Thanks Kees

Looks as if I will have to pay through the nose to get an optocoupler, but
have bitten the bullet and ordered one.
Thanks for everyone's input

Phil
"Kees Verruijt" wrote in message
...
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

Classic combination of mixing (incompatible) RS-232 and RS-422 signals.

If you were trying to do this the other way around (RS-232 to RS-422) you
could try connecting the RS-232 "Ground" to the ST60's "-" input, as the
ST60 has isolated inputs and no ground loop would be created.

In the case of ST60 output to RS232 input i'm not so sure you can do this;
you may end up with damaged hardware if you tried connecting the RS-232
"ground" level to the ST60's "-" output did you. However, as you are
experiencing, connecting just the "A" output doesn't work.

You could risk trying to tie the ST60's "-" output to the RS-232 ground...

The other alternative, if you are not an electrical engineer and unable to
wire up the optocoupler, I suggest you get a RS-422 to RS-232 converter,
for instance as present in the various NMEA multiplexers.

-- Kees