ControlNet (1756-CNB, 1756-CNBR) as the primary deterministic network for I/O and controller-to-controller messaging. The second phase (2004–2012) saw EtherNet/IP (1756-ENBT, 1756-EN2T) emerge as the dominant peer-to-peer and supervisory network, while ControlNet retained its role in time-critical and redundant topologies. The third phase (2012–present) introduced the 1756-EN2TR (ring topology EtherNet/IP) and the 1756-CN2RXT (extended-temperature ControlNet redundancy bridge), addressing both network resilience and environmental extremes.

The 1756-CN2RXT specifically addresses the gap between standard ControlNet bridge modules (1756-CNB Series D, 1756-CNBR Series C) and the requirements of redundant controller systems operating in high-temperature enclosures or outdoor installations. It supports the ControlNet redundancy protocol required by the 1756-SRM (System Redundancy Module) architecture and is compatible with RSNetWorx for ControlNet scheduling software.

1756 ControlLogix Full Catalog & Functionalities (SKU List)

ControlNet Communication Modules

• 1756-CN2RXT: ControlNet redundant bridge, extended temp (-25°C to +70°C), dual-port, Series B
• 1756-CN2R: ControlNet redundant bridge module, standard temp, dual-port coax
• 1756-CN2: ControlNet bridge module, standard temp, single scheduled network
• 1756-CNB: ControlNet bridge module, Series D, legacy scheduled/unscheduled messaging
• 1756-CNBR: ControlNet redundant bridge, Series C, predecessor to CN2R family

EtherNet/IP Communication Modules

• 1756-EN2T: EtherNet/IP bridge, 10/100 Mbps, single port, standard DIN rail mount
• 1756-EN2TR: EtherNet/IP bridge, dual-port, Device Level Ring (DLR) topology support
• 1756-EN2TXT: EtherNet/IP bridge, extended temperature, single port
• 1756-EN3TR: EtherNet/IP bridge, dual-port, gigabit-capable, DLR support
• 1756-ENBT: EtherNet/IP bridge, legacy 10/100 Mbps, Series B, EOL

Controller (CPU) Modules

• 1756-L71: ControlLogix 5570 controller, 2 MB memory, standard chassis mount
• 1756-L73: ControlLogix 5570 controller, 8 MB memory, high-capacity applications
• 1756-L83E: ControlLogix 5580 controller, 20 MB memory, integrated EtherNet/IP port
• 1756-L85E: ControlLogix 5580 controller, 40 MB memory, dual integrated EtherNet/IP ports

Redundancy Modules

• 1756-RM2: ControlLogix redundancy module, Series B, fiber-optic chassis interconnect
• 1756-RM2XT: ControlLogix redundancy module, extended temperature, fiber interconnect

Digital I/O Modules

• 1756-IB16: 16-point 24V DC digital input module, sourcing, standard density
• 1756-OB16E: 16-point 24V DC digital output module, electronically fused, sourcing
• 1756-IA16: 16-point 120V AC digital input module, isolated

Analog I/O Modules

• 1756-IF16: 16-channel analog input module, 4–20 mA / 0–10V, differential
• 1756-OF8: 8-channel analog output module, 4–20 mA / 0–10V configurable

Power Supply Modules

• 1756-PA75R: 75W AC redundant power supply, 120/240V AC input, hot-swap capable
• 1756-PB75R: 75W DC redundant power supply, 18–32V DC input, hot-swap capable

Sourcing Hard-to-Find & Obsolete 1756 ControlLogix Parts

The 1756-CN2RXT and its related redundancy-capable ControlNet modules are stocked specifically for customers maintaining redundant controller architectures in refineries, power generation facilities, and continuous process plants where a planned migration to EtherNet/IP is not operationally feasible within the current maintenance cycle. DriveKNMS can supply individual modules, matched redundant pairs, and complete chassis assemblies pre-configured for drop-in replacement.

All sourced units are accompanied by full traceability documentation, including original manufacturer date codes, firmware revision records where applicable, and test certification reports. Emergency urgent RFQ handling is available for critical process shutdowns.

Quality Control for the 1756 ControlLogix Range

The 1756 ControlLogix backplane operates at a proprietary high-speed serial bus that requires functional validation beyond standard power-on testing. DriveKNMS applies a structured test protocol to all 1756-series modules prior to dispatch:

• Backplane communication verification: Each module is seated in a live 1756 chassis and polled via RSLogix 5000 / Studio 5000 to confirm correct module identity, firmware revision, and backplane slot addressing.
• ControlNet network scheduling test: For CN2RXT and all ControlNet bridge modules, RSNetWorx for ControlNet is used to schedule a live network, verify NUT (Network Update Time) compliance, and confirm scheduled and unscheduled bandwidth allocation.
• Redundancy switchover test: CN2RXT units are tested in a paired redundant chassis configuration to verify seamless switchover behavior under simulated primary controller fault conditions.
• Extended temperature burn-in: XT-rated modules undergo a 48-hour thermal soak at +65°C to validate component stability at rated operating extremes.
• Visual and mechanical inspection: Backplane connector pin integrity, faceplate label legibility, locking tab function, and capacitor condition are inspected under magnification.
• Firmware audit: Firmware revision is recorded and compared against Rockwell Automation's published compatibility matrix for the target controller series (5560, 5570, 5580).