Project: Linux Autopilot for Yamaha HARMO

Goal: Control a Yamaha HARMO electric drive via NMEA 2000 using a Linux computer and Open Source software (Signal K / PyPilot) with a focus on low cost and high reliability.

Important Note on HARMO: The Yamaha HARMO system is a sophisticated drive-by-wire system. While it utilizes NMEA 2000, Yamaha sometimes utilizes proprietary PGNs (Parameter Group Numbers). Successful integration depends on the Linux stack's ability to send standard "Heading/Track Control" messages (PGN 127252) that the HARMO ECU accepts.

The Cheapest Reliable Solution: SocketCAN

The absolute cheapest way to interface a Linux computer with NMEA 2000 is to use a generic USB-to-CAN adapter supported by the Linux kernel's SocketCAN subsystem. This avoids expensive proprietary gateways.

1. Hardware Interface

You need a device that bridges USB to the CAN-Bus physical layer.

Recommended Device: CANable / CANable Pro

Cost: ~$25 - $35 USD.
Chipset: STM32 with CandleLight firmware (Native Linux USB support).
Why: It appears as a standard network interface (can0) in Linux. It is open-source hardware.
Where to buy: Amazon, Mouser, or directly from the manufacturer (Linklayer Labs).

Alternative: Any adapter using the PCAN-USB driver (Peak System clones) or Lawicel protocol. Avoid cheap ELM327 OBD2 scanners; they are too slow for autopilot control loops.

2. Cabling (DIY)

NMEA 2000 is electrically compatible with CAN Bus, but the connectors differ.

Step A: Purchase a standard NMEA 2000 Drop Cable (Male or Female depending on your backbone).
Step B: Cut off one end of the cable. Strip the wires.
Step C: Identify the CAN wires (usually Blue for CAN High, White for CAN Low. Note: Verify your specific cable datasheet).
Step D: Connect the Blue and White wires to the CAN H and CAN L terminals on your USB-CAN adapter. Connect the Shield/Ground wire to the adapter ground.

3. Software Stack

This is where the magic happens. We will use Signal K as the translation layer.

Step 3.1: Configure Linux Network

Plug in the USB-CAN adapter. Check if it is recognized:

ip link show

You should see an interface like can0. Set the bitrate (NMEA 2000 runs at 250kbps):

sudo ip link set can0 up type can bitrate 250000

Tip: Add this command to your rc.local or systemd startup script so it runs on boot.

Step 3.2: Install Signal K Server

Signal K is the modern standard for marine data on Linux. It handles the heavy lifting of converting NMEA 2000 (binary) to JSON and vice versa.

Install Node.js on your Linux computer.
Install Signal K Server: sudo npm install -g signalk-server
Run the server: signalk-server

Step 3.3: Enable the NMEA 2000 Plugin

Open the Signal K dashboard in your browser (usually http://localhost:3000).
Go to Appstore -> Install signalk-to-nmea2000.
Go to Settings -> Plugin Config.
Enable the plugin and ensure it is linked to your can0 interface (SocketCAN).

4. Autopilot Control Logic (PyPilot)

PyPilot is a fantastic open-source autopilot, but it is typically designed to control a servo motor via a motor controller.

Since you have a HARMO, you do not need a servo; you need to send data commands. You have two paths here:

Path A: PyPilot + Signal K (Recommended)

PyPilot acts as the "Brain". It calculates the required Rudder Angle or Heading.

Install PyPilot on the Linux computer.
Configure PyPilot to output the "Autopilot State" and "Commanded Rudder Angle" to Signal K.
In Signal K, map the steering.autopilot.active and steering.rudderAngle paths to NMEA 2000 PGNs.

Path B: OpenCPN (Simpler Navigation)

If you just want the boat to follow a route calculated by OpenCPN:

OpenCPN can send "Track Control" messages directly via Signal K.
PGN 127252 (Heading/Track Control) is the standard message the HARMO needs to listen to in order to turn the motor.

The "Cheapest Reliable" Shopping List

Item	Est. Cost
CANable USB Adapter (USB-CAN)	$30 USD
NMEA 2000 Drop Cable (to hack)	$15 USD
Linux Computer (You already have this)	$0
Total	~$45 USD

Why this is better than serial converters

Cheap serial converters (like NGT-1 clones) often buffer data too slowly, causing lag in the autopilot loop. SocketCAN puts the CAN interface directly into the Linux networking stack, providing the lowest possible latency and high reliability for a fraction of the cost of a commercial gateway like the Actisense NGT-1 ($300+).