Firewire: Will firewire bus power be sent to a firewire device that does not require

[musicguy7]

Hi,

I have a 2010 2.8 mac pro quad with only firewire 800 9 pin ports. I have a 6 pin firewire device that "cannot be" bus powered and has its own ac adaptor. I was wondering when hooking up these to with a 9 to 6 pin firewire cable, if firewire bus power will still be sent to the device? Or is there something built into the firewire protocol where it will "determine" and not send bus power to devices that don't require it and have there own power source? I am concerned about the extra bus power, because I know issues of unnecessary rises in cpu temps and noisier fans and potential damage to firewire devices.

 

[alust2013]

If a device doesn't require power, it won't draw any, simple as that.

 

[gr8tfly]

The voltage is available to the device, but if it doesn't use it, there's no current draw from the MacPro.

Power(Watts) = current(Amps) * voltage, so if current = zero, then power consumed = zero. Drives that use their own supply will simply not use the FW power.

Even if you use a bus powered drive, there's no hit to CPU temps, and shouldn't cause any excessive heating in the DC power components. Bus powered drives are usually 2.5" drives, and they typically only draw 1A at 5V max (5W).

 

[musicguy7]

Hmm, I read from this guy's past post it still is an issue, he has a firewire soundcard with it's own power adaptor, and the firewire bus is still sending power as well resulting in problems:

http://www.macosxaudio.com/forums/viewtopic.php?f=7&t=49264

Perhaps he was mistaken there was no bus power being sent, and something else is causing his issues?

 

[gr8tfly]

Perhaps he was mistaken there was no bus power being sent, and something else is causing his issues?

That's what I was suggesting.

In any event, even if bus power were being supplied, it wouldn't affect CPU temps.

 

[musicguy7]

Actually i was more concerned about the effects of excess power on my firewire soundcard. I understand firewire port isolators are available to prevent damaging peripherals with unwanted bus power, so are you saying as well that an ac powered firewire device also getting firewire bus power will not experience any damage/side effects as well? Or are you standing by the fact that it will never accept bus power in the first place?

 

[Lennyvalentin]

There's no such thing as "unwanted" or "excess" bus power. That's not how electrical systems work. Firewire bus power will have no effect on your externally powered sound card, so you can stop worrying now.

 

[Fishrrman]

I was wondering when hooking up these to with a 9 to 6 pin firewire cable, if firewire bus power will still be sent to the device?

Standard firewire cables have lines that route power to the connected device, whether or not that power is actually "used".

Although I've never had one "in-hand", I understand that there are some specialized firewire cables available that _do not_ include the power wires inside. You'll have to look around to find these, and you may have to pay a premium to buy them. They were sold once, although they may no longer be on the market.