Question CPU OPT fan not customizable. Help please

rcsverige

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Mar 15, 2021
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I have an ASUS Prime b550m-a
I have a total of 5 fans with 3 intakes, 1 exhaust, and 1 cpu fan
my MB only has 2xCHAFAN 1xCPUFAN and 1xCPUOPT
So what I did was attached my exhaust to my CHA1header
Attached, with a splitter, two intake fans to CHA2 header
Attached cpu cooler fan to CPUFAN
and my left over fan, I attached to CPUOPT
All of my fan, except for the CPU COOLER fan are SP120 RGB ELITE fans and have a maximum RPM of 1500

I decided to check my BIOS now and recognized that my CPU OPT fan is operating at 1400ish RPM... I want to change that but I cant seem to on my BIOS. Was wondering if anyone could help or provide insight as to whether or not it is ok to operate at this speed. Below I have attached a picture that I took of the BIOS Qfan screen.
View: https://imgur.com/a/TSRLVwd
 

rcsverige

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Mar 15, 2021
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thats not bios qfan screen at all

this one is:


Yea I know... I dont have the option to tune the CPU OPT fan tho. Thats what I am trying to figure out. The fan connected to the CPU OPT fan is an intake fan but operating very close to its maximum rpm. And I went to change that but its not available in the BIOS menu. Only chasis fan 1, chasis fan 2, and CPU fan. Thats it :(
I showed this screen to display how significant the difference is between this fan and all of the others. I just want to lower it but I cant it seems.
 
CPU_OPT, for the longest time, has ran off whatever CPU_FAN header is set to.
recently cpu_opt can be configured as pump header...

ea I know... I dont have the option to tune the CPU OPT fan tho. Thats what I am trying to figure out.
can you post picture from your bios? bios can do capture screen as image on inserted usb stick btw
 

Zerk2012

Titan
Ambassador
I have an ASUS Prime b550m-a
I have a total of 5 fans with 3 intakes, 1 exhaust, and 1 cpu fan
my MB only has 2xCHAFAN 1xCPUFAN and 1xCPUOPT
So what I did was attached my exhaust to my CHA1header
Attached, with a splitter, two intake fans to CHA2 header
Attached cpu cooler fan to CPUFAN
and my left over fan, I attached to CPUOPT
All of my fan, except for the CPU COOLER fan are SP120 RGB ELITE fans and have a maximum RPM of 1500

I decided to check my BIOS now and recognized that my CPU OPT fan is operating at 1400ish RPM... I want to change that but I cant seem to on my BIOS. Was wondering if anyone could help or provide insight as to whether or not it is ok to operate at this speed. Below I have attached a picture that I took of the BIOS Qfan screen.
View: https://imgur.com/a/TSRLVwd
Most of the time you have a choice in BIOS to make the CPU OPT fan header for water or air cooling on water it should run full speed as the pump on a AIO should run full speed.

EDIT Read your motherboard manual!!!!!!!
 

rcsverige

Commendable
Mar 15, 2021
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Most of the time you have a choice in BIOS to make the CPU OPT fan header for water or air cooling on water it should run full speed as the pump on a AIO should run full speed.

EDIT Read your motherboard manual!!!!!!!
Ive read the MB manual... And Im sorry but I cant find anything which is why I reached out to TH
 

rcsverige

Commendable
Mar 15, 2021
101
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Most of the time you have a choice in BIOS to make the CPU OPT fan header for water or air cooling on water it should run full speed as the pump on a AIO should run full speed.

EDIT Read your motherboard manual!!!!!!!
https://dlcdnets.asus.com/pub/ASUS/mb/SocketAM4/PRIME_B550M-A/E16550_PRIME_B550M-A_UM_WEB.pdf?model=PRIME B550M-A

Ther is literally nothing about this in the manual. Nothing about changing the OPT header so I dont understand why people are bashing me about this. I literally cant change it and I dont know if I am missing something or not. The fan connected to the CPU fan is different from the fan connected to the CPU OPT fan yet they are running at drastically different RPMs with the latter running close to maximum RPM. For now I unplugged it and bought a splitter cable for the CHA1 fan header. At that point I should be fine. I just find it odd that I cant find control over the CPU OPT header.
 
https://dlcdnets.asus.com/pub/ASUS/mb/SocketAM4/PRIME_B550M-A/E16550_PRIME_B550M-A_UM_WEB.pdf?model=PRIME B550M-A

