Phil Stanton wrote:
I can connect the laptop through a
cigarette type laptop power supply to an external car battery and again no
problem. If I connect the negative of this battery to the negative of the
ship's battery I immediately loose the signal.

Sorry, but this still points to ground loops being the problem. The
optocoupler was supposed to solve this...

If, with the computer isolated and the NEMA going in correctly via the
serial port I connect the optocoupler input across the NEMA out terminals I
again loose the signal. Its as if there isn't enough power in the NEMA
signal to power anything.

Duh, that is strange...

Sorry, no further thoughts! Anybody else?

-- Kees