Product Overview
The Allen-Bradley 1756-OB8I is a ControlLogix isolated DC output module engineered for reliable discrete control in demanding industrial environments. This module delivers 8 individually isolated outputs, each rated for 2 A, making it ideal for driving solenoids, contactors, and pilot-duty loads. As part of the Rockwell Automation ControlLogix family, the 1756-OB8I integrates seamlessly into existing systems while providing robust electrical isolation to protect your PLC backplane.
Technical Specifications
Every parameter below is sourced directly from the manufacturer's data, ensuring accuracy for your engineering decisions.
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Manufacturer: Rockwell Automation / Allen-Bradley
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Part Number: 1756-OB8I
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Series: ControlLogix
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Module Type: Isolated DC Output Module
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Outputs: 8 individually isolated
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Rated Voltage: 12-24 VDC
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Operating Voltage Range: 10-30 VDC
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Current Draw (5.1 V): 165 mA
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Current Draw (24 V): 2 mA
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Max Power Dissipation: 4.6 W
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Thermal Dissipation: 15.7 BTU/hr
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Backplane Power: 0.89 W
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Isolation Voltage: 50 V continuous
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Removable Terminal Blocks: 1756-TBCH, 1756-TBS6H
The module supports a surge current of 4 A for 10 ms per point (repeatable every second at 140 °F) and offers a maximum on-state voltage drop of 0.5 VDC at 2 A. With a maximum off-state leakage current of 0.5 mA per point, it ensures clean signal integrity.
Real-World Applications
In automotive assembly lines, the 1756-OB8I controls pneumatic valve banks and actuator solenoids, where its isolated outputs prevent ground loop interference. In material handling systems, it switches motor contactors for conveyors, benefiting from the module's high surge capability. For water treatment plants, the module drives pilot lights and alarm indicators, and its fault-mode programmability (On/Off/Hold Last State) ensures safe shutdown during PLC faults.
Installation & Setup Guidance
Before installation, ensure the chassis is properly grounded. The 1756-OB8I requires a 36-pin removable terminal block (RTB). Use only the approved 1756-TBCH or 1756-TBS6H RTBs. Insert the module into an available slot in the ControlLogix chassis and secure the RTB. Wire outputs per your wiring diagram, observing polarity and voltage ratings. Because the module is not fused, Rockwell Automation recommends using a fused IFM to protect output circuits.
Maintenance & Reliability
Routine inspection every 6-12 months is recommended. Check terminal block screws for tightness and inspect for signs of overheating or corrosion. The module's thermal design dissipates up to 15.7 BTU/hr, so ensure adequate ventilation around the chassis. Its isolated outputs reduce wear from electrical noise, extending service life. With proper maintenance, the 1756-OB8I provides years of dependable operation, minimizing costly downtime.
Compatibility & Integration
The 1756-OB8I is designed for the ControlLogix platform and supports multiple communication formats, including Listen-Only Rack Optimization, Listen-Only Output Data, Scheduled Output Data, Output Data, and Rack Optimization. This flexibility allows seamless integration with ControlLogix controllers and remote I/O racks. For configuration, use Rockwell Automation's Studio 5000 environment.
Buying Guide
When purchasing the 1756-OB8I, verify the part number and ensure you have the correct RTB (1756-TBCH or 1756-TBS6H) in your inventory. Confirm your system's backplane power capacity (0.89 W per module). For genuine, factory-tested modules, consider PLC Source PRO as a reliable supplier.