1756-CN2R Allen-Bradley ControlLogix ControlNet Bridge Module

Product Overview

The 1756-CN2R is a high-performance ControlNet bridge module belonging to the Allen-Bradley ControlLogix platform. It is specifically designed to facilitate high-speed, deterministic communication in redundant network topologies. Featuring dual BNC ports, the 1756-CN2R allows for a "bumpless" switchover between media cables, ensuring that critical industrial processes remain online even if one network path is compromised.

Supporting up to 99 nodes and a data rate of 5 Mbps, this module is a cornerstone for Distributed I/O architectures, HMI/SCADA connectivity, and peer-to-peer controller interlocking. Its ability to cover up to 1,000 meters per segment (extendable via repeaters) makes it an ideal solution for large-scale manufacturing facilities and process plants that require time-critical data exchange via the Common Industrial Protocol (CIP).

Technical Configuration

The 1756-CN2R is engineered for maximum availability and system flexibility:

• Redundant Media Support: Dual BNC ports provide two independent paths for ControlNet signals, protecting against cable breaks or connector failures.

• High Connection Capacity: Supports up to 128 Logix connections and a total of 131 communication connections, allowing for extensive I/O and messaging tasks.

• Maintenance Port: Includes a front-facing USB 1.1 port (12 Mbps) dedicated to local configuration, programming, and firmware updates (not intended for runtime data communication).

• Robust Environmentals: Designed for a wide operating temperature range (0 to 60 °C) and high vibration resistance (2G at 10-500Hz), suitable for demanding cabinet environments.

• Chassis Versatility: Compatible with all standard 1756 ControlLogix chassis (A4 through A17) and both standard and redundant power supply configurations.

Technical Specifications

Technical FAQs

What cable is required for the 1756-CN2R?

The module requires 1786-RG6 quad-shield RG6 coaxial cable. Proper termination with 75-ohm resistors at the ends of the trunk line is mandatory for network stability.

Can the USB port be used for HMI communication?

No. The USB port is strictly for temporary maintenance tasks, such as configuring the module via RSLinx or flashing firmware. Runtime data communication must occur over the ControlNet BNC ports.

Does this module support "Hot Swapping"?

Yes. Like most 1756 modules, the 1756-CN2R supports Removal and Insertion Under Power (RIUP), allowing you to replace the module without cycling power to the entire chassis.

Engineering & Installation Guide

Redundant Cabling Strategy

To achieve true redundancy, route the two BNC coaxial cables (Channel A and Channel B) through different physical paths or conduits. This minimizes the risk of a single mechanical event (like a forklift accident or fire) severing both communication lines simultaneously.

Network Scheduling

Every ControlNet network containing a 1756-CN2R must be scheduled using RSNetWorx for ControlNet. This software establishes the Network Update Time (NUT) and ensures that deterministic data (Scheduled) and non-critical data (Unscheduled) are prioritized correctly. Failing to "save to the keeper" after making hardware changes will result in I/O communication failures.

Power and Grounding

Ensure the chassis is properly grounded to a low-impedance earth ground. The 1756-CN2R has an isolation voltage of 30V continuous, tested at 500V. Proper grounding is critical for the BNC shields to prevent electromagnetic interference (EMI) from corrupting the 5 Mbps data stream.