Some manufacturers support only Windows or DOS computers. If you have an Apple, Palm, or other computer, you'll need to buy or build an interface cable, or just bridge the gap between the PC connector and a non-PC computer. The second choice can be easier, since GPS units may have unusual connection ports. They also have references for making your own cables, and a complete software package for the Mac. Whether you buy or build, connecting cables is risky.
Problems can arise from voltage differences or pin-assignments.
Always connect cables with both units off. There are many GPS-related computer programs available for DOS, Windows, and Macintosh. Be certain you set the datum before downloading your coordinate data. If you're not sure about the datum, try looking at a downloaded waypoint file on your computer, as described in the next section. If you see the same coordinates as your receiver shows when set for the same datum, all is well.