Allen-Bradley 1756-OB32 ControlLogix 32-Point DC Output Module

Product Overview

The 1756-OB32 is a high-density discrete output module for the Allen-Bradley ControlLogix platform. It provides 32 sourcing outputs in a single-slot module, divided into two isolated groups of 16 points each. Operating within a 12/24V DC range, this module is ideal for driving high-density solenoid valves, relays, and indicator lights in complex automation systems.

Engineered for performance and reliability, the 1756-OB32 features exceptionally fast response times (typically 60 µs from Off to On) and supports Scheduled Outputs, ensuring synchronized operation within the Coordinated System Time (CST) for time-critical manufacturing processes.

Technical Configuration

The 1756-OB32 is designed for high-density control and environmental resilience:

• High Point Density: 32 points per module allow for smaller chassis requirements and reduced cabinet footprint.

• Group Isolation: Outputs are arranged in two groups of 16, providing 125V continuous isolation between groups to protect against cross-talk or different power source issues.

• Robust Diagnostics: Features per-point fault mode states (Hold Last State, On, or Off), allowing for predictable machine behavior during a processor fault or program change.

• Conformal Coating: This module is rated for harsh environments, meeting Severity Level G3 (ISA 71.04) for corrosive atmospheres, making it suitable for wastewater, chemical, or heavy industrial sites.

• Electronic Keying: Software-configurable keying ensures that the correct module type is present in the slot before the system allows operation.

Technical Specifications

Technical FAQs

Does this module have internal fusing?

No. The 1756-OB32 is not internally fused. To protect your outputs from short circuits or over-current conditions, it is highly recommended to use a fused Interface Module (IFM) or external terminal block fuses.

Can I use this module to drive high-inrush loads?

The module supports a surge current of 1 A for 10 ms per point, repeatable every 2 seconds at 60 °C. If your load exceeds this (such as large DC motors), you should use an interposing relay.

What is the "Off-state leakage current"?

The maximum leakage current is 0.5 mA per point. This is important to consider when driving high-impedance loads like small LEDs or sensitive electronic inputs, which might "glow" or stay active even when the output is technically Off.

Engineering & Installation Guide

Thermal Derating

When designing your panel, account for thermal derating. At 50 °C (122 °F), the module can handle 0.5 A per point and 16 A total. However, at 60 °C (140 °F), the capacity drops to 0.35 A per point and 10 A total. Ensure your enclosure has adequate cooling to maintain these limits.

Wiring and Termination

• Terminal Blocks: Requires a 36-pin Removable Terminal Block (RTB), specifically the 1756-TBCH (Cage clamp) or 1756-TBSH (Spring clamp).

• Wire Size: Use 22–14 AWG solid or stranded copper wire rated for at least 105 °C. For double-wire connections, use 22–16 AWG.

• Commoning: Each group of 16 points has its own DC+ (VDC) and DC- (COM) terminals. Ensure both groups are powered if all 32 points are in use.

Software Configuration

In Studio 5000:

1. Fault/Program States: Configure how the outputs should react when the PLC enters "Program Mode" or if a communication "Fault" occurs. The default is "Off," but "Hold Last State" is often used in continuous process applications.

2. Electronic Keying: Set to "Compatible Module" for easier maintenance, or "Exact Match" for high-security applications where only a specific firmware/series is allowed.