Allen-Bradley 1756-CNBR/B ControlNet Redundant Communication Module

The Allen-Bradley 1756-CNBR/B, also cataloged as the 1756-CNBR ControlNet redundant communication module, operates as a dedicated hardware interface for dual-channel ControlNet data exchange within ControlLogix chassis networks. It provides physical layer execution for redundant A/B ControlNet communication paths, supporting deterministic scheduled and unscheduled message transfer across distributed controller nodes.

Suffix Breakdown & Model Matrix

• 1756: ControlLogix platform chassis family identifier

• CNBR: ControlNet Redundant communication bridge module designation

• /B: Hardware revision indicator (revision-specific implementation level)

No additional functional segmentation beyond revision and platform coding is defined in the provided dataset.

Hardware Specifications

ControlNet Redundant Channel Architecture and Backplane Integration

The module implements dual independent ControlNet communication paths (A/B) for fault-tolerant operation. Channel switching and synchronization are handled at the network interface layer while maintaining backplane-level integration with ControlLogix processors.

Backplane communication is executed through the 1756 chassis bus, with deterministic data exchange timing aligned to ControlNet scheduling cycles. The module participates in system-wide I/O and controller messaging through configured connection instances.

Firmware execution and network stack behavior remain aligned with ControlLogix platform compatibility requirements, including revision-dependent Studio 5000 configuration support.

Frequently Asked Questions (FAQ)

Q: Can the 1756-CNBR/B operate in a single-channel mode if one ControlNet path fails?
A: Yes. The module maintains operation using the remaining ControlNet channel, while fault status is reported at the chassis and network diagnostics level.

Q: Is the module hot-swappable within a running ControlLogix chassis?
A: Yes. The module supports hot replacement, but network re-establishment depends on connection reallocation and scheduled ControlNet updates.

Q: Does the redundant A/B design include full electrical isolation between channels?
A: No. Only basic insulation is implemented; there is no full galvanic isolation between ControlNet A and B paths.

Field Installation Guidelines

The module must be installed in a compatible ControlLogix 1756 chassis slot with backplane power removed or under controlled hot-swap procedure. ControlNet coaxial cabling should maintain impedance consistency and proper termination at both network ends.

Shielding must be grounded at a single defined earth reference point to prevent ground loop currents across redundant channels. Cable routing for A/B networks should be physically separated to reduce correlated failure risk.

USB service access (if used for diagnostics) must be isolated from ControlNet trunk cabling to avoid EMI coupling. Slot configuration must match Studio 5000 project mapping before network activation.