Ther is literally nothing about this in the manual. Nothing about changing the OPT header so I dont understand why people are bashing me about this. I literally cant change it and I dont know if I am missing something or not. The fan connected to the CPU fan is different from the fan connected to the CPU OPT fan yet they are running at drastically different RPMs with the latter running close to maximum RPM. For now I unplugged it and bought a splitter cable for the CHA1 fan header. At that point I should be fine. I just find it odd that I cant find control over the CPU OPT header.
wrong manual
here you go
https://dlcdnets.asus.com/pub/ASUS/misc/Manual/PRIME_TUF_GAMING_B550_Series_BIOS_EM_WEB_EN.pdf?model=PRIME B550M-A
ignore tuf gaming name in link, this manual is for multiple mainboards

but yes, yure right, it cant be configured separately, its tied to cpu fan
 
Last edited:
Jan 19, 2023
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It looks like you've connected your fans to the appropriate headers on your motherboard, but are unable to adjust the RPM of the fan connected to the CPUOPT header in the BIOS.

The CPUOPT header on your ASUS Prime B550M-A motherboard is typically used for an additional CPU cooler or water pump, and may not have the same fan control capabilities as the CHA and CPU headers. Some motherboards only allow for basic control (On/Off) of the fan connected to this header, while others do not provide any fan control options at all.

It's not uncommon for the fan connected to this header to run at full speed, especially if it's an additional cooling solution. However, if you want to lower the RPM of this fan, you could try using a fan controller or using software that allows you to adjust the fan speed through the system's software.

As for the RPM being at 1400, it should not be a problem as long as the fan is not making loud noise or producing excessive vibration. Additionally, you should keep an eye on the temperature of your CPU and make sure that it is not running too hot.

Keep in mind that the maximum RPM of the fan is just a reference, and the actual RPM might be different depending on the situation, for example, depending on the temperature of the CPU, the fan can run at a lower RPM.
 

rcsverige

Commendable
Mar 15, 2021
101
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wrong manual
here you go
https://dlcdnets.asus.com/pub/ASUS/misc/Manual/PRIME_TUF_GAMING_B550_Series_BIOS_EM_WEB_EN.pdf?model=PRIME B550M-A
ignore tuf gaming name in link, this manual is for multiple mainboards

but yes, yure right, it cant be configured separately, its tied to cpu fan
But I’m curious as to why the CPUFAN header is operating at a lower rpm while my CPUOPT header is operating at a consistent 1400rpm? Shouldn’t it be basing its rpm on the cpu or am I just misunderstanding this. I bought another splitter for my CHAFAN1 but is that ideal? For example, the two fans I have plugged into the CHAFAN2 are in DC mode. I’m still reading about what that means but, seeing as how I want to solve this so that all fans are operating again, I was wondering if maybe you might be able to provide some insight as to whether fan splitters will be sufficient.
 

rcsverige

Commendable
Mar 15, 2021
101
2
1,585
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It looks like you've connected your fans to the appropriate headers on your motherboard, but are unable to adjust the RPM of the fan connected to the CPUOPT header in the BIOS.

The CPUOPT header on your ASUS Prime B550M-A motherboard is typically used for an additional CPU cooler or water pump, and may not have the same fan control capabilities as the CHA and CPU headers. Some motherboards only allow for basic control (On/Off) of the fan connected to this header, while others do not provide any fan control options at all.

It's not uncommon for the fan connected to this header to run at full speed, especially if it's an additional cooling solution. However, if you want to lower the RPM of this fan, you could try using a fan controller or using software that allows you to adjust the fan speed through the system's software.

As for the RPM being at 1400, it should not be a problem as long as the fan is not making loud noise or producing excessive vibration. Additionally, you should keep an eye on the temperature of your CPU and make sure that it is not running too hot.

Keep in mind that the maximum RPM of the fan is just a reference, and the actual RPM might be different depending on the situation, for example, depending on the temperature of the CPU, the fan can run at a lower RPM.
Thank you so much! I never saw your comment for some reason until now. I bought another splitter so that all 3 intakes and 1 exhaust can be plugged into CHAFAN1 and CHAFAN2. I don’t know if that will be better or not? I’ll check into what my options are tomorrow in regards to software that can do that. Either way, thank you for explaining in detail!
 

Paperdoc

Polypheme
Ambassador
As others have said, you did not recognize what a CPU_OPT fan header does. Normally it can ONLY duplicate exactly what the CPU_FAN header does - you do NOT have any other control over it.

