Hardware

I used the following hardware for this project:

1. Remeha Calenta Ace boiler.
2. Raspberry Pi 3B+.
3. USB CAN bus adapter. I have used this one.
4. RJ12 cable.

Pinout

The pinout for the RJ12 cable appears to be:

Pin	RJ12	USB adapter
1	CAN L	CAN_L
2	CAN H	CAN_H
3	not connected	not connected
4	GND	AGND
5	not connected	not connected
6	24V	not connected

Please be careful creating the cable! Please measure the voltage on the line before connecting it to the CAN USB adapter. Avoid connecting the 24V line!

The USB adapter has a jumper marked "RES". With the jumper in place, the CAN bus end of line resistor is active. I have left it in place and have not had any issues. I'm not entirely sure what the right configuration is...

More information about the physical layer of the CAN bus can be found here.