Overview
The Allen-Bradley 1771-VHSC Series B Very High Speed Counter Module is an intelligent block transfer I/O module engineered for high-frequency pulse processing in industrial automation. This module integrates seamlessly with Allen-Bradley PLC-5 systems to deliver fast, accurate counting and event detection. The 1771-VHSC supports 4 independent input channels configured for encoder mode, counter mode, period/rate mode, and continuous/rate mode, making it ideal for precision applications such as totalizing, motion control, and encoder integration.
Key Features
- 4 Independent Input Channels — Configurable for multiple counting modes (encoder, counter, period/rate, continuous/rate)
- High-Speed Counting — Up to 1 MHz in single-channel counter mode, 250 kHz in quadrature (encoder) mode, and 500 kHz in period/rate or continuous/rate modes
- 8 Digital Output Channels — Current-sourcing outputs operating at 5–24V DC, isolated in groups of two, capable of sourcing 2 amps DC per output
- User-Selectable Input Voltage — Accepts 5V DC or 12–24V DC input signals, providing flexibility for diverse sensor and device integration
- Ultra-Fast Update Period — 100-microsecond module update period for real-time responsiveness
- LED Diagnostics — Status indicators for power, fault, and input/output activity enable quick troubleshooting
Technical Specifications
| Part Number | 1771-VHSC Series B |
| Module Type | Very High-Speed Counter (VHSC) |
| Input Channels | 4 independent channels |
| Maximum Counting Speed | 1 MHz (single-channel); 250 kHz (quadrature/encoder); 500 kHz (period/rate and continuous/rate) |
| Input Voltage | User-selectable 5V DC or 12–24V DC |
| Output Channels | 8 digital outputs (5–24V DC current-sourcing) |
| Output Current Capability | 2 amps DC per output |
| Output Isolation | Groups of 2 |
| Module Update Period | 100 microseconds |
| Backplane Current Draw | 650 mA at 5V DC |
| Wiring Arm Compatibility | 1771-WN wiring arm |
| Signal Types | TTL and RS-422 differential compatible |
| Communication Protocol | Block Transfer (BTR/BTW instructions) |
Typical Applications
- High-speed encoder monitoring and quadrature counting for motion control systems
- Pulse accumulation and frequency measurement from sensors and pulse generators
- Mechanical and proximity switch event counting and totalization
- Real-time monitoring of conveyor speeds, fan RPM, or pump flow rates
- Limit switch detection and interrupt-driven event response
- Distributed control via Remote I/O (RIO) networks in multi-chassis PLC-5 systems
Compatibility & Replacements
Compatible With
- Allen-Bradley PLC-5 processors (PLC-5/40, PLC-5/60, PLC-5/80 series)
- 1771 I/O chassis (all models with block transfer capability)
- 1771 Remote I/O (RIO) communication networks
- Allen-Bradley Bulletin 845H, P, L, E, and K encoders
- 1771-WN wiring arm assemblies
Replaces / Drop-In Replacement For
- Allen-Bradley 1771-VHSC Series A (same footprint and block transfer interface)
- Other PLC-5 counter modules performing equivalent high-speed pulse functions
Works With
- RSLogix 5 PLC programming software (counter mode, encoder mode, and rate measurement configuration)
- Block Transfer Read (BTR) and Block Transfer Write (BTW) instructions
- Remote I/O (RIO) distributed control networks for multi-chassis applications
- TTL and RS-422 pulse sources (encoders, proximity sensors, limit switches, mechanical counters)
