Phil Stanton wrote:
Thanks for that, Dennis.
I have not tried to output to the ST60 Multi - one thing at a time. The +
input from the Multi is connected to pin 2 and the - input from the Multi is
connected to pin 5.
I will try shorting out the pins you suggest

Phil

Aha, this information (- connected to pin 5) combined with the fact that
if the laptop's internal zero is no longer floating compared to the boat
zero (= connect charger) means you are introducing a ground loop.

Face the facts, you need an optocoupler. If you don't know how to build
an appropriate circuit, get a RS-422 input port or an NMEA Multiplexer.

-- Kees