A serial port for the TomTom Bluetooth Receiver

On first sight, why would you need it? Either you have inherited a TomTom Receiver and only own a Palm with serial port or you need to service one of the receivers and don't have a bluetooth compatible device or lacking the software. For more information about the TomTom family of GPS products visit TomTom.
The TomTom receiver is equipped with the SIRF II chipset and uses a serial 19200 baud datastream to communicate with the Samsung bluetooth modul. Fortunately, this signal is available at the small expansion/test port seen here on the low edge of the printed cicuit board.

The serial output is the 3rd pin from the right. This output is inverted when compared to standard RS232, hence a little circuit is added to adjust the polarity and protect the GPS receiver:

Pinouts for the serial port are for the standard PC SubD 9-pin connector and the Palm cradle.
To protect the receiver battery, no current is drawn from it. While the Palm cradle supplies a positive voltage of about
5 Volts on pin 6, the PC doesn't have it, an external 5 volt tap is necessary here. To increase the voltage range for the Palm,
i connected Ground from the GPS receiver to a pin carrying -4 Volts on the Palm, improving RS232 compatibility.
Note that supplying both devices from a single ground would lead to problems. In this case just connect ground to pin 5 of the cradle port instead of pin 2.
The circuit was successfully tested on the PC with MicroDEM, a free program for managing digital elevation models and on the Palm 3 using CetusGPS
Set your GPS software to use Standard NMEA data,19200 baud, 8bit, 1 stop bit and no parity.
Have fun !!

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