Allen-Bradley 1756-CNB/E ControlLogix ControlNet Communication Module

The Allen-Bradley 1756-CNB/E, also cataloged as the 1756-CNB/E ControlNet communication module, operates as a dedicated interface component for deterministic ControlNet frame scheduling between ControlLogix backplane controllers and a single-channel ControlNet network within a ControlLogix chassis architecture. Configured for both scheduled and unscheduled communication handling, the 1756-CNB/E provides direct physical and protocol-level execution of ControlNet data transfer at the network interface layer.

Within the Rockwell Automation ControlLogix system, the module maps controller task execution to ControlNet media timing, enabling synchronized cyclic data exchange between controllers and distributed nodes under a fixed 5 Mbps communication structure.

Hardware Specifications

ControlNet Deterministic Scheduling and Backplane Execution

The 1756-CNB/E executes ControlNet communication through deterministic scheduling integrated into the ControlLogix backplane architecture. Cyclic data transfer is synchronized with controller scan cycles, ensuring predictable timing of input and output updates across the network.

Single-channel ControlNet topology is used without redundant media paths. Network bandwidth allocation is managed via scheduled time slots, where each node is assigned deterministic transmission windows under a fixed 5 Mbps communication rate.

USB service access operates independently of cyclic traffic and is used for configuration and diagnostic-level communication without impacting scheduled data flow.

Frequently Asked Questions (FAQ)

Q: Does the 1756-CNB/E support hot-swapping in a live system?
A: Yes. The module supports hot insertion and removal. Reinitialization occurs through chassis backplane rescan and configuration reload.

Q: What is the maximum number of ControlNet connections?
A: The module supports up to 99 connections, constrained by ControlNet scheduling bandwidth and backplane communication resources.

Q: Does this module support redundant ControlNet paths?
A: No. The CNB/E operates on a single-channel ControlNet architecture without redundancy support.

Field Installation Guidelines

Install the module into any compatible ControlLogix chassis slot ensuring full engagement with the backplane connector. ControlNet coaxial cabling must maintain impedance matching and continuous shielding integrity across trunk and drop segments.

Route communication cables away from high-current conductors to reduce electromagnetic coupling effects. Shield grounding shall be implemented using a single-point earth reference to avoid loop current formation in the network.

Verify firmware compatibility between controller, chassis, and communication module prior to commissioning to ensure correct backplane synchronization and deterministic scheduling behavior.