ControlLogix Communication Module 1756CNB 1756 CNB 1756CNB

Allen-Bradley 1756-CNB | ControlLogix ControlNet Bridge Module — Single Channel, 24VDC

Part Number: 1756-CNB

Manufacturer: Rockwell Automation / Allen-Bradley (USA)

Product Type: ControlNet Interface / Bridge Module

Platform: ControlLogix (1756 chassis)

Network: ControlNet — single channel

Supply Voltage: 24V DC

Bridging ControlLogix to ControlNet

The 1756-CNB is the ControlNet bridge module for the Allen-Bradley ControlLogix platform. It occupies a slot in the 1756 chassis and connects the ControlLogix backplane to the ControlNet industrial network — acting as the scanner that manages all ControlNet I/O and peer communication for the Logix controller in that chassis.

ControlNet is Rockwell Automation's deterministic industrial network, designed for motion control, coordinated I/O updates, and interlocking between PLCs and drives. It guarantees a defined update rate for cyclic data — critical in applications where I/O must be synchronised precisely with motion or where multiple PLCs must exchange data with deterministic timing. 

The 1756-CNB provides the physical ControlNet connection (BNC coaxial connectors, or optional fibre via adapter) and handles the NUT (Network Update Time) scheduling that ControlNet requires.

The single-channel CNB provides one coaxial media connection. In applications requiring redundant media (where a second cable path provides fault tolerance), the later 1756-CN2 module provides dual independent channels. For non-redundant installations, the 1756-CNB remains the standard bridge module.

ControlNet vs EtherNet/IP

Many newer installations use EtherNet/IP rather than ControlNet for controller-to-I/O communication. ControlNet's specific advantage — deterministic, scheduled NUT — remains relevant in existing installations where motion-coordinated I/O, drive-to-PLC interlocking, or multi-PLC synchronisation depend on the scheduled network.

ControlNet infrastructure (cable, repeaters, taps) is not easily substituted with Ethernet hardware without application redesign. The 1756-CNB remains the correct bridge module for any ControlLogix system connected to an existing ControlNet network.

Key Specifications

FAQ

Q1: The ControlLogix controller shows a major fault related to the 1756-CNB and all ControlNet I/O has gone offline. What should be checked first?

Remove and re-seat the 1756-CNB in its slot — module seating issues cause loss of backplane communication identical to a module fault. If the fault persists after re-seating, check the ControlNet cable connection at the module's BNC port for loose taps or damaged cable at the nearest node. A single cable fault can take down all nodes on that ControlNet network segment.

Q2: The 1756-CNB is discontinued. Can the 1756-CN2 directly replace it?

The 1756-CN2 is the designated successor and is a drop-in physical replacement for the 1756-CNB in the same 1756 chassis slot. The CN2 adds a second ControlNet channel for redundant media. In RSLogix 5000 / Studio 5000, the module type in the I/O tree must be changed from CNB to CN2 to match the installed hardware. I/O tags and connection paths remain unchanged.

Q3: How many ControlNet nodes can the 1756-CNB support?

ControlNet supports up to 99 nodes per network segment. The 1756-CNB as scanner can manage the I/O connections and peer connections that the Logix controller's connection budget allows — typically hundreds of connections total across all modules in the chassis. The practical limit on a single ControlNet segment is determined by the NUT and the number of scheduled connections, not the CNB module itself.

Q4: The ControlNet NUT (Network Update Time) alarm is appearing intermittently. The network has not changed. Could the 1756-CNB be failing?

Intermittent NUT alarms on an unchanged network can indicate the 1756-CNB's internal ControlNet processor is experiencing timing errors — a sign of hardware aging. Before replacing the module, verify the ControlNet cable topology (all taps securely connected, no unterminated cable ends, proper termination resistors at both ends of the trunk). If cable checks out and the alarm persists, the CNB has failed.

Q5: Where is the discontinued 1756-CNB sourced?

Specialist ControlLogix spare parts dealers and Rockwell Automation aftermarket suppliers hold exchange stock. Request a copy of the test data confirming the module's ControlNet port was verified on a live ControlNet network before shipment. Untested exchange modules occasionally have failed ControlNet ASICs that are not detectable from a simple power-on check.