Success with ST60 Multi NMEA optoisolator circuit
I have put together a simple output circuit as suggested by Norman and tested it on the boat (see attached JPEG file for diagram).
Some notes – The ST60 Multi’s NMEA out + goes thru a 1K resistor in series with an LED (light emitting diode) which in turn is optically coupled to the transistor section of the optoisolator IC (an NTE3041 - $1.49 @ Fry’s electronics). NMEA – is the return from the LED.
On the PC serial port side (I used a USB Serial adaptor – IOGEAR GUC232A) DTR (data terminal ready – pin 4) provides the +10v power to the circuit (DTR is only positive when the serial port is open – ex. When the application HyperTerminal is connected). A 1K resistor is in series with the collector of the transistor. The emitter is connected to the receive pin of the serial port (RXD pin 2) and in turn goes to another 1K resistor to ground of the serial port (GND pin 5)
I ran my test at 4800 baud and all waveforms and voltage levels looked fine.
The ST60 Multi provided the following NMEA messages every 2 sec (while sitting at the dock in San Francisco Bay with a little under 20 knots of wind).
$IIHDM,185,M*30 (magnetic heading)
$IIMTW,013.4,C*25 (water temp)
$IIMWV,092,R,016.2,K,A*28 (wind speed and angle)
$IIVHW,199,T,184,M,00.00,N,00.00,K*59 (water speed and heading)
So now that I have the electrical interface working it is on to my software projects.