On Tue, 11 May 2004 10:37:31 +0200, "Meindert Sprang"
wrote:

"Dick Locke" wrote in message
.. .

To follow up, the right solution was to hook the "high" output from
the GPS "current loop" to the GPS input of the VHF, and not to hook up
anything else.

Auch! That means that the return for the signal goes through the common
power ground. You want to make sure in this case that the main ground from
the battery goes to the VHF and from there on to the GPS. Otherwise the
voltage drop due to high current might influence the datasignal.

Makes me wonder everytime why manufacturers want to save on a $0.20
optocoupler. The NMEA standard is very clear on the REQUIREMENT for galvanic
isolation....
Too bad...

Meindert

Good idea. I still have to hook up a radar and possibly an autopilot
to the GPS too, so things will probably get more confusing. If I get
an expander, will that also avoid one device "contaminating" the
other?

The VHF is Uniden, I got it because of the WHAM wireless remote
microphone.

I guess this is why they call it a "current loop", the current makes a
tremendous loop.

"Dick Locke" wrote in message
...
Good idea. I still have to hook up a radar and possibly an autopilot
to the GPS too, so things will probably get more confusing. If I get
an expander, will that also avoid one device "contaminating" the
other?

Probably not. This would require an expander where each output is
galvanically isolated from the others. All expanders I know of are only
isolated from the power supply, while all outputs are fed from the same
DC/DC converter.

Meindert