What does the RD (rotation detect) signal indicate?

RD is a fault indicator, not a second tach: it asserts when the fan enters a defined abnormal state — typically a locked rotor, and on some models also under-speed operation. Output style is usually open-drain/open-collector, and polarity differs between makers (low-on-fault and high-on-fault both exist). Scope both fault and healthy states on a sample before trusting it in production logic.

What it asserts — and when

Read the specific model's truth table: some RD outputs go active only at a full stall; others also assert below a minimum speed threshold; a few provide a 'running' output instead (the logical inverse). The trap is testing only the healthy state — a wired-but-never-tested alarm line that is actually inverted turns every future failure into a false-healthy report, discovered only by the customer.

Wiring and logic

Open-drain outputs need a pull-up at the host (value per the fan's sink rating, commonly 10 kΩ to the host's logic rail). Confirm the pull-up rail: mixing a 5 V pull-up into a 3.3 V input can overdrive the input. Decide the power-up masking window: fans take a defined spin-up time, during which RD may assert transiently — the host should mask alarms for that window (typically a few hundred milliseconds to a couple of seconds, per the model's start-up spec).

• Pull-up to the correct logic rail; verify thresholds with a scope.
• Mask the start-up window before enabling the alarm.
• If the platform supports it, pair RD with tach cross-checks: RD idle while tach dead = wiring fault, not airflow fault.

Beyond stall: building real protection

A single RD line protects against one failure mode. Robust designs layer it: RD for hard faults, tach trend for degradation, current monitoring for electrical drift, and temperature shutdown as the final backstop. MEGA Tech supplies fans with RD, FG/TACH or combined-signal interfaces across its DC range — tell us which fault states your controller acts on and we will match the signal set, not just the airflow.