That's not really what it means. There really are two different functions to clear up here.
All fans generate a speed signal consisting of 5 VDC pulses (2 per revolution) sent back to the host header on Pin #3. The header counts the pulses to display speed. It also monitors that signal for NO pulses, which it interprets to mean the fan has FAILED, so it pops up a prominent warning on your screen. In the particular case of the CPU_FAN header it probably would take more drastic action and shut your system down completely pretty soon before even waiting for the temperature sensor inside the CPU chip to show high temps. Assuming that the CPU cooling unit really had failed, this prevents the CPU from overheating rapidly and getting damaged. Other headers like SYS_FAN do not take such drastic actions.
The header can deal with a stream of pulses coming in from only ONE fan. So when you use a Splitter or Hub to connect two or more fans to a single header, the speed of only ONE fan is sent back, and all the other fan speed pulse signals are ignored - they go nowhere and you will never "see" them. You are supposed to assume that those "other" fans are doing exactly the same thing since all fans get the same control signals. BUT that means that the header can NOT monitor all those Ignored fans for FAILURE. So from time to time YOU have to look and verify that all the fans still are working. For a Splitter or Hub that looks like a collection of cable "arms", only ONE of the male output connectors has all four pins in it - the others all are missing Pin #3 so it cannot send back the speed signal from that fan. For a unit that looks like a circuit board or a closed box with ports recessed in holes, ONE of those outputs will be marked in some way to indicate this is the ONLY output that feeds its speed signal back to the host header. You MUST connect one of your fans to this marked port. NOTE that some mark this port for "CPU" but you do NOT need to connect your CPU cooling fan here unless you really ARE using that cooling device on this Hub or Splitter for some reason.
We all speak of fan SPEED control, but the truth is the focus of the automatic control is TEMPERATURE as measured by a relevant sensor. The system controls temperature, and to do that it manipulates the speed of the connected fan(s) to change the air flow over the hot items and cool them sufficiently. In fact it does NOT care what the speed really is and does not use that info for its work. The only use of the speed signal is to monitor for failure. A mobo normally has at least wo different temperature sensors: one inside the CPU chip used for the CPU_FAN header to control CPU cooling, and the other on the mobo where the maker judges it is a good guide to overall device tempertures and cooling needs on the board. Many mobos also have added sensors on specific board components IF you plan to dedicate a fan to that particular component. In general, any units that cool the CPU in some way should be connected to the CPU_FAN header or some related ones. All CHA_FAN or SYS_FAN headers used for general case cooling should use the motherboard temperature sensor.
So pulse counting is the means of measuring the fan speed and monitoring it for failure. A fan header aslo provides POWER to the fan and CONTROL of the fan speed. Older 3-pin fans and the new 4-pin fans are designed and operated differently, so they require some differences in the details of the header output signals. Today almost all headers have 4 pins, but the user gets to specify in the configuration settings of each header exactly which signal set type it will send to its fans. For older 3-pin fans the VOLTAGE supplied on Pin #2 is varied from 12 VDC for full speed down to about 5 VDC for minimum speed without stalling, so BOTH Power and Control are done this way. This is called Voltage Control Mode or DC Mode. For newer 4-pin fans the POWER part from Pin #2 is always 12 VDC, and the CONTROL part is done by sending out the new PWM signal on Pin #4, which is used internally by a special chip in the motor to control the fan speed.
Within the CONTROL task, the header software in BIOS has two sub-tasks. The first is to decide what speed the fan should run and this is the header PROFILE setting. Typically the options here are Normal or Standard, Quiet, Full Speed or Turbo, and Manual or Custom. The first and last of these decide on the basis of the TEMPERATURE at the sensor selected what the speed output signal should be (usually as a "% of Full Speed" kind of signal). The other two allow the used to specify a fixed speed that is NOT adjusted continously. The last option allows the user to specify details of the temperature - speed decision scheme, whereas the first uses a default "fan curve". Once that decision is made, HOW to make the fan do what has been decided is done by the type of signal set sent out. This is the MODE setting: either Voltage Control Mode (aka DC Mode) or PWM Mode.
When you use a Splitter or a Hub to connect two or more fans to a single header, that device shares exactly the SAME signals to ALL its fans, and they all do the SAME thing. (That is, IF the fans all are 3-pin or all are 4-pin so the all CAN do what the shared signals request.) The fact that only ONE of those fans can send its speed signal back to the mobo has NO impact on whether ALL fans can be CONTROLLED in this way.
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