The Polstar PMB-688 is a great GPS receiver. Though its documentation is less than a cup running over, it's knowable nonetheless.
From what I've seen, getting the wires right is the biggest problem. I think that that stems from a general and disheartening disregard for the documentation ( Polstar PMB-688 [PDF Alert] ), substituting for it an intellectually conceited, and lazy, conflation of the silkscreened labels by the square pads above the connector with the wires coming out of the connector assembly. So, here it is, unequivocally: Those labels DO NOT correspond to the connector wires!
Click pic to enlarge.
You don't have to believe me - Get out an ohmmeter and trace the wires to the pads.
The PMB-688 has only TTL I/O available. The Yellow wire is data out (TXD) and the Blue wire is for data in (RXD).
The Red wire is for +5, the Black wire is for Ground.
(It works out like this for the PMB-648, too, but "RS232" TX/RX are available on it.)
Here's the document's wiring diagram and an enhanced version of my devise beside it.
22 OCT 2012 - Had it on the oscilloscope, over the supply range of 3 to 5V, TX was 0/3V signal.
26 OCT 2012 - My only interest for a certain project is the RMC sentence. The '688 default config is with GGA. GLL, GSA, GSV, and RMC enabled. Heretofore, I've used the SiRF Demo, solely, to configure the '688 with. The sentences can be disabled from the SiRF Demo, but if the GPS is left unpowered for 2-3 days then it reverts to stock config.
Q) What to do?
A) Make the config part of the initialisation in the programme.
Q) How's that?
A) Send the following data (4800 8N1):
$PSRF103,0,0,0,1*24<CR><LF> -- disables GGA
$PSRF103,1,0,0,1*25<CR><LF> -- disables GLL
$PSRF103,2,0,0,1*26<CR><LF> -- disables GSA
$PSRF103,3,0,0,1*27<CR><LF> -- disables GSV
and, if for some reason you need to
$PSRF103,4,0,0,1*20<CR><LF> -- disables RMC
<CR> is Carriage Return, ASCII $0D (or 0x0d), or \r
<LF> is Line Feed, ASCII $0A (or 0x0a), or \n
The number after the asterisk is the checksum. I found a great tool, it's an on-line checksum generator
Another invaluable GPS site, most everything about anything NMEA --