"Larry W4CSC" wrote in message
...
I'm building an extensive NMEA network aboard "Lionheart" for its
owner who likes to have 2 of all the toys. I encountered a problem
adding a Yeoman to interface the paper charts to the network,
yesterday, that might be a good idea for everyone to know about.......

snip

Luckily, no damage was done. The Noland had simply latched to protect
itself and recycling the master power relay reset it to working again.

Has nothing to do with protecting itself. Just something that happens if you
overload an *unprotected* output on a CMOS circuit. Eventually the circuit
breaks.

big snip

Now with all the wires disconnected from my bruised NMEA network, I
put the logic probe on the Yeoman's input and output wires. YAAACK!
THE INPUT WIRES ARE HIGH.....BOTH OF THEM!! How can this be? This is
the INPUT, not the output! A reading with my DVM showed battery
voltage, 13.8V with no connection to them...both of them.

And this again proves the nessecity of galvanically isolated inputs on NMEA
devices. But many manufacturers try to save a dime and omit a cheap
optocoupler.
And it gets worse: even Noland dropped the galvanic isolation on their new
"improved" models.

commercial mode on
So, get the best multiplexers you can get: www.shipmodul.com
commercial mode off

Meindert