You will (currently) *need* battery backup with the Oncore. gpsapp will not initialise NMEA mode, and the Oncore will use motorola-binary by default. With a battery you can initialise the Oncore to NMEA from a PC app, and then the battery will remember it across reboots.

I'm kinda hoping that may Jan can add in support for switching various GPSs to NMEA - which would make battery backup un-neccessary. (But still a good idea IMO). My current setup uses 2 AA cells for backup, since I couldn't locally source a suitable lithium cell holder, where-as a 2AA cell holder was 99c. Obviously lithium will be smaller, and notionally might last longer (assuming that the AA cells hit their shelf-life limit), but you can pick up a 2AA holder at RatShack.

The difference between using a DS275 and a MAX233 is little in the empeg world. A DS275 will probably only allow half-duplex communication where-as the MAX chips will allow full-duplex. But as gpsapp only talks briefly to the receiver during startup, and everything after that is the receiver talking to gppsapp, this shouldn't matter.

[edit] Actually the DS275 solution will probably work full-duplex with the empeg anyway. The only reason that it's supposed to work half-duplex only is because it uses the PC's (ie empeg's) RxD line when at -12V and charges a capacitor which it then uses as it's -12V source. But the empeg (or most PCs for that matter) doesn't actually need -12V as a mark - TTL level switching is enough. Case in point, I used a 1489 quad RS232 receiver in my interface (I had one lying around) in both directions with no problems. I'm sure that if I were trying to drive 100ft of serial cable then issues may start to crop up, but for under 10ft it works fine.

[edit2]
and then the battery will remember it across reboots
Obviously, what I meant here was "and then the receiver will remember...". Batterys can't remember that kind of information as they don't have a memory. (*)

*...well, not that kind of memory. Many batteries have been found to contain memory over the decades since their discovery. Sir Nicolas Cad for example, discovered that certain batteries could remember just how much energy they were usually asked to expend, and use this information to achieve their goal of laziness by refusing to store any more energy than necessary. And it is widely believed that the reason that citrus fruits have tough skins is due to the selfish gene and self-preservation, whereby the fruits have attempted to prevent the insertion of metal electrodes by the native indians. **
And as for that poor bunny....

** It has also been speculated that they migrated to Florida for the same reason.