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