Product Overview

The 1756-OB8 is a standard digital DC output module from Rockwell Automation’s Allen‑Bradley ControlLogix® family, providing eight high-current sourcing output channels in a single-slot chassis-based form factor -1-2. Designed for the 1756 ControlLogix platform, the 1756-OB8 accepts 24V DC nominal (10-30V DC range) and delivers up to 2A per point, making it ideal for directly energizing heavy-duty DC loads such as solenoids, motor starters, pilot lights, and hydraulic valves -6-14. This module serves as a robust interface between the controller and field devices, offering reliable discrete output control for demanding industrial automation applications.

The 1756-OB8 installs into any available slot of a ControlLogix chassis (except the left-most slot reserved for the processor or communication adapter) and can operate as either a local I/O component or as part of a distributed I/O system across ControlNet™ or EtherNet/IP™ networks -2-9. The module features two isolated output groups of four channels each, providing flexible wiring options and improved noise immunity -4-6. The 1756-OB8 communicates with the controller via multiple formats including rack optimization, listen-only rack optimization, and scheduled output data -2-6. With a fast 1ms ON transition delay and 2ms OFF transition delay, the 1756-OB8 delivers responsive and predictable performance for a wide range of control tasks -6-11. This module represents a foundational building block in the ControlLogix ecosystem, offering the high-current capacity and configuration flexibility required for complex control architectures.

Technical Specifications

Main Features and Advantages

High-Current Sourcing Outputs: The 1756-OB8 provides eight independent sourcing (current source) output channels, each capable of delivering up to 2A at 60°C, with a total module capacity of 8A -6-14. This high-current capability allows the 1756-OB8 to directly drive large solenoids, motor starters, and other heavy inductive loads without requiring external interposing relays -14. The module’s sourcing architecture is compatible with the majority of standard DC field devices, simplifying system design and reducing component count.

Fast and Predictable Switching Performance: With a maximum ON transition delay of 1ms and OFF transition delay of 2ms, the 1756-OB8 delivers rapid and consistent switching performance suitable for high-speed automation applications -6-11. The 10-30V DC wide operating voltage range ensures reliable operation even with fluctuating field power supplies -2-6. The module’s minimum load current of 2mA per point accommodates low-power loads, while surge current capability of 4A for 10ms per point handles momentary inrush demands -6.

Flexible Communication and Configuration: The 1756-OB8 supports multiple communication formats including rack optimization, listen-only rack optimization, output data, listen-only output data, and scheduled output data, providing flexibility for various system architectures -2-6. The module features configurable fault states and program mode states per point, with options to Hold Last State, On, or Off (default Off), allowing engineers to define safe behavior during controller faults or program mode transitions -6-13. The Output Data Echo feature broadcasts commanded output states to the system, enabling listen-only devices to monitor module status without interrogating the owner-controller -3-5.