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post #9 of Old 09-18-2008
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I've always heard NMEA referred to as RS-232, not RS-422. The former can be implemented on 3 wires and swings from +5 to 0 volts, where the latter is a differential system which swings from +5 to 0 to -5. Macintoshes typically used an RS-422 serial bus, instead of the RS-232 used on the PC, because the more expensive bus allowed longer runs and higher speeds.

I treat NMEA as RS-232, and it works. It couldn't do that if it was RS-422. Typical GPSes (Garmin, etc.) have RS-232 poinouts, and don't support 422.

The biggest confusion over connections, is that most manuals fail to make clear that "RX" and "TX" (aka Receive and Transmit) are relative terms, not absolute. So the RX wire from one unit, goes to the TX wire of the next, when there's only a pair of them. Not "same to same" as most folks would expect.
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