Overview
The Allen-Bradley 1756-CNBR is a ControlLogix communication bridge module that provides redundant ControlNet connectivity for distributed automation architectures. Unlike the non-redundant 1756-CNB, the 1756-CNBR includes two ControlNet BNC ports enabling redundant media operation, protecting against network disruption when tap or trunkline segments are disconnected. This module is essential for mission-critical applications requiring uninterrupted ControlNet communication across multiple I/O racks and devices.
Key Features
- Dual ControlNet Ports: Two BNC connectors for simplex and redundant ControlNet network configurations, plus one RJ-45 Network Access Port (NAP) for programming and diagnostics.
- 5 Mbps Communication Rate: Standard ControlNet protocol transmission speed supporting scheduled and unscheduled messaging.
- High Connection Capacity: Supports 40–48 Logix logical connections and up to 64 total bidirectional connections with other ControlNet devices.
- 99 Node Support: Accommodates large distributed I/O networks across multiple physical locations.
- Low Power Consumption: 970 mA @ 5.1 VDC and 1.7 mA @ 24 VDC, minimal heat dissipation (5.1 W).
- Multiple ControlLogix Rack Compatibility: Works as a local communication interface or primary communication module in distributed racks.
Technical Specifications
- Module Type: ControlLogix Communication Bridge (Redundant)
- Communication Protocol: ControlNet
- Communication Rate: 5 Mbps
- Logical Connections: 40–48
- Total Bidirectional Connections: Up to 64
- Maximum Nodes: 99
- ControlNet Ports: 2 BNC connectors (redundant media capable)
- Network Access Port: 1 RJ-45 8-pin shielded connector
- Trunk Cable Type: RG-6 coaxial (4-layer shielding)
- Current Draw (5.1 VDC): 970 mA
- Current Draw (24 VDC): 1.7 mA
- Power Dissipation: 5.1 W (17.4 BTU/hour)
- Operating Temperature: 0 to 60°C
- Mounting: ControlLogix Chassis
- Certifications: CE, CSA, CUL, KC, UL, MARINE, NRTL, RCM, ATEX, IECEx, China CCC
Typical Applications
The 1756-CNBR is deployed in distributed ControlLogix architectures where network reliability is critical:
- Multi-rack I/O distribution across facility floors or buildings
- Redundant ControlNet networks requiring automatic failover capability
- Large-scale manufacturing plants with 99+ networked devices
- Production lines requiring unscheduled messaging in parallel with scheduled data
- Applications where tap or trunkline disconnection would cause downtime
Compatibility & Replacements
The 1756-CNBR is compatible with ControlLogix racks including the 1756-A13, 1756-A4, 1756-A17, 1756-A7, and 1756-A10. It functions as a local communication interface in the main controller chassis or as the primary communication module in distributed I/O racks. The module bridges both scheduled and unscheduled ControlNet traffic with the ControlLogix backplane using the Logix5550 controller architecture.
End-of-Life & Replacement: The 1756-CNBR has been discontinued by Rockwell Automation effective December 31, 2024, with the engineering replacement being the 1756-CN2R. However, the 1756-CNBR remains widely available in the secondary market for legacy system support and upgrades.
