Help building a CANbus receiver and sender unit

Question

I am an engineer in the marine industry and work as a commissioning engineer and need to build something that I can connect to each sensor for alarm testing. The sensor is a thermocouple that is hard wired into the CANbus node. CANbus is the area where I have the least understanding, not taught when I was at uni.(If anyone has links to indepth information, please send me the link) I would like to be able to connect to the CANbus sensor and read it, while also being able to send a false output temperature CANbus signal to raise the temperature being read by the alarm system. I have looked at what is available of the shelf and can´t find anything that can do this. I can see there is some opensource software and making a CANbus reader seems not so hard, but sending a temperature signal over CANbus seems rather difficult. CANbus system runs on 13v with a 120 Ohm resister at the bus end. There are 4 bus with about 6 nodes on each. There are 6 wires in each bus cable. 2 are shields ard the other 4 which run to each node are 1)CAN Hi 2)CAN Lo 3)return 4)13v

Thanks in advance

Answer 1

To begin with, you cannot work with "CAN bus", since that's only the hardware layer. To be able to communicate over a CAN bus, you must know which higher layer protocol that is used. This can be anything. Some common standards are CANopen (most common by far), J1939 and Devicenet. But it is even more common to use a custom, often proprietary protocol.

Unless you know the protocol, and thereby how the whole bus timing is designed, you cannot and should not connect to a CAN bus. Starting to send data onto a CAN bus which you don't know how it is designed, is simply bad and dangerous practice.

As for how to communicate with a CAN bus from the outside world, such as a PC, that's the easy part. There exists many companies (Vector, IXXAT, Kvaser etc) that make CAN listeners, which is typically an adapter with a DB9 male connector towards the bus, pin-out according to the CANopen DS303 hardware standard, where CAN H, CAN L and signal ground are needed. Shield is optional, supply is not needed (unless galvanic isolated CAN nodes are used). The other side of the adapter will have USB that you connect to a PC with the CAN listener software.

Alternatively you could cook up some custom microcontroller solution of your own, but that requires a whole lot more knowledge about CAN, particularly if you are to design the driver yourself.