Allen-Bradley 1771 Series Modules – 1771-IBN/C Discrete Input Module

Technical Dossier

Product Details And Specifications

Allen-Bradley 1771 Series Technical Notes

The Evolution of 1771 Architecture

The 1771 I/O platform was introduced alongside the original Allen-Bradley PLC-5 controller in the early 1980s, establishing a parallel-bus, slot-addressed I/O architecture that became an industry standard for large-scale discrete and process control. Early revisions (Series A/B) established the physical form factor: single-slot and double-slot modules with a 40-pin backplane connector, housed in 1771-A1B through 1771-A4B chassis configurations ranging from 4 to 16 slots.

Through the 1990s, the platform expanded to include analog I/O (1771-IFE, 1771-OFE), high-speed counter modules (1771-VHSC), and specialty communication adapters (1771-ASB, 1771-DCM). Series lettering (A, B, C, D) on individual modules denotes hardware revisions — firmware updates, component substitutions, or EMC compliance changes — while maintaining backward compatibility within the same chassis slot. The 1771 platform reached end-of-life (EOL) announcement from Rockwell Automation, with the PLC-5 family officially discontinued for new orders, transitioning users toward the ControlLogix (1756) and CompactLogix (1769) platforms. Despite this, the 1771 series remains in active service at thousands of facilities worldwide, driving sustained demand for spare and replacement modules.

1771 Full Catalog & Functionalities (SKU List)

Discrete Input Modules

• 1771-IBN/C: 32-point 24 VDC discrete input module, Series C revision, single-slot, sink/source compatible
• 1771-IB: 16-point 24 VDC DC input module, standard single-slot discrete input
• 1771-IB16: 16-point 24 VDC input, isolated commons, enhanced noise immunity
• 1771-IA: 16-point 120 VAC discrete input module, single-slot
• 1771-IA2: 32-point 120 VAC input, double-slot, high-density AC input
• 1771-IC: 16-point 220 VAC discrete input module, single-slot
• 1771-ID: 16-point 125 VDC discrete input module, isolated, for high-voltage DC systems

Discrete Output Modules

• 1771-OB: 16-point 24 VDC transistor output module, single-slot
• 1771-OB16: 16-point 24 VDC output, isolated commons, sourcing type
• 1771-OA: 16-point 120 VAC triac output module, single-slot
• 1771-OA2: 32-point 120 VAC output, double-slot, high-density AC output
• 1771-OW: 16-point relay contact output module, dry contact, 120/240 VAC or 24 VDC

Analog I/O Modules

• 1771-IFE: 8-channel analog input module, 4–20 mA / 0–10 VDC selectable, 12-bit resolution
• 1771-OFE: 4-channel analog output module, 4–20 mA / 0–10 VDC, 12-bit resolution
• 1771-IFE/B: Enhanced analog input, Series B, improved common-mode rejection

Specialty & Communication Modules

• 1771-ASB: Remote I/O adapter module, connects 1771 chassis to PLC-5 remote I/O network
• 1771-DCM: Direct communication module, DH+ and remote I/O interface
• 1771-VHSC: Very high-speed counter module, up to 1 MHz count rate, quadrature encoder input

Sourcing Hard-to-Find & Obsolete 1771 Series Parts

With the 1771 platform at end-of-life, OEM production of most modules has ceased. Facilities operating PLC-5-based systems face a sourcing gap that cannot be resolved through standard distribution channels. DriveKNMS maintains a dedicated inventory of refurbished, tested, and new-surplus 1771 modules to support lifecycle extension programs for these installations.

Quality Control for the 1771 Range

The 1771 backplane uses a parallel I/O bus architecture with slot-addressed communication. Quality verification for 1771 modules requires testing beyond basic power-on checks. DriveKNMS applies a structured test protocol for all 1771 inventory:

• Backplane connector inspection: 40-pin edge connector examined for pin damage, oxidation, and mechanical integrity
• Functional I/O verification: each discrete input and output point exercised at rated voltage (24 VDC, 120 VAC, or 220 VAC depending on module type) using a dedicated 1771 chassis test fixture
• Analog calibration check: for 1771-IFE and 1771-OFE modules, full-scale linearity and zero-offset verified against NIST-traceable references
• Communication module validation: 1771-ASB and 1771-DCM modules tested for correct remote I/O node addressing and DH+ token-passing behavior
• Thermal burn-in: modules operated at rated load for a minimum of 4 hours prior to shipment to screen latent component failures