In configuration.nix or equivalent, enable legacy compatibility mode in bluetoothd:
systemd.services.bluetooth.serviceConfig.ExecStart = lib.mkForce [
""
"${config.hardware.bluetooth.package}/libexec/bluetooth/bluetoothd -f /etc/bluetooth/main.conf -C"
];
It is probably best to reboot your machine after this. Next, pair and trust your ELM327 device:
tris@dripleaf ~ % bluetoothctl
[bluetooth]# scan on
Discovery started
[NEW] Device AA:BB:CC:11:22:33 OBD-II
[bluetooth]# pair AA:BB:CC:11:22:33
(... you may have to enter a PIN specific to your device, maybe try 1234 ...)
[bluetooth]# trust AA:BB:CC:11:22:33
Changing AA:BB:CC:11:22:33 trust succeeded
Next, discover the serial channel:
tris@dripleaf ~ % sdptool browse --l2cap AA:BB:CC:11:22:33
Browsing AA:BB:CC:11:22:33 ...
Service Name: JL_SPP
Service RecHandle: 0x10004
Service Class ID List:
"Serial Port" (0x1101)
Protocol Descriptor List:
"L2CAP" (0x0100)
"RFCOMM" (0x0003)
Channel: 2
Profile Descriptor List:
"Serial Port" (0x1101)
Version: 0x0102
The number in the RFCOMM Channel output will be needed in the next step; in my case it was channel 2.
Next, bind an rfcomm device node with rfcomm bind 1 AA:BB:CC:11:22:33 2. The 1 is the rfcomm device number (i.e. in this case we'll end up with /dev/rfcomm1) and the 2 is the channel number from the previous step.
Now you can connect to the serial port with a tool like python-obd by specifying the rfcomm device you created in the last step.
Good luck!