Allen-Bradley 1771-VHSC Very High Speed Counter Module

The Allen-Bradley 1771-VHSC, also cataloged as the 1771-VHSC Very High Speed Counter Module, operates as a dedicated hardware component for high-frequency pulse counting and position feedback processing within Allen-Bradley PLC-5 chassis systems.

Hardware Specifications

Profinet / EtherNet/IP Deterministic Networks & Backplane Synchronization

The 1771-VHSC executes real-time pulse accumulation independently of the processor scan cycle, transferring 32-bit count values across the backplane bus. High-speed input channels process quadrature signals from incremental optical encoders, supporting hardware-based compare registers that trigger eight direct 24 VDC outputs within microseconds. This architecture offloads high-frequency pulse processing from PLC-5 processors while maintaining deterministic synchronization across networked remote I/O drops.

Frequently Asked Questions

Q: How does the 1771-VHSC module handle high-speed output actuation relative to the main PLC scan cycle?

A: The 1771-VHSC features direct onboard hardware output control that triggers its 8 digital outputs instantly based on pre-programmed counter match conditions, operating independently of the PLC processor scan time.

Q: What is the backplane current draw required by the 1771-VHSC counter module from the 1771 I/O chassis?

A: The module draws approximately 1.0 A at 5 VDC from the 1771 chassis backplane power supply bus.

Field Installation Guidelines

1. Chassis Slot Insertion: Align the 1771-VHSC circuit board with the chassis card guides in the designated 1771 Universal I/O slot and slide it inward until the rear edge connector firmly seats into the backplane.
2. Field Wiring Termination: Connect encoder quadrature channels and 24 VDC power conductors to the corresponding wiring arm terminals. Secure wiring arm latches to ensure solid electrical contact with the module header.
3. Shielding & Grounding: Route high-frequency pulse inputs using twisted-shielded cable pairs. Connect the cable drain wire directly to the panel chassis earth bus at one end to suppress high-frequency electromagnetic interference (EMI).
4. Wire Separation: Maintain physical separation of at least 200 mm between pulse encoder signal wiring and high-voltage AC or variable frequency drive (VFD) output cables.
5. Thermal Airflow: Ensure panel ventilation slots above and below the 1771 I/O chassis remain unobstructed to allow proper convection cooling across internal circuit components.