Get serial data from USB GPS receiver

Some years ago, I purchased a “GPS mouse”, a device that has magnetic base and connects to PC USB port to give GPS position to navigation program. I remember I purchased, also, a true licensed copy of Microsoft’s Autoroute 2005 program. This means I’m speaking about a device at least 7 years old…

Recently, I reloaded the software on a very old notebook that is normally hidden under the seat in my car ; it is just for emergency situations (I am able to lost even in a well known regions).

Good, having tried a new route and noticed that the way was every minute much harder, and forced to admit that it terminates on the gate of a haunted house, I tried to make a position search using the GPS receiver with the notebook.

But there was a problem… the device, when connected to the notebook, makes it turning off ! It was like a short circuit on USB connector. So, no position, no help, no GPS. I did my route backside until recognized a known road…

Ok, when at home, I decided to open the GPS receiver to look inside and indagate on the reason of such short circuit. The receiver model is BU-303 and has 2 hidden screws under a rubber plate that covers the magnet (I never attached the device on my car’s top, ‘cause it was originally good to connect satellites even if used inside the car). So, after removing the rubber plate and the screws, I opened the box. Oh, there is a very small battery inside, probably to retain some satellites data when powered off. This small battery was rated for 3.3Volts… Hmmm, using my multimeter to measure the residual charge, I read exactly ZERO volt. The battery is gone. Ok, it was directly┬á soldered on the circuit and I removed it easily. Now, connecting the USB to a notebook, all worked fine.

I decided to look inside and discovered, after removing a metallic shield, one unused option. As you can see in the picture below, the device can have USB or RS-232 connection !!! The unmounted IC seems (looking at the connections) very similar to one classic MAX232 or equivalent.

Connecting an oscilloscope and measuring the most probable pins, I discovered that the serial output was present on pin 11 of the missing IC, with amplitude of about 3V and protocol 4800,N,8,1. Also it was evident that with a small wire to connect one pad to the very near opposite, that signal can be “routed” to an easy point of the circuit to solder on, so the shield could be soldered again in its place. So I soldered a very small wire to the contacts as in figure :

The yellow dots are the points connected together. It was not easy… But at the end I mounted again the metallic shield on, and soldered 3 wires to make power and serial connections as in the final picture here :

Now, the circuit can be powered from 5V ( also tested and working at 3.3V ) and on the wire S-out you can get (at TTL level, around 3V) the various NMEA sentences that can be read, for example, from a microcontroller to make a satellite localization system, adding a GSM phone module to send you the actual position of your car via sms.

I also tried to connect the circuit to the debug port ( ttyS0 ) of a cheap Android tablet, but unfortunately the ttyS0, when forced by software commands to run at 4800 BPS, on such hardware (WM8650 based) can’t set the right baud rate and “shifts” automatically to 9600, that obviously isn’t useful for this work.

Anyway, I hope this will be useful for someone….

3 thoughts on “Get serial data from USB GPS receiver

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