Connecting the HP and GPS

If you have PC cables for the HP and GPS, just connect them with a null modem adapter. You can also build a custom cable (look here for more on building cables for the GPS or HP).

Unfortunately, there is one potential complication: the HP 48's serial port uses RS-232 voltage levels while most GPSs use NMEA levels. And of course the two standards are slightly different. Some combinations of HP48/NMEA device work just fine with the simple cable described above, but others don't work at all (you get only timeouts). I haven't figured out yet whether this is due to slight variations between HP 48 revisions or variations between NMEA devices, but if you have this problem, there are two fairly simple fixes:

Connect a small 1.5 V battery in series with the transmit data line coming from the HP (positive polarity in the direction of the GPS). For a compact solution, you could probably even fit the battery inside the housing for the serial connector. Your cable should look like this:

Alternatively, the battery may be replaced by a 1 kOhm resistor in series with a little diode (1N4148 or so) connected from the Garmin's data input line with anode to ground.

Thanks to Detlef Rohde for figuring this out! I've had two reports of this problem and in both cases adding that 1.5 V battery fixed everything.

Here is an article with some more details about this problem and another, more complicated way to fix it.

Cases

Some smaller toolboxes make excellent hard cases for the HP and GPS. The Sears Compact Truck Box (965117) costs $4.99 and is just the right size for my HP and Garmin III. I glued in a wooden divider, some foam padding, and lined the whole thing with crimson felt. It turned out rather well.
Also, Pelican makes some very sturdy, waterproof cases. My GPS and HP 48 fit nicely into the "1200 CASE", which I got at a photography store for about US $40.