Description
Product Introduction
The Allen-Bradley 1756-CN2R is a high-performance ControlNet redundant bridge module designed for the ControlLogix chassis family. Functioning as a high-speed communications link across distributed I/O racks, drive systems, and peer-to-peer processor networks, this module routes deterministic control data without placing overhead on the local CPU.
Built specifically for mission-critical applications, the 1756-CN2R features dual BNC RG-6 coaxial connectors that support automatic media redundancy. If a cable break or tap degradation occurs on Channel A, the card seamlessly switches signal traffic to Channel B in sub-millisecond timeframes without triggering rack faults, lost I/O packets, or process trips in the main controller.
Key Technical Specifications
| Parameter | Value |
| Manufacturer | Allen-Bradley / Rockwell Automation |
| Catalog Number | 1756-CN2R |
| Communication Rate | 5 Mbps |
| Media Type | Dual RG-6 Quad-Shield Coaxial (ControlNet BNC) |
| Connection Capacity | Up to 128 Logix Connections |
| Backplane Current (5.1V DC) | 1000 mA |
| Backplane Current (24V DC) | 3.0 mA |
| Power Dissipation | 5.14 W |
| Slot Width | Single-Slot ControlLogix Chassis Mounting |
| Operating Temperature | 0°C to 60°C (32°F to 140°F) |
Application Scenarios & The “Trench” Experience
When a forklift snags a ControlNet coax drop line in a busy assembly plant, standard single-channel communication cards fail instantly, dropping remote racks offline and triggering an emergency shutdown. In a redundant network running a 1756-CN2R, the card silently shifts traffic to the second coax line, keeps the line running smoothly, and throws a minor diagnostic alert so technicians can repair the damaged drop tap during scheduled maintenance.
- Automotive Body Shops & Weld Lines: Distributed I/O Interlocking – Connects central ControlLogix processors to distant remote I/O chassis across harsh electrical noise zones.
- Oil & Gas Pipeline Pumping Stations: Redundant Control Network – Maintains uninterrupted communication loops across ESD (Emergency Shut Down) systems and valve manifolds.
- Water Treatment Facilities: Basin & Pump Sequencing – Routes real-time telemetry across spread-out plant floors using deterministic, scheduled I/O messaging.
Field Case Study: A continuous chemical processing plant suffered frequent comm loss faults due to aging coaxial taps on its main reactor loop. The plant manager was losing $22,000 per downtime event. By replacing legacy 1756-CNB cards with upgraded 1756-CN2R modules, engineers doubled the connection capacity, enabled dual-channel redundancy, and completely eliminated unscheduled network trips.
1756-CN2R
Transparency SOP: Quality Assurance & Testing
We don’t just ship boxes; we test them on actual ControlLogix chassis racks. Every 1756-CN2R bridge module undergoes our strict 5-step QA process before leaving the facility:
- Inbound Physical & BNC Connector Inspection: Thorough optical check of backplane pins, housing clips, thermal sinks, and front BNC coaxial jack threading.
- Chassis Power-On & Boot Test: Insertion into a 1756-A10 rack to verify module initialization, alpha-numeric diagnostic display bootup, and self-test routines.
- ControlNet Node Assignment & RSNetWorx Polling: Assigning node IDs via front rotary switches and verifying scheduled/unscheduled network traffic over Studio 5000 / RSNetWorx.
- Channel A/B Failover Testing: Active dual-cable communication test where Channel A is physically disconnected under full load to verify instant, hitless failover to Channel B.
- ESD Chemical Wash & Anti-Static Packaging: Ultrasonic cleaning of gold edge contacts followed by vacuum sealing in conductive ESD shielding bags with fresh desiccant.
The Veteran’s Tech Trap Guide
- ⚠️ Match the Rotary Node Switches Before Installation: Always set the physical 2-digit rotary switches on the side of the 1756-CN2R to match your original module’s node address BEFORE sliding it into the rack. Changing switches while the card is powered will not update the node address until the chassis power is cycled!
- ❗ Verify ControlNet Network Keep-Alive (NUT) Settings: When replacing an older 1756-CNB card with a higher-capacity 1756-CN2R, ensure your Network Update Time (NUT) and RPI parameters in RSNetWorx match the original system schedule. Re-reserving the network keeper is required to lock in the new hardware configuration.
- ⚠️ Watch Out for BNC Tap Tightening: Never use pliers to tighten RG-6 coaxial BNC connectors onto the front panel. Over-torquing snaps the internal solder pads off the 1756-CN2R main board, causing permanent, intermittent signal dropouts.
Frequently Asked Questions
Q: What is the main difference between the 1756-CNB and the 1756-CN2R?
A: The 1756-CN2R is a second-generation, high-capacity ControlNet module. It offers double the connection capacity (128 connections vs 64 on the CNB), a faster onboard processor, and improved diagnostics, while retaining full dual-media redundancy.
Q: Is the 1756-CN2R hot-swappable (RIUP supported)?
A: Yes. ControlLogix chassis backplanes support Removal and Insertion Under Power (RIUP). You can safely pull and replace a 1756-CN2R module while the rest of the rack remains energized, provided the area is non-hazardous.
Q: Do I need to re-run RSNetWorx for ControlNet after replacing a failed card?
A: If you are replacing a failed 1756-CN2R with an identical 1756-CN2R set to the exact same node address and firmware revision, you do not need to re-schedule the network. If upgrading from a 1756-CNB, you must update the Studio 5000 I/O tree and re-save the ControlNet schedule.
Q: What condition are these units in?
A: Our inventory includes both factory-sealed New Surplus modules and fully tested Pre-Owned units sourced from clean industrial reserves. Every unit undergoes live rack validation before shipment.
Q: What warranty comes with this module?
A: Every 1756-CN2R module comes backed by our full 12-month replacement warranty. If a hardware fault occurs within one year under normal operating conditions, we send a replacement right away.
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