wired connection to BT GPS problem

leonidm · Occasional Visitor Joined: Apr 21, 2007 Posts: 5

I have the folowing combination:
1) treo 700p
2) seidio car kit G2500
3) Solar Bluetooth GPS Receiver based on MediaTek MT3 BT 2.3MR chipset
http://www.pocketgpsworld.com/SolarBTGPS.php
I beleive GPS can also act as a serial gps -it has a mini-USB connector that has GND, Vcc, Serial Rx andTx at 3.3v

I made a cable and hooked up GPS to the car kit - the charging function works, but the GPS part doesn't. I tried 9600,8,N,1 and pretty much every other speed and combination. If i start ptelnet and configure it to look at serial port , it does see traffic from gps, but it looks like garbage at any speed I tried. It doesn't look like a readable strings serial gps would send. I hooked up gps to a scope and saw a nice square wave with signal level of 3.3v BTW, it works fine in BlueTooth mode.
Have anybody did something similar?

philpugh · Posted: Mon Apr 23, 2007 11:24 am Post subject:

I would expect the data to be 4800 bps if it's transmitting NMEA sentences. It could be transmitting proprietary sentences - you would need to check in the manual.

3.3V is probably on the margin for PC serial input and may benefit from a level shift.

When you say 'square wave' did you mean just that - regular high and low levels with equal timing? I wouldn't expect that - rather a pulse waveform with unequal high/low timings which is psuedo-random due to different 'characters' being transmitted. If you can get timings of the waveforms you should be able to deduce the bit rate of the communications.
_________________
Phil

leonidm · Occasional Visitor Joined: Apr 21, 2007 Posts: 5

Hi,
Thank you for your reply. I'm connecting this to the phone's serial port, no PC's serial port - Treo serial port levels are 3.3v as far as I know. The square wave is not a clock, it's "pseudo random" so it is some kind of signal, and I was hoping it's rs232,It does look like burst of strings, similart to what you'd expect from GPS, but I can be wrong. Manual doesn't mention anything about it other then stating you can buy a cable thet will trnsfwer this to USB (I beleive it's rs232-2-usb cable) I will try to determine baudrate based on the signal frequincy on the clock. however setting phone's terminal emulation to 4800 still prodused garbaged strings on the screen, also the length of the strings looks right.

AllyCat · Posted: Mon Apr 23, 2007 8:02 pm Post subject:

Hi,

It may well be "TTL/CMOS" serial data, which not only has 0/3.3 v dc levels (rather than the +/- of RS232) but inverted relative to normal RS232. You can convert between the two using a generic "MAX232" integrated circuit.

This is the interface on my Holux GPSslim 236 (5-pin type B mini USB), for which the USB-serial cable is available from Clove technology.

Cheers, Alan.
_________________
Garmin GPS72H/76/60/45, Etrex H, Mapsource v6.5.
Acer N50,HP114,Loox N560,Dell x50,CF/SD cards to 4/32GB.
RoyalTek,Holux236,Navman B10 & Copilot(Globalsat) BT GPS,TomTom5/6.
Memory Map (v5.4.2 & v5.1.3 OS & Euro), GPS gate,OSGPSconverter.

leonidm · Occasional Visitor Joined: Apr 21, 2007 Posts: 5

Hi Alan.
I'm connecting my GPS to palm based device not PC ,so it's not RS-232, it should be 0-3.3v, however you might be right about invertion. Is there any way to know? will I see it on the scope? I'l try to decode characters on the scope and see if they make sense after invertion.

AllyCat · Posted: Mon Apr 23, 2007 8:44 pm Post subject:

Hi,

NMEA data consists relatively short sentences of characters, so you should be able to determine the inversion (or not) of the data from the leading edge (start) of each message. Here's a sample found by Google:

http://www.picotech.com/experiments/serial_data/results.html

Cheers, Alan.
_________________
Garmin GPS72H/76/60/45, Etrex H, Mapsource v6.5.
Acer N50,HP114,Loox N560,Dell x50,CF/SD cards to 4/32GB.
RoyalTek,Holux236,Navman B10 & Copilot(Globalsat) BT GPS,TomTom5/6.
Memory Map (v5.4.2 & v5.1.3 OS & Euro), GPS gate,OSGPSconverter.

leonidm · Occasional Visitor Joined: Apr 21, 2007 Posts: 5

ok, I confirmed that the data coming out of GPS is a real simple UART at 9600 8N1 - After conneting it to a standard CMOS/TTLlevel converter I can see NMEA strings on a PC. The problem seems to be on either Treo's serial port or the car kit. Perhaps the car inverts the data. I will do some more tests on Friday. Thank you for your help.

philpugh · Posted: Thu Apr 26, 2007 1:42 pm Post subject:

Glad to hear you are getting somewhere.

The last problem may be really simple...

Your car kit comprises a cable to connect your GPS to your TREO (right?)

Usually they will supply a cable that is expecting to connect to a PC. This won't work with a GPS as it's serial output is expecting to talk to a PC also - you need to insert a null-modem - either by buying one or by simply swapping the transmit and receive pins in one of the connectors - I am assuming that it (GPS) isn't supplying any modem flow-control (RTS, DTR etc) as they don't usually - so you can ignore the other connections in the serial cable. All you need is TRANSMIT, RECEIVE and DATA GROUND.
_________________
Phil

leonidm · Occasional Visitor Joined: Apr 21, 2007 Posts: 5

OK, I found the problem - the car kit does have a small inverter board inside - once I've bypassed it , it started to work fine. Thank you ,everybody!

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