Allen-Bradley 1791 Block I/O Modules | AB 1791-OB32

Technical Dossier

Product Details And Specifications

Allen-Bradley 1791 Block I/O Series: Comprehensive Module Range and Technical Overview

The Allen-Bradley 1791 Block I/O series, manufactured by Rockwell Automation, is a discrete I/O platform engineered for direct field-device wiring in high-density industrial environments. Deployed across chemical processing plants, petroleum refineries, nuclear power generation facilities, and continuous-process manufacturing lines, the 1791 series occupies a foundational position in legacy distributed control architectures worldwide. Its rugged block-style housing integrates terminal blocks and I/O circuitry into a single sealed unit, eliminating the need for separate terminal strips and reducing panel footprint. The series supports 24 VDC and 120/240 VAC field wiring, making it compatible with a broad range of sensor and actuator types used in heavy industry. Installed base estimates place the 1791 platform in tens of thousands of active control panels globally, many of which remain in continuous operation under long-term maintenance contracts.

The Evolution of 1791 Architecture

The 1791 Block I/O series was introduced in the mid-1980s as a companion platform to the Allen-Bradley PLC-5 and SLC 500 controller families. Its original design addressed the need for remote I/O expansion without the cost and complexity of rack-based systems. Communication was handled via the Allen-Bradley Remote I/O (RIO) link, a proprietary serial bus operating at 57.6 Kbps, 115.2 Kbps, or 230.4 Kbps, depending on cable length and configuration.

Through the 1990s, Rockwell Automation expanded the 1791 catalog to include AC output variants, combination I/O blocks, and specialty modules for analog interfacing. The series reached peak deployment during this period, coinciding with widespread PLC-5 adoption in process industries.

By the early 2000s, Rockwell introduced the 1794 FLEX I/O platform as the architectural successor, offering modular I/O on a DIN-rail carrier with EtherNet/IP and DeviceNet support. The 1791 series entered a maintenance phase, with Rockwell formally discontinuing new production on most catalog numbers between 2005 and 2015. The current generation replacement is the 5069 Compact I/O, which supports Studio 5000 Logix Designer and high-speed EtherNet/IP communication. However, due to the scale of installed 1791 infrastructure, demand for original and refurbished 1791 modules remains active in the MRO (Maintenance, Repair, and Operations) market.

1791 Full Catalog & Functionalities (SKU List)

Discrete Output Modules

• 1791-OB32: 32-point 24 VDC sourcing output block, 0.5 A per point, RIO communication.
• 1791-OB16: 16-point 24 VDC sourcing output block, 1 A per point, compact form factor.
• 1791-OA16: 16-point 120 VAC triac output block, zero-crossing switching, 0.5 A per point.
• 1791-OW16: 16-point relay output block, Form A contacts, 2 A per point, AC/DC field compatibility.
• 1791-0B8: 8-point 24 VDC sourcing output block, high-current 2 A per point rating.
• 1791-0A8: 8-point 120 VAC output block, triac switching, panel-mount terminal block.

Discrete Input Modules

• 1791-IB32: 32-point 24 VDC sinking input block, 7 mA per point, high-density field wiring.
• 1791-IB16: 16-point 24 VDC sinking input block, standard density, RIO-linked.
• 1791-IA16: 16-point 120 VAC input block, 10 mA per point, optically isolated.
• 1791-16A8: 16-point AC input block, 8-point group isolation, suitable for mixed-voltage panels.

Combination I/O Modules

• 1791-16BC: 16-point combination block, 8 inputs / 8 outputs, 24 VDC, single housing.
• 1791-16AC: 16-point combination block, 8 AC inputs / 8 AC outputs, 120 VAC field wiring.
• 1791-8BC: 8-point combination block, 4 inputs / 4 outputs, 24 VDC, compact installation.
• 1791-24A8: 24-point combination block, 16 inputs / 8 outputs, 120 VAC, high-density terminal.
• 1791-24B8: 24-point combination block, 16 inputs / 8 outputs, 24 VDC, RIO bus interface.

Specialty / Interface Modules

• 1791-IOBX: Expansion block interface adapter, extends I/O capacity on existing 1791 RIO nodes.

Sourcing Hard-to-Find & Obsolete 1791 Parts

The majority of 1791 catalog numbers have been discontinued by Rockwell Automation. Factory lead times for any remaining new-old-stock units are indefinite, and authorized distributor inventory is largely depleted. DriveKNMS maintains a dedicated inventory of tested 1791 modules sourced through controlled MRO channels, including decommissioned plant equipment and verified surplus stock.

For end-users operating PLC-5 or SLC 500 systems with 1791 Block I/O nodes, DriveKNMS provides lifecycle extension support including: direct replacement units for discontinued catalog numbers, cross-reference assistance for identifying functional equivalents within the 1794 FLEX I/O or 5069 Compact I/O families, and long-term supply agreements for facilities under multi-year maintenance contracts. All units are inspected, functionally tested, and shipped with documentation of test results.

Quality Control for the 1791 Range

The 1791 series presents specific technical challenges in refurbishment and testing due to its integrated backplane-and-terminal design. DriveKNMS applies the following test protocol to all 1791 modules prior to shipment:

• RIO Bus Communication Verification: Each module is connected to a live RIO network segment and verified for correct node addressing, data table mapping, and fault-free communication at all three standard baud rates.
• Point-Level I/O Functional Test: Every discrete input and output point is individually exercised using calibrated signal sources and load simulators. Output leakage current and input threshold voltage are measured against Rockwell's published specifications.
• Isolation Resistance Test: Field-side to logic-side isolation is verified at 500 VDC using a calibrated megohmmeter. Minimum acceptable isolation resistance is 10 MΩ.
• Thermal Cycling: Modules are subjected to a 4-hour thermal cycle between 0°C and 60°C to identify latent component failures before shipment.
• Visual and Mechanical Inspection: Terminal block integrity, connector pin condition, housing seal, and label legibility are inspected and documented.