Are PWM, FG and RD signals interchangeable between fan brands?
Not automatically. The pin names are industry-common, the implementations differ: PWM frequency windows (typically 20–30 kHz, but check), duty-to-speed maps (rarely identical), tach pulses-per-revolution (commonly 2 per revolution but model-specific), and RD logic (open-drain vs push-pull, alarm polarity). Every one of these mismatches has caused a field escape in a fan replacement.
PWM — command input
Most DC fans accept a PWM drive in the 20–30 kHz range — 25 kHz is a common convention (and Intel's 4-wire specification uses 25 kHz) precisely to keep switching out of audible range. But the duty-to-speed mapping is vendor-specific: 50% duty is not 50% speed on every model, and minimum duty for guaranteed rotation varies (commonly 10–20%). Always request the duty-to-RPM table from the candidate fan and compare it with the incumbent.
| Signal | Typical convention | What differs between brands |
|---|---|---|
| PWM | 25 kHz, 0–100% duty | Duty→RPM map; min/max duty; pull-up |
| FG/TACH | 2 pulses/rev, open-collector | PPR; pull-up level; max sink current |
| RD | Alarm output, open-drain | Polarity; asserted states; behaviour at slow spin |
FG/TACH — speed feedback
The tach output pulses at a fixed count per shaft revolution — two pulses per revolution is common on 4-pole and 2-pole designs, but it is model-specific. The host converts: RPM = 60 × f_tach / PPR. A candidate with 2 PPR replacing a 4 PPR incumbent makes the host display double the real speed (or half, depending on direction of the swap). Check the pull-up resistor value and rail too — some hosts pull to 5 V, some to 3.3 V.
- Confirm PPR from the datasheet or by tach-meter on a sample.
- Recompute the host's RPM conversion if PPR differs.
- Verify open-collector sink current stays within the output rating.
RD — rotation detect / alarm
RD indicates a defined fault state — typically locked rotor — but polarity and asserted states differ: some outputs go low on failure, others release; some also assert below a minimum speed, others only at a full stop. Scope it: stop the fan, then run it below minimum duty, and check both states against the host's alarm logic. A wrong RD polarity turns a protection feature into a false-healthy indicator.
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