microATX Motherboard Interface Specification

Version 1.2

Page 22

3.4.1 FanM Signal

The FanM signal is an open collector, 2 pulse per revolution tachometer signal from the power supply fan.

The signal stops cycling during a lock rotor state; the level can be either high or low. This signal allows the

system to monitor the power supply for fan speed or failures. Implementation of this signal would allow a

system designer to gracefully power down the system in the case of a critical fan failure. The monitoring

circuit on the motherboard should use a 1k Ohm to 10k Ohm pullup resistor for this signal. The output should

be fed into a high impedance gate for the motherboard implementation. Figure 11 shows a simple illustration

of the basic circuit requirements. If this signal is not implemented on the motherboard, it should not impact

the power supply function.

Figure 11: Simple Implementation of a Fan Monitor Circuit

3.4.2 FanC Signal

The FanC signal is an optional fan speed and shutdown control signal. The fan speed and shutdown are

controlled by a variable voltage on this pin. This signal allows the system to request control of the power

supply fan from full speed to off. Implementation of this signal would allow a system designer to implement a

request-fan-speed control or shut-down during low power states such as sleep or suspend. The control circuit

on the motherboard should supply voltage to this pin from +12 VDC to 0 VDC for the fan control request.

• If a voltage level of +1 volts or less is sensed by the power supply at pin 2 of the optional connector, the

fan is requested by the motherboard to shut down.

• If a voltage level of +10.5 volts or higher is being supplied to pin 2, the fan in the power supply is

requested to operate at full speed.

The fan control in the power supply can be implemented so that it allows variable speed operation of the fan,

depending on the voltage level supplied. If, for example, a +6 volt signal is sensed at pin 2, the power supply

would operate the fan at a medium speed. If this signal is used for on/off control of the power supply fan, and

speed control is not implemented in the fan control circuit of the power supply, the power supply fan should

operate at full speed for any voltage level over +1 VDC. The power supply should draw no more than 20mA

from pin 2 of the optional power supply connector. A pullup resistor should be used internal to the power

supply for this signal so that if the connector is left open, the fan will be requested to operate at full speed.