To prevent missing counts when a digital input fires every 20 ms on a PLC, which component should be used?

• A. High speed counter module
• B. Standard counter
• C. Debounce circuit
• D. External timer

Answer: (A)

When you need to count every edge from a fast digital input, the counting must be performed by hardware that operates independently of the PLC’s program scan. A high-speed counter module provides a dedicated counter that increments with each input edge in real time, so pulses aren’t lost while the CPU is busy with other tasks. This is exactly what’s needed when the input fires every 20 ms, ensuring every pulse is captured reliably.

A standard counter relies on the PLC’s software and scan cycle, so edges can be missed if they occur between scans or while the CPU is occupied. A debounce circuit is meant to suppress rapid, unintended transitions from noisy switches and isn’t designed to handle high-frequency counting. An external timer measures time intervals or gates events, but it doesn’t provide the raw, uninterrupted counting capability that a dedicated high-speed counter module offers.

So, the high-speed counter module is the best choice to prevent missing counts in this scenario.