(Ben Jackson) writes:

The proprietary 4-pin connector that Garmin uses on most (all?) of its
GPSs are the same. Serial RX/TX, and ground/power.

Actually, there are several styles.

The larger marine and aviation units tend to use that round plug. There
are actually two variants of it. The low-voltage models that need a
voltage regulator in the power cord have an extra plastic pin in the
centre of the socket on the GPS and a matching extra hole in the middle
of the plug. The GPS 12, 38, and 40 (72 also?) are examples of
low-voltage models. Most of the other larger units work up to 30 V at
least, and don't have the extra pin or hole in the plug and socket. The
two plugs are otherwise the same.

And then there are the smaller eTrex and eMap units, which use a
completely different slide-in plug with 4 contacts in a row. These
units all want 3 V power, not 5-8 or 10-30 V.

Dave