Purchase now: Allen-Bradley 1756-CNB ControlNet Module
Purchase now: Atlantechdrives provides you the 1756-CNB, a highly reliable, legacy ControlNet communication bridge module engineered by Rockwell Automation for the widely installed ControlLogix 1756 programmable automation platform. Although newer facilities lean towards EtherNet/IP, countless established industrial plants—including heavy automotive body shops, legacy steel extrusion lines, and sprawling chemical batching facilities—still rely heavily on the ControlNet architecture due to its absolute, uncompromising determinism. Operating at a strict 5 Megabits per second (Mbps) over heavy-duty shielded RG-6 coaxial cables, the 1756-CNB utilizes the Concurrent Time Domain Multiple Access (CTDMA) token-passing protocol. This highly advanced mathematical protocol guarantees that critical I/O data, Variable Frequency Drive (VFD) speed commands, and interlocking machine safety messages arrive at their precise destinations at an exact, predictable interval known as the Network Update Time (NUT). This specific standard module acts as the crucial gateway, reliably routing scheduled and unscheduled CIP messages between the ultra-fast ControlLogix backplane and the deterministic ControlNet factory floor network.
Technical Specifications of the 1756-CNB
The 1756-CNB is a standard (single-media) communication bridge. It is characterized by its specific capacity limitations, making it ideal for medium-sized networks where massive data bandwidth is not the primary constraint.
- Part Number: 1756-CNB
- Platform: ControlLogix 1756 System
- Network Protocol: ControlNet (CTDMA token-passing)
- Communication Speed: 5 Mbps
- Connection Capacity: Supports up to 40 ControlNet (CIP) connections
- Media Type: Standard RG-6 Coaxial Cable (Quad-shield recommended)
- Connector Type: Single BNC Connector (plus an RJ45 Network Access Port)
- Backplane Current Draw: 970 mA at 5.1V DC; 1.7 mA at 24V DC
Expert Tips: The 40-Connection CIP Limit
Understanding the hardware limitations of legacy equipment is absolutely critical when upgrading or expanding a factory line. Expert Tip: The most significant bottleneck of the legacy 1756-CNB module is its internal connection capacity. It can only support a strict maximum of 40 simultaneous ControlNet (CIP) connections. If you attempt to add too many new remote I/O chassis, intelligent motor starters, or smart VFDs to an existing CNB module, you will instantly max out its processor. The module will reject the new connections, leading to confusing I/O fault triangles in your Studio 5000 software. If your network requires more than 40 connections, you must completely replace the 1756-CNB with the modern, high-capacity 1756-CN2 module, which comfortably supports up to 100 simultaneous CIP connections.
Installation Advice: Node Addressing and Network Scheduling
Integrating a 1756-CNB requires strict adherence to legacy configuration protocols. Before inserting the module into the energized ControlLogix chassis, you must physically assign its unique MAC ID (Node Address) using the two rotary switches located on the side of the printed circuit board. Valid addresses range from 01 to 99. Furthermore, ControlNet is not a «plug-and-play» network like standard unmanaged Ethernet. Once the physical coaxial wiring, taps, and terminating resistors are correctly installed, you must use Rockwell’s RSNetWorx for ControlNet software to calculate the routing schedule, set the Network Update Time, and save the «Keeper» signature to the module, allowing it to govern the token-passing network.
Frequently Asked Questions (FAQs)
Q: What is the RJ45 port on the front of the 1756-CNB used for?
A: It is a common misconception that this is an Ethernet port. It is actually a Network Access Port (NAP). It is used strictly by maintenance technicians to plug a laptop (using a specialized 1786-CP cable) directly into the ControlNet network for localized diagnostics and programming without interrupting the coaxial trunk line.
Q: Can the 1756-CNB be used in a highly available ControlLogix redundant architecture?
A: No. The standard 1756-CNB is specifically designed for single-media, non-redundant systems. If you are designing a high-availability redundant architecture, you must purchase the 1756-CNBR (Redundant Media) module.
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