Next item many do not understand: although we all talk about fan SPEED control, in fact the system is a TEMPERATURE control. Each fan header is focused on the TEMPERATURE at its associated sensor, and will manipulate the speed of its fan to meet that temp target. It does NOT know or care what that speed is - it only will alter the signal to the fan according to temperature. It does that by altering the Voltage or the PWM signal sent to the fan. Even though it can show you the speed of its fans it does NOT use that or care what it is for its temperature control functions. As an important secondary function, though, it DOES monitor that speed for NO speed, which would indicate the fan has failed and there is poor to NO actual cooling happening. That situation will pop up a warning on your screen and maybe more drastic protective actions.

IF you connect two DIFFERENT fans to the SAME header, they will receive identical control signals (either Voltage or PWM), BUT they will NOT run at the same SPEED! Each fan design has its own range of speeds for the range of input signals it gets. That is why your one case vent fan connected to the CPU_OPT header ran at a different speed from the fan that was part of your CPU cooler system.

The CPU_FAN and CPU_OPT headers on your mobo do all their work using a temperature sensor inside the CPU chip. The CHA_FAN headers use a different sensor on the motherboard, and so these headers are where you should plug in all your case ventilation fans.

When you have to connect several fans to one header using Splitter(s), the normal limit on one header is no more than 1.0 A max current total to all the fans connected to that header. So you look up the max current spec for each fan you are using. Most these days pull at max 0.10 to 0.25 A, so three to four such fans on one header is OK, but you DO need to check that detail.

3-pin, 4-pin and MODES. Older fans use 3 wires and have 3 holes in the female connector on the end of the wires. To control the speed of this fan type, the header they use MUST be configured to use Voltage Control Mode (aka DC Mode). IF you connect them to a header using the newer PWM Mode, they will always run full speed. Newer fans have FOUR wires and holes / pins, and are called PWM Fans. If you have this type their header really should be configured to use PWM Mode for best control. They will work using DC mode, but it's not ideal.
 
Last edited:
Reactions: rcsverige

rcsverige

Commendable
Mar 15, 2021
101
2
1,585
0
As others have said, you did not recognize what a CPU_OPT fan header does. Normally it can ONLY duplicate exactly what the CPU_FAN header does - you do NOT have any other control over it.

Next item many do not understand: although we all talk about fan SPEED control, in fact the system is a TEMPERATURE control. Each fan header is focused on the TEMPERATURE at its associated sensor, and will manipulate the speed of its fan to meet that temp target. It does NOT know or care what that speed is - it only will alter the signal to the fan according to temperature. It does that by altering the Voltage or the PWM signal sent to the fan. Even though it can show you the speed of its fans it does NOT use that or care what it is for its temperature control functions. As an important secondary function, though, it DOES monitor that speed for NO speed, which would indicate the fan has failed and there is poor to NO actual cooling happening. That situation will pop up a warning on your screen and maybe more drastic protective actions.

IF you connect two DIFFERENT fans to the SAME header, they will receive identical control signals (either Voltage or PWM), BUT they will NOT run at the same SPEED! Each fan design has its own range of speeds for the range of input signals it gets. That is why your one case vent fan connected to the CPU_OPT header ran at a different speed from the fan that was part of your CPU cooler system.

The CPU_FAN and CPU_OPT headers on your mobo do all their work using a temperature sensor inside the CPU chip. The CHA_FAN headers use a different sensor on the motherboard, and so these headers are where you should plug in all your case ventilation fans.

When you have to connect several fans to one header using Splitter(s), the normal limit on one header is no more than 1.0 A max current total to all the fans connected to that header. So you look up the max current spec for each fan you are using. Most these days pull at max 0.10 to 0.25 A, so three to four such fans on one header is OK, but you DO need to check that detail.

3-pin, 4-pin and MODES. Older fans use 3 wires and have 3 holes in the female connector on the end of the wires. To control the speed of this fan type, the header they use MUST be configured to use Voltage Control Mode (aka DC Mode). IF you connect them to a header using the newer PWM Mode, they will always run full speed. Newer fans have FOUR wires and holes / pins, and are called PWM Fans. If you have this type their header really should be configured to use PWM Mode for best control. They will work using DC mode, but it's not ideal.
Thank you. I have scoured the internet but I always end up finding conflicting information so I appreciate that you took the time to help me understand this better. Thank you. I ended up buying two splitters 1 for CHA1 and 1 for CHA2. I noticed what you were talking about in regards to PWM and DC so I switched to DC and optimized the fan controls so that they ramp up in certain temps. So far so good
 

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