Description
1. At-A-Glance Executive Brief
- Model: 1756-CNB
- Brand: Allen-Bradley (Rockwell Automation)
- Part Type: ControlNet Bridge Communication Module
- Lifecycle Status: Mature / Legacy Support
- System Compatibility: Allen-Bradley ControlLogix PLC Racks (1756 Chassis)
- Core Protocol: ControlNet (Coaxial BNC Media)
2. Technical Introduction
Network obsolescence in legacy manufacturing facilities often creates significant integration roadblocks when older distributed I/O drops fail unexpectedly. The Allen-Bradley 1756-CNB serves as the core networking bridge within ControlLogix chassis architectures, orchestrating deterministic, high-speed data exchanges across ControlNet coaxial backbones. By linking local rack controllers to remote I/O islands and peer-to-peer processors, this module maintains the strict scheduling windows required for time-critical automation processes without demanding a complete system redesign.
3. Core Technical Specifications
| Parameter | Value |
| Communication Rate | 5 Mbps |
| Media Type | RG-6/U Coaxial Cable (BNC Connection) |
| Chassis Current Draw | 1.2 A at 5 VDC; 3 mA at 24 VDC |
| Backlog Buffer | Optimized for scheduled and unscheduled messaging |
| Node Capacity | Up to 99 nodes per ControlNet network segment |
| Isolation Voltage | 30V (continuous), basic insulation type (port to backplane) |
| Slot Location | Any slot in a ControlLogix chassis (consumes 1 slot) |
| Operating Temperature | 0°C to 60°C (32°F to 140°F) |
4. Field Engineer’s Tech Notes
- Node Address Switch Setting: Always verify the rotary node address switches on the front panel match the required network configuration in RSLogix 5000 or Studio 5000 before inserting the module into a live rack. Mismatched node IDs will lock the card out of the scheduled network scan list.
- Coaxial Tap and Cable Attenuation: ControlNet networks are highly sensitive to impedance mismatches. Ensure your BNC drop lines use proper 75-ohm RG-6 cable and verified taps. Excessive signal attenuation or loose terminators will trigger intermittent fault codes and rack drops.
5. Adaptive QA & Testing Protocol
Every pre-owned or New Surplus (NSFP) 1756-CNB module goes through a strict validation procedure on our dedicated ControlLogix test chassis.
- Testing Actions: We power the module up in a multi-chassis test rack to verify backplane power consumption and indicator LED sequencing. Using RSLinx and RSLogix 5000, we test the BNC physical layer by forcing a network schedule, verifying node communication stability at 5 Mbps, and executing explicit messaging read/write cycles to confirm full transceiver health.
- Documentation: Detailed functional check reports and video verification logs are available upon request prior to dispatch to guarantee seamless out-of-box performance.
6. Buyer’s FAQ
- Can I replace a standard 1756-CNB with a 1756-CNBR?The 1756-CNB is a standard single-media module, whereas the CNBR features redundant media ports. If your system relies on dual redundant media cables, you must replace it with a CNBR unless you reconfigure the physical drop.
- What is the warranty coverage for this legacy part?All units include a comprehensive 12-month operational warranty covering hardware function and network communication stability.
- How quickly can an urgent replacement ship for plant shutdowns?We offer same-day shipping from our US warehouse on all in-stock items to minimize downtime during emergency maintenance windows.
- Are firmware revision updates required when swapping this module?While backward compatibility is generally robust across major series revisions, checking your project file’s electronic keying settings (e.g., Compatible Module vs. Exact Match) helps prevent minor revision conflicts during installation.
