Firewire fried a PC motherboard

[haklesup]

Has anyone ever heard of a firewire connection between two PCs causing one of the motherboards to get fried. He is working on a various hardware drivers and using a firewire network to monitor the host PC remotely. It sounds like one PC was on and the other was off when he plugged in the firewire cable.

This is what my software contractor told me today:

> I got a firewire card, but I'm not sure it's going to work. I think it
> fried the motherboard on the computer I put it in. To debug device
> drivers you're supposed to run a firewire connection between the
> computers, but it looks like the firewire card I got is not buffered, so
> when connected to another PC, there was enough power on the PCI bus to
> spin all the fans in the computer with the power supply unplugged.
>
> I believe the fans run on 12V, so it must have been powering the 12V
> rail and nothing else. That probably caused something bad to happen on
> the motherboard. It's definitely not a state the board is designed to
> handle.

If you know more information of know of any relevant links, point me that way. I'm trying to understand what happened.

In any case, if anyone is considering using firewire or maybe even USB to network between two computers, make sure both are turned on or the connection is current limited or buffered somehow

Can I somehow disable power over the firewire and still use the port for communication?

[haklesup]

I looked around and although there are plenty of examples of firewire apparently blowing out individual devices, external HDDs and occasionally internal HDDs I couldn't find anything quite like what happened here.

I did discover that firewire power pin has 30V on it. That's a lot higher than I expected and I don't even know where it comes from since a PC shouldn't have anything more than 12V in it. There would need to be a DC-DC on the firewire hardware somewhere to pull that off. I have yet to make a direct measurement to confirm this is not just a spec differing from typical reality. I'm also not sure about the current available from firewire but it must be significant to spin up all the fans in an unplugged PC.

I am also dumbfounded that a firewire PCI card would not have a blocking diode or at least an overvoltage shunt protection diode. Perhaps the shunt burned out then power was allowed to back-flow into the system. In any case, I probably won't get the failed hardware to examine. I think he will try to return it to the store for a refund.