Converting GPS Serial Output to TTL levels

Not realizing the output voltage of the GPS we are using, I wired it directly up to our new Arduino. After three hours of it not working, it occurred to me that the voltage output may be at RS232 levels (plus or minus 3 to 15 volts) instead of TTL (0 through 5 volts) levels. A quick check on the oscilloscope confirmed that the Garmin unit we are using outputs at plus and minus 6 volts. I ordered a sample of an ADM232 which converts between RS232 levels and TTL levels. Temporarily, we are using an OPA2350 rail to rail op amp to limit the output from 0-5 volts. This solution works, and we are now able to read NMEA data through the Arduino.
Here is the schematic of the circuit that we are using:

Reading NMEA data from the Garmin 76CSx

Knowing that the Garmin 76CSx is capable of outputting NMEA data, I needed to figure out how to obtain such data in a way that is easy for a microcontroller to process. The 76CSx has 2 ports on the back: a USB port, and a proprietary serial port. Without any modifications, ussing PuTTY to establish a connection via USB failed. The reason for this is USB is, although a serial connection, is not a true serial port. A workaround to this problem is to use a program from Garmin called Spanner. Spanner creates a virtual serial port from a USB port, and allows a terminal session to send/receive data over USB. Although Spanner is not officially supported by Garmin for use with the 76CSx, it worked perfectly and NMEA data instantly filled my terminal window. While this established that it was possible to read NMEA data from the unit, it does not meet my requirement of being able to read the data from a microcontroller as Spanner requires Windows to run. The obvious solution would be to try reading from Garmin's proprietary serial port, but the cable is $40! Before I spend any kind of money on a glorified (simplified actually, the Garmin connector is only 4 pins) RS232 cable, I wanted to make sure that I could get the data I needed from the serial port.

Using the diagram below, I attached the 76CSx to a logic analyzer.

The waveform I captured is below.

The way NMEA data is formatted dictates that each command should begin with a dollar sign followed by a name identifier that is specific to the type of device; thus, the first 3 characters of each message should have been "$GP" without the quotes. Translated form ASCII to binary, the expected result is "0010 0100 X 0100 0111 X 0101 0000 X" where "X" is a stop bit. If you take the time to analyze the output from the logic analyzer, it does not appear that I received the correct data at all. There may be something I don't know about this type of serial connection; for example, the data may be sent last bit first, or I my waveform may be from the middle of a command etc.

Since I was encouraged that SOME kind of data was being output, Matt and I decided to fashion a very crude Garmin -> RS232 cable (not anything close to a permanent solution, we literally held the wires with our fingers), and see what happened. The attempt was successful and once again, I saw a terminal window fill with, what appeared to be, valid NMEA data.
(Pictures soon to follow)

Knowing that we could now use the Garmin serial connection, I decided to make a more permanent Garmin -> RS232 cable. I found a website that will send you a custom made, injection molded, silicon connector for any Garmin model, as long as you promise to send them some form of compensation The average user sends a paypal payment of $10, but they will accept other amounts. (tangible goods are also accepted). I ordered 2 plugs and will document the construction of my cable when they arrive.

NMEA 0183

NMEA 0183 (or NMEA for short) is a combined electrical and data specification for communication between marine electronic devices such as echo sounder, sonars, anemometer (wind speed and direction), gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined by, and is controlled by, the U.S.-based National Marine Electronics Association.


Link to the wikipedia article.
Arduino NMEA device driver library.