Allen-Bradley 1771 I/O Chassis Modules – 1771-A4B

Technical Dossier

Product Details And Specifications

Allen-Bradley 1771 Series Technical Notes

The Allen-Bradley 1771 I/O system is one of the most widely deployed modular I/O platforms in global heavy industry. Installed across petrochemical refineries, nuclear power facilities, offshore platforms, and continuous-process chemical plants, the 1771 backplane architecture established the foundational standard for rack-based programmable controller I/O in the 1980s and remained in active production through the early 2000s. The 1771-A4B — a 16-slot I/O chassis — represents the largest capacity variant in the 1771 chassis family, designed for high-density I/O applications where a single rack must accommodate the full complement of analog, digital, and specialty modules required by a process unit.

The Evolution of 1771 Architecture

The 1771 I/O system was introduced by Allen-Bradley in the early 1980s as the I/O subsystem for the PLC-2 and PLC-3 families, later becoming the primary I/O platform for the PLC-5. The architecture uses a passive backplane with a parallel I/O bus; modules are addressed by slot position and rack number, with the processor scanning the I/O table at each program scan cycle.

Early 1771 chassis (A1, A2 series) supported 4 and 8 slots respectively and were designed for single-processor configurations. The introduction of the A3B (12-slot) and A4B (16-slot) variants expanded capacity for complex process applications. The 1771-ASB (Remote I/O Adapter) enabled distributed rack configurations over Allen-Bradley's proprietary Remote I/O (RIO) network, allowing 1771 racks to be located up to 10,000 feet from the host processor.

Compatibility constraints are significant: 1771 modules are not electrically or mechanically interchangeable with the later 1746 (SLC 500) or 1756 (ControlLogix) I/O platforms. Migration from 1771 to 1756 requires either a 1771-to-ControlLogix bridge adapter or a full I/O replacement project. For facilities still operating PLC-5 systems, the 1771 chassis and its associated modules remain the only compatible hardware path.

Allen-Bradley officially discontinued the PLC-5 and 1771 product lines, with end-of-life announcements issued progressively from 2012 through 2022. The platform is now in the obsolescence phase, making third-party spare parts sourcing the primary procurement channel for maintenance operations.

1771 Full Catalog & Functionalities (SKU List)

I/O Chassis

• 1771-A1B: 4-slot I/O chassis, single-density, local rack configuration
• 1771-A2B: 8-slot I/O chassis, standard capacity local or remote rack
• 1771-A3B: 12-slot I/O chassis, mid-capacity for process I/O grouping
• 1771-A4B: 16-slot I/O chassis, maximum capacity, primary SKU for high-density applications

Power Supplies

• 1771-P4S: 120/240 VAC input power supply for 1771 chassis, 5 VDC output
• 1771-P6S: 120/240 VAC input, extended current capacity power supply for full 16-slot chassis
• 1771-P7: 24 VDC input power supply, field-power applications

Digital Input Modules

• 1771-IA: 120 VAC 16-point discrete input module, standard process sensing
• 1771-IB: 24 VDC 16-point discrete input module, low-voltage field devices
• 1771-ID: 125 VDC discrete input module, nuclear and high-reliability applications
• 1771-IG: TTL-compatible discrete input module, encoder and logic-level signals

Digital Output Modules

• 1771-OA: 120 VAC 16-point discrete output module, solenoid and relay actuation
• 1771-OB: 24 VDC 16-point discrete output module, low-voltage actuator control
• 1771-OD: 125 VDC discrete output module, high-reliability DC switching
• 1771-OW: Relay contact output module, isolated dry-contact switching

Analog Modules

• 1771-IF: 4-channel analog input module, 4–20 mA / 0–10 VDC, 12-bit resolution
• 1771-OF: 4-channel analog output module, 4–20 mA / 0–10 VDC, 12-bit resolution
• 1771-IFE: 8-channel analog input module, enhanced resolution, thermocouple/RTD compatible

Communication & Adapter Modules

• 1771-ASB: Remote I/O (RIO) adapter, connects 1771 chassis to PLC-5 over RIO network
• 1771-DCM: Data communication module, DH+ network interface for 1771 rack

Specialty Modules

• 1771-QA: Stepper motor controller module, open-loop positioning
• 1771-QB: Servo positioning module, closed-loop axis control

Quality Control for the 1771 Range

The 1771 backplane uses a parallel bus architecture with slot-addressed I/O communication. Quality verification for 1771 modules requires hardware-level testing against a live 1771 chassis and a compatible PLC-5 processor, as software-only diagnostics cannot validate backplane signal integrity or module addressing logic.

DriveKNMS applies the following test protocol to all 1771 inventory:

• Physical inspection: backplane connector pin condition, module locking tab integrity, faceplate label legibility, and capacitor condition assessment for aged units
• Power-on self-test: module LED status verification against known-good reference states
• Functional I/O test: point-level verification of all discrete and analog channels using calibrated signal sources and loads
• Backplane communication test: block-transfer read/write verification via PLC-5 processor to confirm module addressing and data integrity
• Analog calibration check: for 1771-IF and 1771-OF modules, full-scale linearity and offset verification against NIST-traceable references