Allen-Bradley 1771-IJ High-Speed Encoder/Counter Module

The Allen-Bradley 1771-IJ is a high-speed encoder/counter module for the PLC-5 programmable logic controller system. It accepts TTL (5V) encoder signals and provides two independent 32-bit counter channels. Each channel can count up to 250 kHz, making it suitable for high-speed position feedback from incremental encoders. The module can also function as a frequency measurement device. It mounts in a 1771 I/O rack and communicates with the PLC-5 processor via the backplane. The 1771-IJ is commonly used in motion control applications such as cut-to-length, web registration, and high-speed packaging.

Technical Specifications

Number of channels: 2 independent counters. Input type: TTL (5V differential or single-ended). Maximum count frequency: 250 kHz (channel A and B combined). Count resolution: 32-bit (0 to 4,294,967,295). Counter modes: quadrature, pulse/direction, up/down. Encoder inputs: A, B, Z (index) for each channel. Gating inputs: for start/stop counting. Outputs: 2 discrete outputs (24V DC, 0.5 A) per channel for preset or limit detection. Power supply: 5V DC from backplane (300 mA), plus external 24V DC for outputs. Dimensions: single-slot 1771 module. Operating temperature: 0°C to 60°C. Storage: -40°C to 85°C. The module includes LED indicators for power, count activity, and outputs.

Key Features

• 2 independent 32-bit counter channels, 250 kHz max

• Accepts TTL (5V) differential or single-ended encoder signals

• Quadrature (×1, ×2, ×4) or pulse/direction modes

• Index (Z) pulse capture for homing

• Discrete outputs for preset or limit comparison

• Configurable as frequency meter

• Compatible with PLC-5 processors (1771 rack)

Applications

The 1771-IJ is used for high-speed counting and position feedback in applications such as flying shears, print registration, cut-to-length control, and conveyor tracking. It can interface with TTL incremental encoders directly.

Configuration and Programming

The module is configured using the PLC-5 programming software (RSLogix 5). The user selects the counter mode, scaling, and preset values. The module's status and count values are read as 32-bit integers in the PLC's input data table. Outputs are controlled by ladder logic.

Installation and Wiring

Insert the module into any slot of a 1771 I/O rack (except slots used for processor). Connect encoder signals to the 36-pin Berg connector (pins A+, A-, B+, B-, Z+, Z-). For single-ended, connect the negative to ground. Connect the external 24V DC supply for outputs. The module includes removable terminal blocks (TB) for outputs. Use shielded twisted-pair cables for encoder wiring.

Maintenance

No routine maintenance. Check for loose connections. The module has no field-replaceable fuses. Replace the module if faulty.

Conclusion

The Allen-Bradley 1771-IJ high-speed counter module provides precise position feedback and frequency measurement for PLC-5 systems. Its dual 250 kHz channels and flexible counting modes make it a valuable component for legacy motion control applications.