Allen-Bradley 1756 ControlLogix Modules – 1756-CN2R/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 heavy industrial sectors including petrochemical refineries, nuclear power generation facilities, offshore oil & gas platforms, pulp and paper mills, and large-scale water treatment infrastructure. Its modular backplane design supports mixed I/O, motion, safety, and communication modules within a single chassis, making it the dominant DCS/PLC hybrid platform in North American and global process industries. Installed base estimates exceed several hundred thousand chassis worldwide, with active deployments spanning systems commissioned from the mid-1990s through current greenfield projects.

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 backplane (1756-A series) that decoupled the controller from I/O and communication modules. The original 1756-L1 and 1756-L20 controllers used a 1756-PA72/PB72 power supply and communicated via ControlNet (1756-CNB) or DeviceNet (1756-DNB).

The architecture evolved through three major generations: Series A/B (1997–2005) established the physical form factor and ControlNet-centric topology; Series C/D (2005–2012) introduced EtherNet/IP as the primary network with the 1756-ENBT and 1756-EN2T modules, alongside expanded controller memory in the L6x family; Series E/F and the L8x generation (2012–present) brought 64-bit processing, SD card-based firmware storage, and the 1756-EN3TR dual-port EtherNet/IP module for Device Level Ring (DLR) topology support. The 1756-CN2R/B specifically belongs to the ControlNet redundant media communication module line, designed for dual-cable ControlNet networks requiring fault-tolerant communication in safety-critical environments. Compatibility constraints exist between Series A backplanes and Series E controllers; firmware version alignment between the controller (L5x vs L8x) and communication modules is mandatory for stable operation.

1756 ControlLogix Full Catalog & Functionalities (SKU List)

Communication Modules — ControlNet

• 1756-CN2/B: Single-port ControlNet communication module, Series B, 5Mbps NUT
• 1756-CN2R/B: Dual-port redundant ControlNet module, Series B, fault-tolerant media
• 1756-CNB/E: ControlNet bridge module, Series E, scanner/adapter dual-mode
• 1756-CNBR/E: Redundant ControlNet bridge, Series E, dual-cable topology

Communication Modules — EtherNet/IP

• 1756-ENBT/A: 10/100 Mbps EtherNet/IP bridge module, Series A, single port
• 1756-EN2T/D: 10/100 Mbps EtherNet/IP module, Series D, 256 CIP connections
• 1756-EN2TR/C: Dual-port EtherNet/IP module, Series C, DLR ring topology support
• 1756-EN3TR/B: Dual-port EtherNet/IP, Series B, 256 TCP connections, DLR
• 1756-EN2TXT/A: Extended-temperature EtherNet/IP module, -25°C to 70°C

CPU / Controller Modules

• 1756-L61/B: ControlLogix L61 controller, 2MB user memory, Series B
• 1756-L63/B: ControlLogix L63 controller, 8MB user memory, Series B
• 1756-L71/B: ControlLogix L71 controller, 2MB user memory, L7x generation
• 1756-L73/B: ControlLogix L73 controller, 8MB user memory, L7x generation
• 1756-L83E/B: ControlLogix L83E controller, 40MB user memory, L8x generation

Digital Input (DI) Modules

• 1756-IB16/A: 16-point 24VDC digital input module, Series A, sinking
• 1756-IB32/A: 32-point 24VDC digital input module, Series A, high-density
• 1756-IA16/A: 16-point 120VAC digital input module, Series A

Digital Output (DO) Modules

• 1756-OB16E/A: 16-point 24VDC electronically fused digital output, Series A
• 1756-OA16/A: 16-point 120/240VAC digital output module, Series A

Analog I/O Modules

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

Power Supply Modules

• 1756-PA72/C: 120/240VAC power supply, 72W output, Series C
• 1756-PB72/C: 18–32VDC power supply, 72W output, Series C

Sourcing Hard-to-Find & Obsolete 1756 ControlLogix Parts

Rockwell Automation has formally discontinued multiple 1756 Series A and Series B modules, including the 1756-CNB/E, 1756-ENBT/A, 1756-L61, and 1756-L63 controller families. End-of-life (EOL) notifications were issued between 2015 and 2022, with last-time-buy windows now closed for most Series A communication modules.

Quality Control for the 1756 ControlLogix Range

The 1756 ControlLogix backplane operates on a proprietary high-speed serial bus at 200Mbps. Communication modules such as the 1756-CN2R/B interface directly with this backplane and require verification of both the module-side connector integrity and the network-side port functionality. DriveKNMS applies the following test protocol to all 1756 communication and controller modules:

• Visual inspection of backplane connector pins for oxidation, mechanical deformation, and solder joint integrity
• Powered bench test using a 1756-A7 or 1756-A13 chassis with a known-good 1756-PA72 power supply
• Firmware version verification and module self-diagnostic LED status confirmation (OK, I/O, NET indicators)
• For ControlNet modules (1756-CN2R/B, 1756-CNB): dual-port loopback test using a ControlNet analyzer to verify NUT scheduling and redundant media switchover
• For EtherNet/IP modules: CIP connection establishment test, ping response verification, and web server access confirmation
• Burn-in cycle: 48-hour continuous operation under load prior to shipment
• Final packaging in anti-static ESD bags with desiccant, per IEC 61340-5-1 handling standards