Darkies Bum :

Its data console however doesn't produce the wind dir and speed (there are
constraints in CPN on some consoles if no SOG/COG is avail to calc
apparent wind etc. which is understandable) for say "Ground Wind".

Yes, it might be the cause too. Even though CPN should be able to display
the apparent wind without speed information, it may be that the programmers
were a bit lazy. There is a similar bug in MaxSea*: if MaxSea has not
received any GPS localisation sentence, it won't display the magnetic
heading from the compass... (I once lost an entire afternoon trying to
figure out why that new compass would not work...)

I'll look at the translation if required althought this may be beyond my
expertise just now.

I have a piece of code to multiplex and analyse the nmea sentences on the
PC, it should take only a few extra lines to make it perform this
translation, so let me know.

I will also check that the 3 wires for nmea inputs are similar - I only
used the first one I came to.

3 wires? Normally, there are 2 nmea wires per device. Is one of the data
wires shared with the ground?

Can one wire more than one pair of listeners to the "ISO OUT 1"?

Yes, the listeners are passive devices that just measure the difference of
potential between the two ports, so it's essentially like pluging voltmeters
in parallel. The NDC4 manual encourages doing it.

It's pluging several talkers on one listener that is dangerous*: there, the
output ports of the talkers, whose potentials vary independantly, are
connected by simple wires with a tiny resistance...