Allen-Bradley 1756-CNBR | Dual BNC Port 5.5W Max Power Dissipation ControlNet Communication Card

Description

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Product Introduction

When a critical distributed automation system loses all ControlNet communication mid-operation and triggers unplanned full process shutdown, the root cause almost always traces back to a non-redundant ControlNet module that cannot maintain stable operation after a single network trunkline or tap segment fault.

Purpose-built for Allen-Bradley ControlLogix PAC systems, this redundant ControlNet bridge module features two independent BNC ControlNet ports that support both simplex and fully redundant media configurations. It delivers standard 5Mbps ControlNet protocol transmission speed, natively supporting both scheduled real-time I/O data exchange and unscheduled messaging operations. The dedicated RJ-45 Network Access Port (NAP) allows direct local programming and diagnostics access without requiring connection to the main ControlNet network. It installs in a single standard 1756 ControlLogix chassis slot, and is fully compatible with all standard and redundant 1756 series power supplies.

1756-CNBR

1756-CNBR

1756-CNBR

1756-CNBR

Key Technical Specifications

Field Application & Technical Pitfalls

This part runs three non-negotiable production-critical roles.
First, it acts as the core redundant ControlNet communication unit for large petrochemical plant distributed control systems, maintaining uninterrupted real-time I/O data exchange across multiple remote ControlLogix racks.
Second, it serves as the network bridge for automotive assembly line high-speed motion control systems, delivering zero-latency scheduled motion command transmission between the PAC and all connected servo drive nodes.
Third, it functions as the direct retrofit replacement for older non-redundant 1756-CNB ControlNet modules, upgrading existing systems to full redundant network protection without requiring major network architecture rework.

‌⚠️ ControlNet Node Address Conflict Risk‌: Every device on the same ControlNet bus must have a unique node address. If two 1756-CNBR modules are assigned the exact same node ID, you will trigger severe network conflict that causes all connected devices to lose communication.
‌⚠️ Coaxial Cable Termination Mismatch‌: The ControlNet bus requires proper 75Ω termination resistors at both physical ends of the trunkline. If you remove the terminator, you will trigger severe signal reflection that causes random communication dropout.
‌⚠️ Backplane Power Budget Overrun‌: The module draws 970mA from the ControlLogix 5.1V DC backplane. If you install too many high-power communication modules on the same chassis, you will exceed the total backplane power budget and cause the entire rack to fault.

I watched a senior network tech skip the node address uniqueness check last summer. He installed this 1756-CNBR module on a ControlNet network, accidentally assigned it the same node ID as an existing PLC, and the entire network crashed immediately. The team spent 2 hours troubleshooting before they found the address conflict issue. That 1-minute oversight forced the production line to stay offline for half a shift.

Quality Control (QC) SOP & Transparency

1. ‌Inbound Verification‌: Cross-check the OEM serial number against Rockwell Automation’s official ControlLogix part revision database, run anti-counterfeit validation on the module’s laser-etched serial label, and confirm no damaged BNC connectors or bent backplane pins.
2. ‌Live Rig Testing‌: Mount the module in a certified ControlLogix test rack, connect it to a fully populated ControlNet test network with 64 simulated nodes, and verify 4 hours of stable continuous 5Mbps communication operation.
3. ‌Parameter Testing‌: Confirm 5Mbps transmission speed stability, verify dual redundant BNC port failover performance completes in under 100ms, and validate all 64 network connections maintain zero packet loss.
4. ‌Final QA & Packaging‌: Seal the fully tested module in a static-shielding anti-static bag, apply a unique QC passed serial label, and pack it in ESD-dissipative foam with a printed hard copy of the full network communication performance test report.

Frequently Asked Questions

• ‌Can I hot-swap this module while the ControlLogix rack is powered on?‌ Yes, it supports certified hot-plug operation. The ControlLogix PAC will automatically maintain all existing ControlNet connections without interrupting real-time I/O data exchange.
• ‌What exact Allen-Bradley systems does this part work with?‌ It is fully compatible with Allen-Bradley ControlLogix PAC platforms. It will not establish native stable communication with older PLC-5 or SLC 500 non-ControlLogix systems.
• ‌How long is the standard warranty for this new surplus unit?‌ It carries a full 12-month functional warranty. If it fails within that window under specified operating conditions, we cover all return shipping and bench replacement costs.
• ‌How do I tell a genuine OEM unit apart from a refurbished clone?‌ Genuine units have the Allen-Bradley hologram sticker on the side of the PCB, and the main ControlNet communication chip carries the official Rockwell Automation part number marking. Refurbished clones usually skip the official hologram and use generic lower-performance communication chips that cannot deliver the rated 5Mbps zero-latency scheduled transmission performance.
• ‌Will this 1756-CNBR directly replace an older 1756-CNB non-redundant ControlNet module?‌ No. Older 1756-CNB variants may have different firmware versions. You will need to re-verify all ControlNet node address and scheduled connection configuration settings in the RSLogix 5000 engineering software after swapping.
• ‌What’s the maximum safe continuous operating temperature at full 64-connection load?‌ It will hold stable network performance up to 60°C, though your mileage may vary if the control cabinet cooling airflow drops below 15 CFM and internal communication circuit temperatures spike 15°C above ambient.