Allen-Bradley 1756 ControlLogix Modules: 1756-OB32/A Output Module

Technical Dossier

Product Details And Specifications

Allen-Bradley 1756 ControlLogix Series Technical Notes

The Allen-Bradley 1756 ControlLogix platform is one of the most widely deployed programmable automation controller (PAC) architectures in global heavy industry. Installed across petrochemical refineries, nuclear power generation facilities, offshore oil & gas platforms, pulp and paper mills, and large-scale water treatment infrastructure, the 1756 backplane-based system has served as the backbone of process and discrete control since its commercial introduction in the late 1990s. Its modular chassis architecture — supporting 4, 7, 10, and 17-slot configurations — allows engineers to construct highly scalable control systems with deterministic scan times and redundant processor options. The 1756-OB32/A is a 32-point 24V DC sourcing digital output module within this ecosystem, designed for high-density discrete output applications requiring solid-state switching at the field device level.

The Evolution of 1756 ControlLogix Architecture

The ControlLogix platform was introduced by Rockwell Automation in 1997 as a successor to the PLC-5 and SLC 500 families, addressing the growing demand for multi-discipline control (process, motion, discrete, and safety) within a single backplane. The original 1756 series used a proprietary ControlBus backplane operating at 100 Mbps. Early I/O modules (Series A) established the physical form factor and connector standards still in use today.

With the introduction of Series B and C revisions across various module types, Rockwell improved noise immunity, expanded operating temperature ranges, and added diagnostic capabilities such as field-side power loss detection and blown fuse indication. The architecture transitioned from RSLogix 5000 programming software to Studio 5000 Logix Designer beginning with firmware v20, introducing Add-On Instructions (AOIs) and enhanced tag-based programming. Modules from the original Series A generation remain backward-compatible at the backplane level with current 1756-L8x and 1756-L7x processors, though firmware dependencies must be verified for specific feature sets. As the platform enters its mature lifecycle phase, long-term maintenance sourcing for Series A modules — including the 1756-OB32/A — has become a primary concern for asset managers operating legacy installations.

1756 ControlLogix Full Catalog & Functionalities (SKU List)

Digital Output Modules

• 1756-OB32/A: 32-point 24V DC sourcing output module, 0.5A per point, Series A
• 1756-OB32E: 32-point 24V DC sourcing output, electronically fused, diagnostic per point
• 1756-OB16D: 16-point 24V DC sourcing output, individually isolated, diagnostic-grade
• 1756-OB16I: 16-point 24V DC sourcing output, individually isolated per point
• 1756-OA16: 16-point 120/240V AC output module, triac-based solid-state switching
• 1756-OA8: 8-point 120/240V AC output, high-current 2A per point rating
• 1756-OW16I: 16-point relay output module, individually isolated, 120/240V AC/DC

Digital Input Modules

• 1756-IB32: 32-point 24V DC sinking input module, standard density
• 1756-IB16: 16-point 24V DC sinking input, individually isolated
• 1756-IA16: 16-point 120V AC input module, group isolated
• 1756-IB16D: 16-point 24V DC diagnostic input, fault detection per point

Analog I/O Modules

• 1756-IF16: 16-channel analog input, 4–20mA / 0–10V configurable, 16-bit resolution
• 1756-OF8: 8-channel analog output, 4–20mA / 0–10V, 16-bit resolution
• 1756-IF8: 8-channel analog input, high-speed, 16-bit, differential inputs

Controller / CPU Modules

• 1756-L71: ControlLogix 5570 processor, 2MB memory, EtherNet/IP port onboard
• 1756-L73: ControlLogix 5570 processor, 8MB memory, dual EtherNet/IP ports
• 1756-L83E: ControlLogix 5580 processor, 20MB memory, enhanced cybersecurity features

Communication Adapter Modules

• 1756-EN2T: EtherNet/IP communication module, 10/100 Mbps, single port
• 1756-EN2TR: EtherNet/IP module with dual-port DLR (Device Level Ring) topology support
• 1756-DNB: DeviceNet scanner module for field device integration
• 1756-CN2: ControlNet communication module, redundant media support

Power Supply Modules

• 1756-PA72: 120/240V AC input power supply, 10A backplane output
• 1756-PB72: 18–32V DC input power supply, 10A backplane output
• 1756-PA75R: Redundant-rated 120/240V AC power supply for 1756-SRM redundancy systems

Quality Control for the 1756 ControlLogix Range

The 1756 backplane architecture presents specific test requirements that differ from standard I/O verification. Each module interfaces with the chassis backplane via a proprietary high-speed bus; functional verification must therefore include both field-side I/O point testing and backplane communication validation. DriveKNMS applies the following test protocol to all 1756 series modules:

• Backplane communication handshake verification using a reference 1756-L7x or 1756-L8x processor chassis
• Point-by-point output switching test for digital output modules (including the 1756-OB32/A), verifying all 32 output channels under load
• Field-side voltage and current measurement at rated load conditions
• Thermal cycling inspection for capacitor and relay component integrity on aged units
• Firmware revision identification and compatibility cross-reference against current Studio 5000 project requirements
• Visual inspection of backplane connector pins, module locking tabs, and label integrity