SeaTalk1 to NMEA 0183 converter, DIY

Updated 2024-08-30 ! 

!! Another SW solution is available for RPi 5 here ! using the same HW.

Thanks to Thomas, (the guy with the nice marine interface board), it's now possible to decode, one way, the SeaTalk1(ST1) sentences in a pretty simple way. You just use OpenPlotter(OP) and/or SignalK(SK) and an Optocoupler.

The thing that made it, is that Thomas has released a "how to" and Python source code, that will read the data from a GPIO pin on the Raspberry Pi, (RPi), and format it so the standard parser in SignalK can translate it to SignalK delta format. It is then available as NMEA 0183. It's even possible to send the data in to a NMEA 2000 network, with a proper hardware interface.

Hardware

Prerequisites is a Raspberry Pi up and running OpenPlotter and/or Signal K and an interface circuit according to the picture.

The circuit is referring to this optocoupler board, but a similar product can of course be used. The LED in the circuit will flicker when there is ST1 traffic. Choosing an optocoupler as the hardware interface is a smart way to avoid ground loops and creates electrical isolation from hazardous voltages. This interface is not inverting the signal.

If you are building the interface yourself use the second circuit instead. If you don't want any flickering just drop the LED at the input. 

The most simple interface, works well on our boat, is where You use a small signal NPN transistor which shifts the DC level, from 12 V DC to 3,3 V DC, and inverts the signal.

Software

Install

Start, in a terminal/ssh session, with an update and install 

sudo apt-get update && sudo apt-get install pigpio python-pigpio python3-pigpio

and check with

pigpiod -v

which will return something like

79

which is the version number, 7.9. Go on getting the Python code with

wget https://raw.githubusercontent.com/MatsA/seatalk1-to-NMEA0183/master/STALK_read.py

The program is set up to match the opto circuit above.

pigpio

The Python program is dependent on that the "pigpio" library is launched as a demon. Start it with 

sudo pigpiod Which starts a socket interface on port 8888 !! which is used by the Python program.

then start the program with 

sudo python STALK_read.py

if You succeeded You will see something like

STALK,11,1,4,5 

STALK,25,4,e8,51,12,33,3

STALK,24,2,0,0,20 

STALK,26,4,0,0,0,0,20   

STALK,9c,31,13,fe 

showing the decoded ST1 messages. Next step is to check that the messages is sent out as an UDP stream at local host, port 4141. This is done with opening a new terminal/ssh session and issue the command

nc -ulkw 0 127.0.0.1 4041

and the same list of ST1 messages will appear. Terminate with "Ctrl + c".

SignalK

Connection

To get the ST1 messages into SignalK, using the optocoupler setup, You have to set up a new connection in SignalK. Setup according to the picture below. After restarting SK, check out the ST1 messages in the "Data Browser".

Terminate the Python process with "Ctrl + c" and kill the "pigpio" daemon with

sudo killall pigpiod

Prepare for "production"

Move the Python program to a better place 

sudo mv STALK_read.py /usr/local/sbin

systemd service

We will set up "STALK_read.py" as a systemd service, autostart at boot and restart at failure, which also will start the "pigpio" daemon. Get the service file

wget https://raw.githubusercontent.com/MatsA/seatalk1-to-NMEA0183/master/SeaTalk.service

and move it to the right place with

sudo mv SeaTalk.service /etc/systemd/system

then activate it with 

sudo systemctl daemon-reload && sudo systemctl enable SeaTalk && sudo systemctl restart SeaTalk

Check ST 1 messages

Check with

sudo systemctl status SeaTalk

and something like this will be listed

sudo systemctl status SeaTalk

● SeaTalk.service - SeaTalk daemon

   Loaded: loaded (/etc/systemd/system/SeaTalk.service; enabled; vendor preset: enabled)

   Active: active (running) since Sun 2020-06-14 11:04:50 CEST; 44s ago

     Docs: http://pysselilivet.blogspot.com/2020/06/seatalk1-to-nmea-0183-converter-diy.html

  Process: 534 ExecStartPre=/usr/local/bin/pigpiod (code=exited, status=0/SUCCESS)

 Main PID: 544 (python)

    Tasks: 8 (limit: 4035)

   Memory: 7.1M

   CGroup: /system.slice/SeaTalk.service

           ├─539 /usr/local/bin/pigpiod

           └─544 /usr/bin/python /usr/local/sbin/STALK_read.py

Jun 14 11:05:29 zest SeaTalk[544]: $STALK,9c,31,18,ff

Jun 14 11:05:29 zest SeaTalk[544]: $STALK,57,70,2

Jun 14 11:05:29 zest SeaTalk[544]: $STALK,58,25,38,1b,64,c,8d,d0

Jun 14 11:05:29 zest SeaTalk[544]: $STALK,84,36,98,88,40,0,ff,2,17

Jun 14 11:05:29 zest SeaTalk[544]: $STALK,25,4,e8,51,12,33,3

Jun 14 11:05:29 zest SeaTalk[544]: $STALK,24,2,0,0,20

Jun 14 11:05:29 zest SeaTalk[544]: $STALK,26,4,0,0,0,0,20

Jun 14 11:05:29 zest SeaTalk[544]: $STALK,20,1,0,0

and then check the ST1 messages in "Data Browser". 

If You don't want to clutter Your log with the ST1 sentences, just edit the Python code 

sudo nano /usr/local/sbin/STALK_read.py

and make the "print (data)" line start with an "#" and reactivate the service with the above commands.

Now You are ready for "production" !

Remarks

Change port

If You want to change the daemon to another port You have to change the pigpiod.service file with the -p option, and the Python program. 

Signal K

Please note ! With help from @tkurki I added this conversion feature to the standard SK server, V1.32. Documentation found here.

Open plotter

This setup will soon be added to the documentation but in the meantime please check this post in the OP forum.

Fixes

2020-08-18 The "STALK_read.py" program is updated with 2 bug fixes !! These are also applied to the SK server version.

2021-02-18 The "STALK_read.py" updated.

2022-11-03 SeaTalk.service updated.

2023-07-29 Hardware interface updated at input to the optocoupler board. 

WiFi converter

Why don't use the above setup with a Raspberry Zero W, with the one transistor interface, to get ST1 input, via WiFi, to Your SK server ?

Hardware tuning

The mentioned hardware solution worked in my environment but Marco have done a deeper analyze, with an oscilloscope, which can be checked out here. This post update, done 2023-07-29, is taken that info in consideration.