1756-DNB Allen-Bradley ControlLogix DeviceNet Communication Module

Overview

The 1756-DNB Allen-Bradley ControlLogix DeviceNet Scanner Module serves as the bridge between ControlLogix rack architectures and field-level DeviceNet networks. Residing in any standard 1756 chassis slot, this network scanner acts as a master/scanner or slave node, linking controller tag databases with up to 64 DeviceNet field devices (such as motor starters, proximity sensors, pushbuttons, and VFDs). Featuring a front-panel 12 Mbps USB programming port for local configuration and diagnostic access via RSNetWorx for DeviceNet, the unit delivers high-throughput I/O messaging, explicit data transfers, and seamless backplane routing without burdening the host CPU.

Network Architecture & Communications Topology

• Allen-Bradley ControlLogix 1756 Platform
  • DeviceNet Scanner Interface [1756-DNB]: 1-Slot Master/Slave Network Controller
  • Maximum Network Node Capacity: Up to 64 Physical DeviceNet Nodes per Channel
  • Selectable Baud Rates & Distance Limits:
    • 125 Kbps up to 500 Meters (Trunkline Limit)
    • 250 Kbps up to 250 Meters
    • 500 Kbps up to 100 Meters
  • Local Programming Interface: 1 x USB Port (Full Speed 12 Mbps Data Transfer)
  • DeviceNet Bus Voltage Requirement: 11 to 25 Vdc Class 2 External Field Power

Engineering Specifications & Power Parameters

Operational Applications & Field Deployments

• Compatible Field Hardware:
  • PowerFlex Series Variable Frequency Drives (VFDs) equipped with 20-COMM-D network adapters.
  • E300 and E3/E200 Electronic Overload Relays and ArmorStart distributed motor controllers.
  • DeviceNet block I/O blocks (1792D, 1799), photoelectric sensors, limit switches, and valve manifolds.
• Target Industry Deployments:
  • Automotive body shops and high-speed material handling conveyor networks.
  • Packaging lines, bottling facilities, and palletizing cell automation.
  • Mining, aggregate, and bulk material handling systems utilizing long-distance field bus layouts.

Diagnostic & Predictive Monitoring Capabilities

The 1756-DNB Allen-Bradley module features a two-digit numeric LED status display combined with Module/Network (MOD/NET) status indicators. The display outputs alphanumeric error codes (such as Node 00 Fault, Error 72 Bus Off, Error 78 Network Power Lost, or Error 91 Bus Heavy) to accelerate hardware troubleshooting. The scanner firmware actively monitors field trunk voltage levels (detecting drops below 11V DC), measures bus communication error counters, and detects missing slave nodes or baud rate mismatches before communication drops break active control loops.

Global PLC Spare HUB's Insight

During a material handling conveyor upgrade, commissioned field devices suffered intermittent "Error 72" bus-off shutdowns when multiple motor starters energized simultaneously. Maintenance staff suspected a faulty scanner board. Our technical support team evaluated the scanner's internal error logs via RSNetWorx and determined that the DeviceNet 24V DC auxiliary power supply experienced voltage drops below 11V DC at the farthest drop line during inductive load surges. Installing a dedicated 24V trunk power tap near the distant nodes stabilized the network immediately without replacing hardware. Always verify bus termination resistors (121-ohm at both trunk ends) and trunkline voltage drop before condemning scanner electronics.

Warranty, Packaging & Fast Shipping

• Product Supply Condition: 100% Brand New and Original Factory Sealed Hardware
• Warranty Guarantee: Comprehensive 12-Month Quality Warranty from dispatch date
• Order Processing Window: Express handling, dispatching within 24 to 48 hours post-payment
• Global Express Logistics: Fast door-to-door air freight via FedEx, UPS, and DHL networks

Engineering Selection & Operational FAQ

What is the primary cause of an "Error 78" code on the front 2-digit LED display?

Error 78 indicates that 24V DC network power is missing from the 5-pin DeviceNet sealed connector on the front panel. Check the external 24V DC power supply, inline fuses, and cable connections leading to the network tap.

How do I configure the baud rate for the 1756-DNB network scanner during initial commissioning?

The scanner baud rate (125K, 250K, or 500K) is software-configured using RSNetWorx for DeviceNet or Studio 5000 module properties. Ensure Auto-Baud is disabled on critical master nodes to maintain consistent startup speed.

Can the front USB port be used to browse and configure other nodes across the DeviceNet network?

Yes. Connecting a laptop to the module's USB port allows RSLinx to bridge through the scanner backplane straight onto the DeviceNet trunk, enabling online configuration and firmware flashes of downstream field devices.

What physical cable guidelines must be followed when running 500-meter networks at 125 Kbps?

Use thick DeviceNet trunk cable (Class 1) for main runs up to 500 meters. Drop lines to individual nodes must not exceed 6 meters (20 feet) each, with total cumulative drop length kept under 156 meters at 125 Kbps.

How many total bytes of input and output data can a single scanner module handle from its slave scanlist?

The module supports up to 1235 words (2470 bytes) of input data memory and 1235 words (2470 bytes) of output data memory, permitting extensive field I/O mapping within a single ControlLogix project.

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