[clug] Dumb USB question

Brad Hards bradh at frogmouth.net
Tue Aug 8 07:34:14 GMT 2006

On Tuesday 08 August 2006 13:14, steve jenkin wrote:
>    The Dumb User method is to use a USB F-F joiner to join the M-M
>    USB output cable from U-KVM to the USB-PS/2 adaptor. Alternate
>    would be to plug both devices into powered hub (no host).
>    It's gonna do one of three things:
>    - work :-)
>    - not work :-(
Not work.
>    - fry everything is sight (or on site) :=[
Possible, but unlikely. Design normally accounts for this. More likely if 
things are earthed though.

USB isn't a peer-to-peer arrangement. It has host and device. Hosts provide 
power (always at least some, since this is how the device detection works) 
using two of the four wires/pins. The other two are D+ and D- (differentially 
signalled, more like RS422, except that the same wires are used for comms in 
both directions).  Hosts tell devices what they want, and devices answer. 
[You might have heard of "interrupt transfers" - USB interrupt means "host 
polls device at specified interval"]

Some hardware can be either a host or a device (aka "USB on the Go"), but not 
at the same time.

Different conversion boxes work differently, so I don't have a clear answer - 
I'd try avoiding the PS/2 part, and just put a USB card in the linux box if 
it doesn't already have such a port.


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