We use MAX3232 chips to convert from RS232 signal levels to 5V ttl levels, digikey part number MAX3232CPE+-ND. It requires 5 external stabilizing and charging capacitors.
The Sparkfun boards stopped working; this might have been because TX was connected to TX, which I've found a source (somewhere, I believe it was a LadyAda serial tutorial) that says this will damage them. In future, make sure we know which pin goes where.
With no signals connected, 12V power connected for the wind sensor's power, and 5V to power the compass and MAX3232 chip, the following voltages are observed:
Compass DB9 Connector
Pin 2 - 0V Pin 3 - 7.4V Pin 4 - 5V (power to compass) Pin 5 - 0V (ground)
Wind DB9 Connector
Pin 2 - 0V Pin 3 - 7.4V Pin 4 - 5V Pin 5 - 0V (ground) Pin 7 - 15V (power to PB100)
TTL level connector
RX lines - 5V TX lines - 0V
To build the RS232 converter in small size took 6 people hours.