Purchase now: Allen-Bradley 1756-CN2 ControlNet Module
Purchase now: Atlantechdrives provides you the 1756-CN2, a highly robust, advanced ControlNet communication bridge module meticulously engineered by Rockwell Automation for the ControlLogix 1756 programmable automation platform. While EtherNet/IP is the modern standard, thousands of massive industrial facilities—including steel manufacturing plants, continuous chemical refineries, and heavy automotive assembly lines—rely extensively on ControlNet due to its absolute, uncompromising determinism. Operating at 5 Mbps over heavily shielded RG-6 coaxial cables, the 1756-CN2 utilizes a strict Concurrent Time Domain Multiple Access (CTDMA) token-passing protocol. This means that unlike standard Ethernet where data packets can collide and cause micro-delays, the 1756-CN2 guarantees that critical I/O data and interlocking safety messages arrive at their exact destination at a mathematically precise, highly predictable time interval (the Network Update Time, or NUT). This specific module acts as the critical bridge, routing scheduled and unscheduled messages between the high-speed ControlLogix backplane and the sprawling, deterministic ControlNet factory floor network.
Technical Specifications of the 1756-CN2
The 1756-CN2 represents a significant hardware upgrade over the legacy 1756-CNB module, offering vastly increased connection capacities and processing power to handle much larger arrays of remote I/O and Variable Frequency Drives without bottlenecking the network.
- Part Number: 1756-CN2
- Platform: ControlLogix 1756 System
- Network Protocol: ControlNet (CTDMA token-passing)
- Communication Speed: 5 Megabits per second (Mbps)
- Connection Capacity: Supports up to 100 ControlNet connections (CIP)
- Media Type: RG-6 Coaxial Cable (Single Media)
- Connector Type: Single BNC Connector (plus an RJ45 Network Access Port for diagnostics)
- Backplane Current Draw: 1100 mA at 5.1V DC
Expert Tips: Coaxial Taps and Terminating Resistors
ControlNet is notoriously sensitive to physical layer (wiring) imperfections. Expert Tip: Unlike Ethernet switches where you simply plug in a Cat6 cable, the 1756-CN2 connects to the main coaxial trunk line via a specialized device called a «Tap» (e.g., the 1786-TPR). You must never connect the module directly to the trunk cable without a tap. Furthermore, because ControlNet is a linear bus or tree topology, the physical extreme ends of the main trunk cable must be terminated with highly specific 75-ohm terminating resistors (1786-XT). If a resistor is missing, damaged, or of the wrong impedance, the high-frequency 5 Mbps signal will reflect backwards down the copper core, causing massive packet collisions, MAC frame errors, and causing the 1756-CN2 module to drop offline and halt production.
Installation Advice: Scheduling the Network with RSNetWorx
Integrating a 1756-CN2 into a control system is not «plug and play.» After physically installing the module and assigning it a unique MAC ID via the rotary switches on its printed circuit board, you must schedule the network. You must utilize Rockwell’s RSNetWorx for ControlNet software. This software calculates the precise timing algorithms, determines the Network Update Time (NUT), and generates the highly rigid «Keeper» signature. The 1756-CN2 stores this schedule and ensures that all scheduled I/O data takes absolute priority over unscheduled traffic (like HMI read requests or laptop program downloads), guaranteeing that heavy background data never interrupts critical machine control.
Frequently Asked Questions (FAQs)
Q: What is the primary difference between the older 1756-CNB and this 1756-CN2 module?
A: The 1756-CN2 is the modern, high-capacity replacement. While the legacy CNB could only handle 40 ControlNet connections, the CN2 features a vastly superior internal processor capable of managing 100 simultaneous connections, allowing you to control significantly more VFDs and remote I/O chassis from a single module.
Q: Can I use the 1756-CN2 in a ControlLogix Redundancy system?
A: No. The standard 1756-CN2 is designed for single-media, non-redundant architectures. If you are building a high-availability redundant system (using the 1756-RM2 cross-loader), you must explicitly purchase the 1756-CN2R module, which features dual BNC ports for redundant media.
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