ultimate gps and pc serial port

[bilsch]

Has anyone tried using the ultimate gps module on a standard pc serial port?

All I get from minicom is a bunch of jibberish

I have verified the serial config in minicom - 9600 8n1 no flow control

[tastewar]

I know all the doc says 9600 and that seems to be standard now for gps, but have you tried any other rates? 4800 was standard for gps for a long time; I've seen 19200...

[bilsch]

I did try other rates out of curiosity however i know for sure it is 9600

With the ftdi adapter 9600 8n1 works but I want to use the hardware serial port on a linux x86_64 box

Out of curiosity I tried another usb to serial adapter ( olllld adapter :-> ) I get the same thing - just a bunch of jibberish

[bilsch]

I'm not sure if using a multimeter will be accurate enough but thought it worth a shot.

So, when I hook the positive end of my multimeter to the tx out from the gps I see it get to around 3 volts. Is it possible the hardware serial adapter is giving me garbage because the signal voltage is too low?

http://en.wikipedia.org/wiki/RS-232#Voltage_levels

Valid signals are either in the range of +3 to +15 volts or the range -3 to -15 volts; the range between -3 to +3 volts is not a valid RS-232 level

[tastewar]

Sounds quite plausible. There's a link to a little board that would help in this thread:

http://forums.adafruit.com/viewtopic.php?f=10&t=32437

[adafruit_support_bill]

Yes a TTL to serial adapter is needed for interfacing with an RS232 serial port. The signal voltages on RS232 can cause damage to the GPS module.

[bilsch]

Yes a TTL to serial adapter is needed for interfacing with an RS232 serial port. The signal voltages on RS232 can cause damage to the GPS module.

What is the voltage of the gps module and what voltage is a pc serial port? Assuming the voltage off of rs232 serial port is higher than the gps module I suppose I could try to limit the voltage?

[tastewar]

But you have to boost the voltage in the other direction. There's a whole family of chips (e.g. MAX232) that bi-directionally convert between TTL level voltages and RS-232 levels. That little board I linked to (here it is again: http://www.suntekstore.com/goods-14002601-mini_rs232_to_ttl_converter_module_board_.html) has the support circuitry built-in, which is nice, but it's a crappy little thing otherwise. I went through 3 of them trying to solder wires to the little pads, and having the pads lift off the board :-/

Hence my asking if Adafruit would consider producing a similar product. Cause their stuff rocks.

[bilsch]

tastewar,

I completely missed your previous posting thanks I'll see if I can dig one of those up!

[bilsch]

tastewar,

That maxim chip would very likely resolve the rx/tx side of the equation ( my only stated need in this thread ) but since this is for a gps unit I plan to also use pps as a part of the time sync for additional accuracy.

Oddly, with pps connected to the cts pin on the ftdi board I don't actually get the pulse registered in the kernel pps driver. I'm going to try out the gpio approach on my pi but my C foo is ... well, very very limited so far. i had posted on this previously and no one responded :-< ( viewtopic.php?f=19&t=36030&start=0&hilit=pi+gpio+pps )

Thanks for the hint about the maxim chip!

[tastewar]

I wonder if this tutorial, that I bookmarked earlier for myself, would help. It talks about using the GPIO serial port. Definitely avoids the need for level shifting!

http://learn.adafruit.com/adafruit-ultimate-gps-on-the-raspberry-pi/using-uart-instead-of-usb

[bilsch]

I wonder if this tutorial, that I bookmarked earlier for myself, would help. It talks about using the GPIO serial port. Definitely avoids the need for level shifting!

Yes, that does work though note they are not touching the pps side of the equasion.

Here are the links I'm looking over:
http://www.raspberrypi.org/phpBB3/viewt ... 8&p=129224
http://www.raspberrypi.org/phpBB3/viewt ... 06#p142606

It seems as though others have had the same idea re using the pi and gpio for the pps ( and hardware serial uart for comms ).

Curious this has not been merged in to the raspberry pi kernel yet. Guess if I can get it working I'll send a pull request.