There are commercial solutions availabe, ready to use.
But I would like to know which pins are used on the
IPAQ 3900 side for a two wire communication with the GPS
and which pins are used for the +5V power supply.

The IPAQ connector has some 22 pins and I guess that for
power pins 19 gets +5V and pin 21 gets ground.

A standard serial cable may connect pins 6,7,8,9,10,11,12,13,14,
whereas the USB cable connects to what? Is it pin 17 and 22 or
some of the serial pins?

Can someone explain the pin usage for a 4 wire power/com cable
on the IPAq side?

Thanks a lot
Michael