No, I didn't use a search engine first.
I have a Magellan SporTrak GPS receiver. It has a serial output which is ... out of spec. The space (0) is a sufficiently positive voltage (I haven't measured, but judging from the brightness of the LEDs on a breakout box, it's ~3V). The mark (1) is ground.
This is good enough for the UART on my desktop, but I already know where that is! It doesn't satisfy the pl2303 chip in the only USB serial converter I have at my disposal.
Could someone please point me towards a circuit I could build in a D-sub jumper box? The only lines coming out of the GPS receiver are Tx, Rx, and Gnd. Only the data line coming from the GPS receiver needs level shifting, and that only for spacing.