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

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

Within the Rockwell Automation ControlLogix platform, the module maps controller task execution to ControlNet media scheduling, enabling synchronized data propagation between controller nodes and distributed devices over a single deterministic channel.

Hardware Specifications

ControlLogix Backplane Deterministic Communication Behavior

The 1756-CNB/D executes ControlNet communication through deterministic scheduling aligned with ControlLogix controller scan cycles. Data transfer timing is governed by backplane arbitration logic, ensuring consistent frame execution for both scheduled and unscheduled messaging.

The module supports single-channel ControlNet topology without redundant media paths. Network synchronization is maintained through cyclic scheduling, where controller output data and input sampling are aligned to fixed network time slots.

USB service interface operates independently of cyclic ControlNet traffic and is used for configuration and diagnostic-level access without disrupting scheduled communication flows.

Frequently Asked Questions (FAQ)

Q: Does the 1756-CNB/D support hot-swapping in a live ControlLogix system?
A: Yes. The module supports hot insertion and removal. Reinitialization is handled through chassis backplane rescan and controller reconfiguration synchronization.

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

Q: Does the CNB/D provide redundancy across ControlNet channels?
A: No. The CNB/D operates on a single-channel ControlNet architecture without redundant media paths.

Field Installation Guidelines

Install the module into a compatible ControlLogix chassis slot ensuring full engagement of the backplane connector to maintain deterministic timing alignment. ControlNet coaxial cabling must maintain impedance consistency and continuous shielding integrity across trunk and drop segments.

Route ControlNet media away from high-current conductors to reduce electromagnetic coupling effects. Shield grounding must be implemented using a single-point earth reference to prevent ground loop formation in the communication network.

Verify firmware compatibility between controller, chassis, and communication module prior to commissioning to ensure proper backplane synchronization and network scheduling consistency.