[TriLUG] why does usb card on PCIe bus need extra power to detect a disk?

[Stephen Bryant via TriLUG]

In my experience with external drives, a lot of them use bus power for the controller, even if the drive itself is run on external power.

My guess, given the “lowest common spec” so much hardware is built to these days:

External drive powers HDD off the power brick, but expects the controller to get its (relatively minuscule) power from the USB power pins. This would make sense, since the USB3 spec requires the host to supply 150mA for a “low-power” device, and 900mA for a normal device. This allows the manufacturer to have two separate (and therefore multipurpose) boards - one for drive power supply, and one for USB<—>SATA.

The PCI-e card is designed to have a power input, so instead of having a transformer to run the USB (5v) from the PCI-e (3.3v+12v) supply, they use the 5v pin from the Molex connection. Saves on complexity and BOM when you’re gonna have that 5v supply anyway.

That’s my perhaps-underinformed guess, anyway :)

— Stephen

> On Dec 13, 2018, at 5:56 PM, Joseph Mack NA3T via TriLUG <trilug at trilug.org> wrote:
> 
> On Thu, 13 Dec 2018, Brian Henning wrote:
> 
>>> I'm wondering why this PCIe card is recognised by the kernel, but needs power for the external usb disk to be detected.
> 
> looking at a bunch of these cards, some of them need power (either via a Molex ATA type connector, or through a SATA power connector), some of them proudly say they don't need extra power and say they get it off the PCIe bus.
> 
> Some of the ones that say they need external power say they need it to power the external disk(s). Well I already know that, but I don't know why the external disk needs power and I don't know why the power can't come off the PCIe bus.
> 
> Joe
> 
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