Allen-Bradley 1771 Series Modules – 1771-ASB I/O Adapter 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 Rockwell Automation's PLC-5 programmable controller family. Deployed across global heavy industry — including petrochemical refineries, nuclear power generation facilities, offshore platforms, and continuous-process chemical plants — the 1771 chassis-based architecture has accumulated decades of installed base across North America, Europe, and Asia-Pacific. Its modular backplane design, standardized 1771 I/O chassis, and deterministic scan-cycle behavior made it the reference architecture for large-scale discrete and process control from the 1980s through the 2000s. Despite the platform's transition to end-of-life status, tens of thousands of 1771-based systems remain in active service, creating sustained global demand for spare parts, replacement modules, and lifecycle extension support.

The Evolution of 1771 Architecture

The 1771 platform was introduced by Allen-Bradley in the early 1980s as a rack-and-slot I/O subsystem designed to interface with the PLC-2 and later the PLC-5 processor families. The architecture is built around a passive backplane chassis (1771-A1B through 1771-A4B series) that accepts up to 16 I/O module slots per rack. Communication between the processor and remote I/O racks is handled via the 1771-ASB Remote I/O Adapter, which connects to the Allen-Bradley Remote I/O (RIO) network — a proprietary token-passing serial link operating at 57.6K, 115.2K, or 230.4K baud.

Over successive hardware generations, Rockwell Automation expanded the 1771 catalog to include analog I/O, specialty function modules (PID, motion, counter), and communication adapters for DeviceNet and DH+. The platform reached peak catalog breadth in the mid-1990s. Rockwell formally announced end-of-life for the PLC-5 and 1771 I/O platform in 2015, with last-time-buy windows closing progressively through 2022. Current installed systems are supported exclusively through third-party spare parts distributors and certified repair facilities. Migration paths include the ControlLogix 1756 platform with 1771 I/O interface bridges (1756-DHRIO, 1771-to-1756 conversion kits) and the CompactLogix 5380 series for smaller footprint replacements.

1771 Series Full Catalog & Functionalities (SKU List)

Remote I/O & Communication Adapters

• 1771-ASB: Remote I/O adapter module; connects 1771 I/O chassis to Allen-Bradley RIO network at up to 230.4K baud.
• 1771-DCM: Data communication module; DH+ network interface for 1771 chassis, supports peer-to-peer messaging.
• 1771-SDN: DeviceNet scanner module; manages up to 64 DeviceNet nodes from within a 1771 chassis slot.
• 1771-KA: PLC communication adapter; bridges 1771 I/O to Data Highway Plus (DH+) network segments.
• 1771-KA2: Enhanced DH+ communication adapter; supports higher node counts and extended cable distances.

Digital Input Modules (DI)

• 1771-IA: 16-point 120VAC discrete input module; standard industrial AC input for field device interfacing.
• 1771-IB: 16-point 24VDC discrete input module; sinking input configuration for DC field devices.
• 1771-IC: 16-point 48VDC discrete input module; for higher-voltage DC input applications.
• 1771-ID: 16-point 125VDC discrete input module; used in substation and high-voltage DC environments.
• 1771-IG: 16-point TTL-level discrete input module; interfaces with 5VDC logic-level field signals.

Digital Output Modules (DO)

• 1771-OA: 16-point 120VAC discrete output module; triac output for AC load switching.
• 1771-OB: 16-point 24VDC discrete output module; sourcing transistor output for DC loads.
• 1771-OBD: 16-point 24VDC sinking discrete output module; for NPN-type field device compatibility.
• 1771-OW: 8-point relay output module; isolated relay contacts rated for mixed AC/DC loads up to 2A per point.
• 1771-OX: 4-point isolated relay output module; heavy-duty contacts for high-current switching applications.

Analog Input/Output Modules (AI/AO)

• 1771-IFE: 8-channel analog input module; 4–20mA or 0–10VDC, 12-bit resolution, for process variable measurement.
• 1771-OFE1: 4-channel analog output module; 4–20mA current output, 12-bit resolution, for control valve and actuator drives.
• 1771-IFE/B: Enhanced 8-channel analog input; improved noise immunity and extended input range options.

Specialty & Function Modules

• 1771-VHSC: Very high-speed counter module; up to 1MHz count rate for encoder and pulse-train applications.
• 1771-QA: Servo positioning module; closed-loop axis control interface for legacy servo drive systems.

Quality Control for the 1771 Series Range

The 1771 backplane uses a passive bus architecture where each module communicates via a shared parallel I/O bus. Quality verification for 1771 modules at DriveKNMS includes: functional power-on testing in a live 1771 chassis with a PLC-5 processor executing a diagnostic ladder program; backplane bus integrity verification using a logic analyzer to confirm correct data, address, and control line behavior; for the 1771-ASB specifically, RIO network communication testing at all three supported baud rates (57.6K, 115.2K, 230.4K) with rack address configuration verification; analog module calibration verification against NIST-traceable reference standards for all AI/AO modules; and visual inspection per IPC-A-610 Class 2 criteria covering solder joint integrity, connector pin condition, and conformal coating status. Modules that fail any test stage are quarantined and not returned to inventory.