Allen-Bradley 1771 Series Modules: 1771-OAD Digital Output Module

Technical Dossier

Product Details And Specifications

Allen-Bradley 1771 Series Technical Notes

The Allen-Bradley 1771 series is the foundational I/O platform for the Rockwell Automation PLC-5 programmable controller family. Deployed across heavy industrial sectors — including petrochemical refineries, nuclear power generation facilities, offshore oil platforms, and continuous-process chemical plants — the 1771 backplane architecture has accumulated decades of installed base globally. Its modular slot-based design allows mixed analog, digital, and specialty modules within a single chassis, making it the reference architecture for legacy DCS migration projects and long-term maintenance programs in facilities where full system replacement is not operationally viable.

The Evolution of 1771 Architecture

The 1771 I/O platform was introduced alongside the PLC-5 processor family in the early 1980s as a replacement for the earlier 1771-series relay-equivalent modules used with PLC-2 and PLC-3 systems. The architecture uses a parallel backplane bus operating at TTL logic levels, with each module slot addressed via a physical keying and addressing mechanism. Early modules (circa 1982–1990) used discrete wire-wrap termination and DIP-switch addressing. Mid-generation modules (1990–2000) introduced removable terminal blocks (RTBs) and improved noise immunity for industrial environments. Late-generation modules (2000–2012) added diagnostics, HART pass-through on analog variants, and improved surge protection.

Rockwell Automation officially announced end-of-life (EOL) for the PLC-5 processor platform in 2015, with last-time-buy periods expiring by 2017 for most catalog numbers. The recommended migration path is to the ControlLogix 1756 platform using the 1771-to-1756 migration bridge (1756-DHRIO or third-party adapters). However, the installed base remains active in facilities where migration capital expenditure has been deferred, creating sustained demand for spare modules, tested replacements, and refurbished units.

Compatibility note: 1771 modules are not electrically or mechanically compatible with 1746 (SLC 500) or 1756 (ControlLogix) I/O systems. Chassis, power supplies, and processors are series-specific.

1771 Full Catalog & Functionalities (SKU List)

Digital Output Modules

• 1771-OAD: 16-point 120V AC digital output module, triac output, 0.5A per point, for PLC-5 chassis
• 1771-OA: 8-point 120V AC digital output module, triac output, standard density
• 1771-OB: 8-point 24V DC digital output module, transistor sink output
• 1771-OBD: 16-point 24V DC digital output module, transistor sink, high-density
• 1771-OW: 8-point relay contact output module, 120/240V AC or 24V DC, isolated contacts
• 1771-OWN: 16-point relay contact output module, normally-open contacts, mixed voltage

Digital Input Modules

• 1771-IA: 8-point 120V AC digital input module, standard density
• 1771-IAD: 16-point 120V AC digital input module, high-density, screw terminal RTB
• 1771-IB: 8-point 24V DC digital input module, sink/source selectable
• 1771-IBD: 16-point 24V DC digital input module, high-density
• 1771-IM: 8-point 220V AC digital input module, for European mains voltage applications

Analog I/O Modules

• 1771-OFE1: 4-channel analog output module, 4–20mA current output, 12-bit resolution
• 1771-OFE2: 4-channel analog output module, ±10V DC voltage output, 12-bit resolution
• 1771-IFE: 8-channel analog input module, 4–20mA or 1–5V DC, 12-bit resolution
• 1771-IFMS: 8-channel analog input module with multiplexed scanning, thermocouple/RTD compatible

Specialty & Communication Modules

• 1771-ASB: Remote I/O adapter module, connects 1771 chassis to PLC-5 via Remote I/O (RIO) link
• 1771-DMC: Dual-axis motion control module, for servo and stepper axis coordination
• 1771-IXHR: High-resolution thermocouple input module, ±0.1°C accuracy, 8 channels

Quality Control for the 1771 Range

The 1771 backplane uses a parallel TTL bus with slot-specific addressing — a design that requires module-level testing under live backplane conditions to validate correct operation. DriveKNMS applies the following test protocol to all 1771 modules prior to dispatch:

• Backplane communication test: Module is seated in a live 1771 chassis with an active PLC-5 processor. I/O image table read/write cycles are verified at rated scan rates.
• Point-level functional test: For digital output modules (including 1771-OAD), each of the 16 output points is individually energized and de-energized. Output voltage, switching time, and leakage current are measured against Rockwell specification 1771-6.5.12.
• Thermal soak: Modules are operated at rated load for a minimum of 2 hours at 50°C ambient to screen for latent component failures.
• RTB and keying inspection: Removable terminal block connector pins are inspected for corrosion, deformation, and contact resistance. Keying bands are verified for correct slot-type encoding.
• Isolation test: For AC output modules, field-side to logic-side isolation is verified at 1500V AC for 1 minute per IEC 61131-2.