[lug] USB to USB as serial connection

Nate Duehr nate at natetech.com
Thu Jul 1 22:17:44 MDT 2004

On Thursday 01 July 2004 21:59, John Dollison wrote:
> I learned from personal experience that if you try to connect two USB ports
> directly to each other, you will fry the USB controller chips, at a minimum
> (smoke and all!).  In some cases, you may even damage your motherboard or
> power supply.  But I've heard good things about the "null-modem" or "file
> transfer" USB cables.
> John

The USB spec includes a 5V power supply present on certain pins on the USB 
port.  The spec calls for a minimum of 500 mA of current to be drawn, but 
many cheaply (poorly) designed USB ports can source much much more than that.  

I'd have to look at the pinouts, but depending on what type of cables you used 
to connect the two systems, you could have shorted the 5VDC line to ground, 
and if the port was capable of sourcing a few amps of current, that could be 
the source of the damage you experienced.

Many electronics experimenters and numerous small devices like LED lights, 
fans, and even small coffee-cup warmers use the 5VDC present at the USB port 
for power.  (And no, there's usually not enough power available to warm a 
cold cup of coffee to hot, that silly thing can only slow down how fast the 
cup of joe cools down.)  

Devices are SUPPOSED to register with the laptop's USB controller and tell it 
how much power they are going to use, but rarely do the cheap devices do 

Nate Duehr, nate at natetech.com

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