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

Configured for deterministic ControlNet frame exchange within ControlLogix chassis architectures, the Allen-Bradley 1756-CNB/B (1756-CNB/B ControlNet Communication Module) provides direct physical and protocol-level interfacing between ControlLogix backplane logic and a single-channel ControlNet network operating at scheduled data transmission rates. The module supports up to 99 ControlNet connections with coordinated cyclic and unscheduled traffic handling.

Within the Rockwell Automation ControlLogix communication stack, the module executes backplane-to-network mapping for ControlNet nodes using deterministic scheduling logic managed through the chassis communication subsystem.

Hardware Specifications

ControlLogix Backplane ControlNet Scheduling Architecture

The 1756-CNB/B executes ControlNet communication through a deterministic scheduling engine integrated into the ControlLogix backplane. Cyclic data exchange is synchronized with controller task execution, enabling predictable frame delivery across a single-channel ControlNet segment.

The module supports both scheduled and unscheduled messaging, with network bandwidth allocation managed through ControlNet media access control mechanisms. Unlike redundant CN2R architecture, the CNB/B operates on a single communication path without dual-channel fault tolerance, resulting in a simplified network topology with fixed node addressing up to 99 devices.

USB service access operates independently from cyclic network traffic and is used for configuration and diagnostic-level communication without affecting scheduled data exchanges.

Frequently Asked Questions (FAQ)

Q: Does the 1756-CNB/B support hot-swapping in an active chassis?
A: Yes. The module supports removal and insertion under power. Reinitialization is handled via ControlLogix backplane rescan and module registration.

Q: How many ControlNet nodes can be configured?
A: The module supports up to 99 nodes, constrained by ControlNet scheduling bandwidth and backplane resource allocation.

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

Field Installation Guidelines

Install the module into any available ControlLogix chassis slot ensuring full engagement with the backplane connector to maintain deterministic communication timing. ControlNet coaxial cabling must maintain impedance continuity and proper shielding integrity along trunk and drop segments.

Route ControlNet communication cables separately from high-power conductors to minimize electromagnetic coupling effects. Shield grounding should be implemented using a single-point earth reference to avoid ground loop formation.

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