I use an ASRock B550 Extreme 4 motherboard in combination with an active USB-Switch with 2 PC-connections (ABLEWE USB Switch 3.0, 4 Ports USB). I have connected the USB-Hub to the top USB-Ports of my Motherboard because my microphone (Rode NT-USB mini) won’t work on the other ports properly (robotic voice after a few seconds). Power to the USB Ports while the PC is offline is switched off in the Bios (v1.8).
The Problem was that even the PC is switched off the USB hub rapidly flashes the input-device light for this port. The laptop that is on the other port won’t get through its BIOS while the PC is connected to the switch.
My suspicion: The switch uses mosfets to detect if the PC on any output is turned on and the B550 board puts a voltage on the port that’s just enough to turn the mosfet on. Then the current will be dumped though the hub and the mosfet turns off again. The board then charges up the capacitor on the USB output until the turn-on voltage of the mosfet in the hub is reached and the cycle continues.
The hub meanwhile tries to connect to input 1, can’t and switches back to input 2. And this multiple times a second. No wonder the BIOS from the second PC (Lenovo Laptop) hangs.
How to check: I build a breakout-cable that I put between the motherboard and the hub and measured it. There were 1.641V on the USB-line even it should be (near) 0!
(The voltage was there even when I had the hub removed)
After this I calculated which minimum resistor I need, to drop the voltage below the activation-voltage from the USB hub.
Consideration:
– The cable must work when the PC is turned on (5V).
– the value should be as high as possible because 1. so it stays cold, 2. the voltage drop while the PC is on is as low as possible and 3. the power consumption is as low as possible.
resistor-rating = 0.25W
0.25W / 5V = 0.05A (MAX continuous amp through resistor – we want to keep it way under this)
5V / 0.05A = 100 Ohm (absolute minimum resistor-value for continuous usage)
I tested 220 Ohm, 1k, 2k and 10k.
The 10k ohm wasn’t enough ‘current dumping capacity’ to work. But I saw a significant slowdown between ‘activations’ on the hub. The 2k ohm resistor works most of the time but every few tens of seconds I saw a short blinking of the USB hub trying to make a connection.
So I settled for a 1k ohm resistor to be safe. It works well within specs (1/10 of it) and should do the job just nice.
cont. A through resistor while PC is on: 5V / 1000 ohm = 5mA => 25mW
cont. A through resistor while PC is off: 1.641V / 1000 ohm = 1.6mA => 8.2mW
PC on: 1.82€ / year
PC off: 0.63€ / year
(The cost is in the middle of these 2 values – depending on the running time of the PC)
The cable worked like a charm. It would be nice if the ASRock board hadn’t this flaw in the first place.
