Allen Bradley 1756-CN2 Communication Module

Introduction

The Allen Bradley 1756-CN2 is a communication bridge module designed for the ControlLogix® system. It serves as a dual-channel interface for the ControlNet network, enabling high-speed deterministic communication between controllers, I/O modules and remote devices. With its capacity for redundant channel operation, the 1756-CN2 is suitable for demanding automation systems where reliability, network performance and system integrity matter.

In a ControlLogix rack, the 1756-CN2 integrates seamlessly with other 1756-series modules and provides the backbone for ControlNet segments, making it a key element in architectures that require real-time deterministic control at the network layer.

Product Parameters

Application Scenarios

The 1756-CN2 is typically employed in automation systems requiring real-time, deterministic communication via ControlNet. Typical scenarios include:

• Redundant ControlNet networks in critical process plants (chemicals, petro-chemical, oil & gas) where uptime is essential.

• Large machine automation installations where multiple controllers, I/O racks and drives communicate over ControlNet segments.

• Integration of legacy ControlNet installations into newer ControlLogix architecture, using the 1756-CN2 as bridge module.

• Distributed I/O systems where remote racks need to be interconnected reliably with minimal latency and robust network design.

• Control systems requiring dual-channel (A/B) ControlNet redundancy for fault tolerance and high availability.

Advantages (Key Strengths)

• Dual Channel Support & Redundancy: The 1756-CN2 supports two independent ControlNet channels (A and B), enabling redundant network paths and improved system reliability.

• High Node & Connection Capacity: With support up to ~99 nodes and up to ~128 Logix connections, it scales to large systems with many devices and I/O points.

• Seamless Integration in ControlLogix: As a part of the 1756 series, it plugs into the standard backplane of ControlLogix chassis, simplifying installation and system design.

• Deterministic Network Performance: Utilizes ControlNet protocol with fixed timing and priority scheduling, beneficial in time-critical control loops.

• Robust Industrial Design: Rated for industrial ambient temperatures, vibration and shock; suitable for panel installation in real-world environments.

• Simplified System Architecture: By using one communication bridge module for dual channels, wiring and system layout can be optimized.

FAQ (Technical Questions)

1. Q: Can the 1756-CN2 be used for non-redundant ControlNet segments?
   A: Yes — although it supports dual channels, one channel may be used, but full redundancy benefits require both channels populated.

2. Q: What is the maximum number of Logix communication connections this module supports?
   A: The technical specification states up to 128 Logix connections.

3. Q: Does the module support ControlNet repeaters or hubs upstream?
   A: Yes — standard ControlNet network design rules apply (repeaters, taps, segment length) and the 1756-CN2 functions as the controller-side interface.

4. Q: What is the slot width of the module in a 1756 rack?
   A: The module occupies one slot width in a standard ControlLogix chassis.

5. Q: Can I use both channels (A and B) concurrently for separate networks?
   A: Yes — you may use separate ControlNet channels for A and B for segregated traffic or redundancy as system design dictates.

6. Q: Is firmware required for compatibility between the 1756-CN2 and controllers?
   A: Yes — ensure that the firmware revision of the 1756-CN2 is compatible with controller firmware and Logix Designer version.

7. Q: What is the maximum number of nodes supported per channel on ControlNet?
   A: The module indicates support up to 99 nodes across the channel, but actual node count depends on the network design and load.

8. Q: What is the approximate power dissipation of the module?
   A: Approximately 6.7 W under typical load conditions.

9. Q: Does the 1756-CN2 support peer-to-peer messaging over ControlNet?
   A: Yes — it supports messaging via Logix connections and ControlNet scheduled data; design should align with network architecture.

10. Q: Are there conformal-coated or extended temperature variants of this module?
    A: Yes — variants such as 1756-CN2RXT and others exist for extended or rugged environments.

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