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Author Topic: Problems receiving data from GPS Garmin 18X LVC  (Read 1226 times)
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Hi Arduino Community :-) :-) :-)

I'm a newbie to Arduino but I have already had quite a bit of fun programming it in the past few days.

I'm trying to connect a GPS module (Garmin 18X LVC) to the Arduino (Mega) but I cannot manage to successfully receive the NMEA sentences transmitted by the module.
The Garmin 18X LVC doesn't have a USB or Serial connector but has just 6 bare wires (Vin, GND, TXD, GND, RCV and a wire conveying a 1 pulse-per-second signal). Vin is 5V and the specs say that the "CMOS Serial Output Level" is "0 Vdc to Vin (Asynchronous serial TIA-232 F compatible polarity)". The baud rate is 4800 by default.
From that I deduce that the communication should be compatible with the TTL signal levels (am I wrong???) the board can take.

I tried to follow what described in "Arduino Playground: Connecting a GPS Parallax to Arduino" but I get a series of gibberish in both cases and nothing intelligible...

Has anybody any suggestion about what I could be doing wrong?
I'm afraid I'm missing something pretty fundamental here.  

Thank you heaps!!!

LucaQ
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It does look like you are barking up the right tree. Do you have anything else you can try connecting the gps module to? Maybe a serial to TTL cable connected to something else? You need to figure out if your trouble is the GPS module or the Arduino Mega. You could also use a second Arduino to generate 4800 baud ttl serial and see if you can receive that at all on the Arduino Mega.
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The GPS18 interface is non-standard.  It's RS232 levels, but TTL voltages.  You need to run it through an inverter (e.g. 7404) before connecting it to the arduino.

-j

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LucaQ,

Regarding the Garmin 18x LVC interfacing problem:

The data output on pin 1(whi wire) of the Garmin actually swings from -5V to +5V, if you look at it on a scope. The data sheet says that it "transmits voltage levels from ground to the input voltage", but this is not true!!!

One way to interface the Garmin with an Arduino is to run the +_5V output of the Garmin through a MAX232 chip to convert it to TTL levels, then add an 7404 inverter to reinvert it to normal polarity.

The output of the 7404 should give you a good signal. Or try using the uninverted signal to see which one works.

Dave
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Quote
One way to interface the Garmin with an Arduino is to run the +_5V output of the Garmin through a MAX232 chip to convert it to TTL levels, then add an 7404 inverter to reinvert it to normal polarity.
The MAX232 is inverting - you don't want to use the 7404 in conjunction with the MAX232.

-j
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