Thread: Cheap GPS (Requires some modding)

Originally Posted by SniperXX

Well I found this link.

http://www.hackaday.com/entry/1234000147047359/

Basically you are modifing one of the GPS units for an old Palm III into a serial GPS. Just posting it if some of you needed a cheap GPS and had one of the Palm ones lying around.

Looks interesting. And since it is serial, you won't have hibernation/resume issues.

Michael

Originally Posted by stash182

I'm trying to do this project, but I'm confused as to how would you get it to interface with the PC? Can you convert it to run with USB?

http://www.ramelectronics.net/html/usb_serial.htm

Originally Posted by stash182

serial port nubbies?

What is a "serial port nubbie"

AFAIK the original recvr was meant to directly plug into the Palm III serial port which isnt a standard PC serial port. Am I wrong?

What do you mean by "standard serial port". The signals are the same, they are serial. Your computer "understands" serial, and the Palm Pilot "understands" serial, that is why you can connect it to the computer without additional circuitry and chips and things.

The connectors are different, Yes.

Michael

P.S. I found the thread after the PM.

Originally Posted by Wiredwrx

The signals are the same, they are serial. Your computer "understands" serial, and the Palm Pilot "understands" serial, that is why you can connect it to the computer without additional circuitry and chips and things.

The connectors are different, Yes.

how much have you played with serial

RS232 uses +12v and -12v logic levels, with -12v being a 1, and +12v being a 0. Most modern serial devices just use +/- 5v, because the serial ports still register the logic levels.

the output of the device is likely to be TTL serial, which is the same data format, but with 0v being a 0 and 5v being a 1, so the logic levels are completely different voltages, as well as inverted.

that's what RS232 level shifters are for... you can build a fancy one with a MAX232 chip, or you can build one out of cheap discrete components with this schematic: (which I recommend)
http://www.sparkfun.com/datasheets/P...32-Shifter.pdf

Evan,

Before I mistakenly "go off" on you, can you tell me why you posted a dissertation about Serial communications. Also, why are you linking him to information that he doesn't need, and is not relevant to the topic of discussion?

Michael

Originally Posted by evandude

how much have you played with serial

RS232 uses +12v and -12v logic levels, with -12v being a 1, and +12v being a 0. Most modern serial devices just use +/- 5v, because the serial ports still register the logic levels.

the output of the device is likely to be TTL serial, which is the same data format, but with 0v being a 0 and 5v being a 1, so the logic levels are completely different voltages, as well as inverted.

that's what RS232 level shifters are for... you can build a fancy one with a MAX232 chip, or you can build one out of cheap discrete components with this schematic: (which I recommend)
http://www.sparkfun.com/datasheets/P...32-Shifter.pdf

Stash182 asked if he can connect the device directly to the serial connector. You said yes. my "dissertation" was an explanation of why the answer may be "no", under the assumption that the output of the GPS receiver does not use RS232 logic levels, in which case the circuit I linked would take care of that.

Since there weren't any pictures of the unit before disassembly that I could see, I didn't realize it connected to the same connector as the hotsync cradle, in which case it probably does deal with RS232 levels, in which case my explanation, as you put it, is irrelevant.

I wasn't trying to be a dick, so thank you for giving me a chance before "going off on me". I made a wrong assumption, and I apologize.

Originally Posted by evandude

Stash182 asked if he can connect the device directly to the serial connector. You said yes. my "dissertation" was an explanation of why the answer may be "no", under the assumption that the output of the GPS receiver does not use RS232 logic levels, in which case the circuit I linked would take care of that.

However, my explanation is in fact irrelevant if the palm does deal directly with RS232 logic signals.

I will assume that it does "directly deal with RS232 logic signals", because every website I found that had instruction on using these units with a computer make no mention of a MAX232 or similar chip. If you would have followed the link in the first post and read the article, you would have known this.

Michael