Cable, Compaq/HP iPAQ h3800, 3900, 2200, 4000, or 5000 series Pocket PC to Cambridge DDV 302 or Goddard PS-5a, iPAQ 3800, 3900, 2200, 4000, or 5000 series connector -> DB-9m, This cable provides complete communication between the units as well as passing +5 VDC power to the Pocket PC.  You can select from various lengths upon ordering.  I try to keep 0.3m (11.8 in), 0.5m (19.7 in), 1m (3.281 ft), 2m (6.56 ft) and 3m (9.84 ft) units in stock.
Note: This is a very nice cable, but it does extend out from the bottom of the iPAQ and could easily be damaged by your leg in a cramped sailplane cockpit.  For that reason, I sell many of the Nimbus Cradles - it has an integral cable that exits the back of the cradle.  The integral cable connects to a Cambridge product or Goddard PS-5a and is a great alternative to this cable.

Important Note for GPS-NAV Owners:

GPS-NAV Owners - Read This
GPS-NAV units can provide enough 5V power (500 mA) to run a Compaq Aero 1500 without any issues.  However the power supply is only marginal for powering an iPAQ.  I have been told by customers that their iPAQ h3800 units run fine with the GPS-NAV as long as they fully charge the iPAQ before connecting it to the GPS-NAV.  I use my GPS-NAV to test iPAQ cradles all the time, so I know it can power an iPAQ.  However, if the iPAQ battery is more than a little bit discharged it can draw up to 1A of power.  The GPS-NAV cannot supply that much current.  There is a "resettable" fuse in the GPS-NAV that will stop current flow to the iPAQ.  I believe there is a chance that the GPS-NAV may not log your flight while the fuse is tripped.  A good practice with any PDA is to keep it powered in its cradle on your desk at home when not flying so that the battery will never completely lose its charge.  If you follow that practice you should be fine connecting an iPAQ h3800 or similar directly to a GPS-NAV.  If you don't always keep your iPAQ fully charged, I highly recommend using a Goddard PS-5a and either a PC-x or PGNAV-1 cable.  The  PS-5a takes 12V power from your glider battery and provides 5V power to the PDA.  The PC-x cable provides a data connection from the GPS-NAV to the  PS-5a.  The PGNAV-1 provides a data connection from the GPS-NAV to the  PS-5a and also provides 12V power from the  PS-5a to the GPS-NAV - which is nice for portable use.  In my cable and cradle selection table below I recommend the use of a  PS-5a.  If you don't use the PS-5a I highly recommend that you keep your iPAQ fully charged.  Newer iPAQs such as the hx4700 or hx4705 draw much more current than the h3800 units so a PS-5a is an absolute necessity with those units.  Note: This is not a problem with the Cambridge 302 and 302A because they can supply much more current to the PDA (900 mA).