TIDA-00527: Power over CAN

Hi Linus,

Sadly RS-485 and CAN are not very similar. RS-485 transceivers use the push pull architecture in order to drive their signal. They are able to actually supply power when they are driving the line high or low.

CAN will only supply current when it is driving the line dominant. When the CAN bus is recessive there is very little current (typically 1 mA). I'm not sure if you will ever be able to achieve 2A with either of the systems.

Is there a reason why you don't want to use an RS-485 transceiver? I assume it is because of the reduced common mode range.

Best,

Chris

Linus,

To add to what Chris said, with CAN this is a bit more difficult because the CAN differential signal is not standard differential. That is, it's not centered around 0V and the two signals do not "swap" polarities when communicating. There is a DC bias of 2.5V ( CAN common-more/recessive level) that is built in to the CAN standard and thus CAN transceivers, so the removal of the DC bias on the receiving end and then interpreting the message correctly can be tricky.

We do have a power over communication device released, the THVD8000. This device uses more of an RS-485 signaling scheme, but is differential and communicates over the power signal.

Regards,

Eric Hackett

Hey Guys,

Thank you for your fast answers! :-)

We need to use CAN, because the used µCs communicate via CAN.

My goal is to reduce the number of wires and contacts in the connector.

So 2 wires for CAN and tow for the Power supply.

Can I use RS485 with the CAN interface on the µC, I guess not?

Greez Linus

Hi Linus,

I'm going to look into this and I will get back to you with an answer ASAP.

Best,

Chris

Hi Chris,

that would be nice :-) I am looking forward for your idea.

Greez,

Linus

Hi Linus,

I talked to my colleagues about it and the answer is that it may be possible to use an RS-485 device to communicate on a CAN bus. However, this is going to prove to be very difficult and you will have to be smart about how to program the controller. There will be many problems you will have to overcome including implementing arbitration on the device. RS-485 devices are also not designed to communicate at the same time so we are unsure how the device will handle receiving messages from multiple devices at the same time.

There seems to be two options here. The first is to implement an RS-485 system which would probably be less work than trying to figure out how to program an RS-485 device to work with CAN.

The second option is to use a transceiver like the DCAN500. This isn't a TI device so I can't comment on the functionality of it or how it even is implemented. But it is designed for CAN over DC power lines. This seems like an promising option.

Best,

Chris