Phil,
What you need is a Seatalk to NMEA converter. The Raymarine ST60 output
is seatalk NOT NMEA. If you use the Raymarine navigation software
package then it will decode the seatalk data stream from the ST60
directly without the converter.
krj.

Phil Stanton wrote:
Can anyone help
I have a Raymarine ST60 Multi and want to get NMEA data (Wind & Depth) into
the 9 pin serial port on my laptop. If the laptop is not connected to
anything at all (including power supplies) this works fine. If I connect it
to the boats 12v supply via a cigarette type laptop power supply, or an
inverter using the normal laptop power lead I get nothing.
Equally if I connect the USB port via a Serial to USB lead to the GPS and
AIS input I immediately loose the NMEA input.
I have been told I need to use an optocoupler (optical isolator) and have
bought a 4N25 chip.
Can anyone suggest what the problem is and / or give me a circuit diagram of
how to use this optocoupler

many thanks

Phil