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Old 06-14-2019, 02:16 AM   #1
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Garmin Echomap 70DV NMEA0183 RS232 or RS422

Hello, I have a Matsutec HP-528A AIS transponder that I want to connect to my Garmin Echomap 70DV chartplotter via NMEA0183


The HP-528A has RS232 or RS422 TX ports for NMEA0183 but I do not know if the Garmin NMEA RX wiring is RS232 or RS422? does anyone know which one it is?



I have included pictures of the NMEA ports


Top cable is GARMIN
Bottom pins are HP-528A



Thank Ian
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Old 06-14-2019, 05:07 AM   #2
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NMEA 0183 is based on RS-422, so that's what you want to use. Electrically, 0183 and RS-422 are the same thing. 0183 additionally defines a bunch of messages that you send over that RS-422 connection.


Lots of people connect RS-422 and RS-232 devices together, but it's a really bad practice since there is no assurance that it will work reliably. Given their specifications, they are NOT compatible. To work, it depends on data being valid at a time and voltage levels when it is undefined and may or may not be valid. But it works well enough, often enough, that people commonly do it.


To illustrate, I tried it once when the only interface I could get for my nav computer was an RS-232 USB adapter. It seemed to work fine, which is the lure of it all. But when I looked at the detailed data being received by Coastal Explorer, along with the correct messages being received were a steady trickle of corrupted, junk messages. CE was dutifully discarding them, perhaps because they made no sense, or perhaps because their CRC check failed. But do you really want a steady flow of corrupted data in your nav system?
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Old 06-14-2019, 06:44 AM   #3
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Hi, thanks for your reply, so to use NMEA port one on my Garmin, I wire AIS pin 6 - Garmin wire 2 Black and AIS pin 7 - Garmin wire 4 Brown?



would that be correct to view AIS data on the Garmin?


cheers Ian
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Old 06-14-2019, 01:23 PM   #4
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Quote:
Originally Posted by Yobbo View Post
Hi, thanks for your reply, so to use NMEA port one on my Garmin, I wire AIS pin 6 - Garmin wire 2 Black and AIS pin 7 - Garmin wire 4 Brown?



would that be correct to view AIS data on the Garmin?


cheers Ian

Partly, maybe. How's that for a non-answer?


Normally you need to wire the two Tx wires from the AIS to the two Rx wires on the Garmin. And when doing it, you connect plus and plus together (sometimes called A rather than plus), and connect minus and minus together (sometimes called B rather than minus).


But this raises an issue that I didn't notice earlier. This is one of the Garmins that doesn't implement proper 0183/RS-422, and doesn't have true differential signals. Hence only a single Tx and Rx for each port. Shame on them. Some old VHFs are like this too. You are going to have to find the Garmin manual and see how they say to wire it up.
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