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I was reading this Three Monitors For Every User and there were some comments about rear USB ports being able to deliver more power than front USB ports because they are directly connected to the motherboard and closer to the power supply (by circuit board runs). Even though the front USB ports may have connectors farther from the power supply, and there are cables from the motherboard to the front ports, I think that the difference in power would be negligible (unless the case is over 5 meters long). Anyone know for sure if they are the same or different?

Note that I'm not talking about an older case where the front might have been USB 1.1 and the rear USB 2.0. A modern case would have USB 2.0 on all ports. And of course using a powered hub would deliver plenty of power.

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4 Answers 4

up vote 5 down vote accepted

The location on the case is not going to determine whether it is a high power or low power port. The motherboard manufacturer can choose to provide high power or low power ports and they may be on the motherboard directly (at the rear) or just headers that are connected via a cable to the front ports on the case. It would vary depending on the motherboard and how things are hooked up inside the case.

True, there is certainly a tiny bit of voltage drop over a longer distance, but that's not going to affect the overall output much.

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I thought the USB 2 standard for a USB port is 5v @500mA for 2.5 watts. Why would they make some to spec and others with less power on the same motherboard? The on-board (rear) and headers (front) should both be 5v/500mA, making the only difference a foot or two of wire, which has hardly any resistance. Of course it would also depend on how much stuff was connected to each location, but I would make the comparison with one same device on each. –  Bratch Apr 8 '10 at 22:23
    
There's also a battery charging specification which could provide up to 1500mA in some situations. I've seen motherboards which provide that on the actual ports, but not on the headers which are typically hooked up to the front ports. I believe this is even more common in laptops, only some ports able to charge. –  kbyrd Apr 9 '10 at 0:21

My vote goes to "negligible".

If we started losing power on such distances, where would our high-power electricity network wind up ?

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-1. You do lose power along any transmission line. The power grid uses much higher voltage and much larger conductors so that it's more efficient. –  kbyrd Apr 6 '10 at 3:10
4  
@kbryrd - I didn't say you don't. I just said the loss was negligible. –  ldigas Apr 6 '10 at 11:17

Best choice:

Rear ports come directly from the mainboard - best signal quality, best power quality (trough the mainboard layers)

Second choice:

Front ports require extra cabling - from the mainboard to the connector in the case. Thus those cabling and additional connectors have impact on the signal quality (480 MBits/s require high frequency!) and power (every cable is like a resistor - some 0.1 Volts will left on the cable)

Note: 0.5 Ampere are available from USB 2.0 ports. But some hardware designers do not implement a circuit to prevent the port from to much Current, thus it is possible to use a 0.8 A DVD-ROM drive on a single USB port.

but anyhow most of them will work - until you discover problems ;-)

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I've always found the front and back connectors on a system to be equivalent, but there is considerable difference between systems; I have an external hard-drive with a Y-cable to allow a second USB port to be plugged in for extra power,and if I need that on the front of a system I need it on the back also.

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