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post #3 of Old 02-23-2010
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That's right, though there are a few things to watch for. Chances are it will work fine first off. If not, here are some things I've encountered:

The NMEA standard actually calls for balanced (neither side grounded) signals. You can usually connect NMEA - to PC ground, but there are exceptions.

A Raymarine ST60 Multi, for example, sources steady voltage on NMEA+ and switches the NMEA- output. I use a multi at the nav station as a convenient source of NMEA, and found that it worked if the PC (with a USB-serial dongle) was connected to only the ST60, but when I also plugged in the USB cable from my Actisense mux, it died. Reason was that the mux grounded the PC to the boat's negative, effectively sorting out the Multi's NMEA.

I also ran into an Eagle GPS which had some "protection" circuitry in its serial output cable, consisting of a diode and series resistor. This did not provide enough signal to drive another device, in his case a VHF radio's DSC input.
The circuit would have made more sense on the input, rather than output, but I didn't design it. Maybe it was a manufacturing error.

(If anyone wonders why I'd use the Multi's output when I have an Actisense mux, it's because the latter is used to combine VHF DSC out with the Autopilot's 10 Hz Fast Heading output for the E120 Multifunction Display. The combined output is nearly flooded by HDG sentences. I also use th Multi's output to send wind speed/direction to a serial terminal server which is on my WiFi, so I can check on conditions when I'm away from the boat, when the Chartplotter and Mux are off but the Wind instrument and Multi stay on. They're on now, even as the boat is on its cradle, and I can check wind with NavMonPC or PolarCom.

A lot of words to qualify which it will "probably" work. Good luck.
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