How to Use NMEA-0183 With Raspberry Pi

NMEA-0183 Is an electrical standard to connect GPS, SONAR, sensors, auto pilot units etc. in ships and boats. In difference to the newer NMEA 2000 standard (based on CAN) the NMEA 0183 is based on EIA RS422 (some older and/or simple systems use RS-232, or a single wire).

I want to show you how to connect a Raspberry Pi 3B to any NMEA-0183 device with differential output. Although the standard calls for isolated inputs and outputs its useful to use a RS422 / RS485 HAT with isolated interface.

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Step 1: Tools and Software

Materials:

Raspberry Pi

RS422 / RS485 HAT

a serial NMEA0183 device

Software:

Raspbian Stretch

NMEA simulator

Step 2: Connection to NMEA 0183

In the picture above you can see a typical NMEA device with differential output. The terminals are NMEA OUT+ and NMEA OUT- or TX+ or TX-. The NMEA IN+ and NMEA IN- wires are optional.

if you have a single transmit wire from your device (most likely labeled
TX or NMEA OUT or something like that), then your device uses the RS-232 protocol. In this case you will need a simple RS232 converter.

Step 3: DIP Switch Settings

Step 4: Free Up the Serial Line and Enable UART of the Raspberry Pi

The easiest way is to use the raspi-config tool to switch the UART to the GPIO14/15 pins.
take a fresh Raspbian image

sudo raspi-config

goto '5 Interfacing Options'

goto 'P6 Serial'

'Would you like a login shell to be accessible over serial?' --> NO

'Would you like the serial port hardware to be enabled?' --> YES

Finish raspi-config

reboot the Raspberry Pi

Now you can access the UART via /dev/serial0

Step 5: Firmware

You can find a lot of different NMEA-0183 software Python stacks for Raspberry Pi A very simple solution is the NMEA library by Nick Sweeting:

https://github.com/nsweeting/NMEA0183

Please note: pyserial is required for serial connections:

https://github.com/nsweeting/NMEA0183

Step 6: Test Run

The Python program will decode incoming NMEA protocols. If you have no NMEA device at home, you can also use a Simulator on your PC and a simple USB to RS485 adaptor instead of a real device.

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