Overview
What are ControlNet Modules?
ControlNet modules are specialized communication interfaces designed for the ControlNet network—a high-speed, deterministic fieldbus protocol used primarily in Rockwell Automation environments. Unlike standard Ethernet, ControlNet provides scheduled, repeatable data delivery, making it ideal for time-critical applications where jitters or communication delays could compromise machine safety or process integrity. These modules act as bridges, connecting PLC controllers (like ControlLogix) to distributed I/O, variable frequency drives, and other controllers across long distances using RG-6 coaxial cabling.
Main Manufacturers
Palm Parts Solution specializes in high-availability communication hardware, focusing specifically on:
- Allen-Bradley (Rockwell Automation): The primary developer of the ControlNet protocol. We stock modules for both legacy and current-generation chassis, ensuring compatibility with existing control architectures.
Typical Applications
ControlNet modules are preferred in complex industrial environments that require precise synchronization and high reliability. Common use cases include:
- Redundant Control Systems: Utilizing dual-media (1756-CN2R) for seamless failover in oil and gas or water treatment facilities.
- Distributed I/O: Connecting a central ControlLogix processor to multiple 1794 FLEX I/O or 1797-series racks in hazardous areas.
- Long-Distance Networking: Spanning distances up to 30km (with repeaters) where standard copper Ethernet would fail.
- Peer-to-Peer Interlocking: High-speed data exchange between multiple PLC-5 or ControlLogix processors in synchronized manufacturing lines.
Selection Guide
When sourcing a replacement or expansion module, consider the following technical specifications:
- Media Redundancy: Determine if your network requires a single-channel (CN2) or dual-channel redundant (CN2R) module.
- Capacity and Throughput: Older 1756-CNB modules support fewer connections compared to the 1756-CN2 series, which handles significantly more data and larger I/O configurations.
- Chassis Compatibility: Ensure the module is compatible with your backplane (typically ControlLogix 1756).
- Software Requirements: Verify that your version of RSLogix 5000 / Studio 5000 and RSNetWorX for ControlNet supports the hardware revision you are purchasing.
Replacement & Compatibility
Many facilities are currently transitioning from ControlNet to EtherNet/IP. If you are maintaining a legacy system, direct replacements like the 1756-CN2R are the most cost-effective solution. However, if you are planning a phased migration, you may require bridge modules or gateway devices to link ControlNet segments to an EtherNet/IP backbone. Always check the firmware revision of replacement modules, as newer hardware may require a minor update to your controller's project file.
23 products
Frequently asked questions
What is ControlNet and how does it differ from EtherNet/IP?
ControlNet is a deterministic, high-speed fieldbus network used for real-time I/O control and peer-to-peer messaging. It differs from DeviceNet (which is lower speed for simple devices) and EtherNet/IP (which uses standard Ethernet hardware) by providing a predictable, repeatable delivery of critical control data regardless of network load.
Why is my ControlNet module showing a media fault?
Common causes of the 'Red LED' or 'Media' fault include loose BNC connectors, damaged RG-6 coaxial trunk lines, or incorrect termination. ControlNet requires 75-ohm terminators at both ends of a segment. If migrating to a redundant setup, ensure the 'A' and 'B' channels are not swapped.
Is ControlNet obsolete?
While Rockwell Automation has shifted focus toward EtherNet/IP, ControlNet remains widely used in legacy and high-availability systems. We recommend the 1756-CN2R or 1756-CN2 series for existing racks. For new builds, engineers often select EtherNet/IP modules (like the 1756-EN2T) unless deterministic delivery over long distances is a specific requirement.
How do I configure a new 1756-CN2R module in RSLogix 5000?
To add a ControlNet module, you must use RSNetWorX for ControlNet software to schedule the network. This software calculates the Network Update Time (NUT) and ensures that all scheduled data (I/O) is prioritized over unscheduled data (messaging/HMI).
What is the difference between 1756-CNB and 1756-CNBR?
The 1756-CNB is the standard non-redundant bridge, while the 1756-CNBR supports redundant media (two physical cables). Modern equivalents include the 1756-CN2 and 1756-CN2R, which offer higher capacity for connections and faster processing speeds.