Question about rear USB ports

Gas · Jan 31, 2025

Hi all, I am going to build a new PC, the motherboard will be an (MSI MAG B850 TOMAHAWK MAX WIFI) on the manual I see that some of the rear USB ports are from CPU and some are from chipset:
● 1x Clear CMOS Button
● 1x Flash BIOS Button
● 1x HDMI™ port
● 3x USB 10Gbps Type-C ports (From CPU)
● 2x USB 10Gbps Type A port (From CPU & B850 Chipset)
● 1x USB 5Gbps Type A (From B850 Chipset)
● 4x USB 2.0 connectors (From HUBGL850)
● 1x 5G LAN (RJ45) port
● 2x Wi-Fi antenna connectors
● 2x audio jacks
● 1x Optical S/PDIF Out connector
I wonder if there is any difference in performance between these types of USB ports. I did some web search but i did not find any clear answer, I am sure that in the forum there is someone informed about this matter.
Thanks in advance for any help/clarification
Gas

Helmut · Jan 31, 2025

The potential performance/speed is as stated in Gbps, the only omission is the 4 x USB 2.0 ports/connectors which are 480 Mbps Type A.

The unclear bit is what hardware you may choose to connect to those ports.

e.g. The illustration on p15 of your manual hints at what sort of devices you will likely connect e.g. keyboards, mice, printers etc to the USB 2.0 ports, that hardware does not need to use the higher performance/speed USB ports.

abactuon · Jan 31, 2025

USB 2.0 = 2,5 W
USB 3.0/3.1 = 4,5 W
USB 3.2/4 = 7,5 W

It's more important what you have on your computer case, more often than not you plug something in there.

badrobot · Jan 31, 2025

~~I believe the rear USB ports are from CPU which will be faster than the mobo chipset. The CPU will handle the processing power of the rear USB. The front USB will be connected to the mobo chipset.~~

****

Sorry I stand corrected. I personally never asked this question myself.

I wonder why the mobo specs say "from CPU"?

Gas · Feb 1, 2025

badrobot said:
Sorry I stand corrected. I personally never asked this question myself.

I wonder why the mobo specs say "from CPU"?

Hello, and ty for your interest about this matter and i am also surprised to see that many X/B800 family motherboards have this kind of specs and you got the point of the question: is there any performance difference between USB ports managed from CPU and the ones managed from the chipset independently of the devices connected and/or the USB Type. So please readers of this thread, get it as a pure technological curiosity.

hsehestedt · Feb 1, 2025

Let me start by saying that I am not an authority on this topic, but I can tell you this:

I know that often USB ports serviced by the chipset share bandwidth. As an example, say you have two ports each rated at 10 Gbps. If the two ports share bandwidth and you try to use high speed devices on both ports at the same time then that 10 Gbps is shared between the two devices.

This was a bigger deal with USB 2 because you only had 480 Mbps and sharing that among multiple could be a real world bottleneck.

Gas · Feb 1, 2025

Thank you for your interesting clarification, no bandwidth sharing on USB ports, only 1 PCIE and one M2 connections share the bandwidth, see the attached diagram.

MrPig · Feb 1, 2025

The GL850 is a USB hub controller, drawing is a bit misleading.

This is a fun read:

USB in a NutShell - Chapter 1 - Introduction

Introduces the Universal Serial Bus covering the various chapters of the spec and what is required to be read.

beyondlogic.org

Gas · Feb 2, 2025

MrPig said:
The GL850 is a USB hub controller, drawing is a bit misleading.

This is a fun read:

USB in a NutShell - Chapter 1 - Introduction

Introduces the Universal Serial Bus covering the various chapters of the spec and what is required to be read.

beyondlogic.org

Thank you so much, this is a kind of an encyclopedia about USB, I really appreciate.

Yearofthegoat · Oct 9, 2025

In case anyone else arrives here as I did, I have an RME audio interface, which is USB 2.0 with custom drivers and RME's advice - certainly for AM4 and maybe this applies to AM5 also - was to connect the device to USB ports that were connected to the CPU.

I'm about to upgrade to the B850 Tomahawk so that means my audio interface goes into USB 9, as that one is the only USB-A that is connected directly to the CPU.

Back when AM4 was relatively immature there were lots of posts of USB ports dropping out, causing audio stutters etc. and the only ports that didn't do this on my X570 Tomahawk were those that were connected directly to the CPU. Since then the situation has been improved via BIOS updates, but for audio interfaces I think using the CPU-connected USB ports is a sensible approach.

KevTech · Oct 9, 2025

Only sharing on the MSI MAG B850 TOMAHAWK MAX WIFI is the PCIe and M2 slots.

M.2_3 (4x2) shares bandwidth with PCI_E3 (4x4); using this slot will make PCI_E3 run at x2. Switching PCI_E3 to x4 in the BIOS will completely disable M.2_3.

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Question about rear USB ports

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