interupting a CAN bus

[cybermaus]

I am looking for a way to interrupt (not even intercept) a CAN bus to my mini-camper car radio.
Basically, I want to feed it from the aux battery, and keep it playing. And CAN keeps telling it to shut down.

Not wanting to use a physical dual pole switch, I was thinking of simply getting 2 jellybean CAN transceivers like SN65HVD230 or TJA1051, very easy to get on cheap arduino modules.
Hook them both back to back, Rx to Tx and TX to Rx, and use the enable signal to switch.

Another option is to use some analog switch like CMOS 4066, or the TTL version of it.

Unless of course if someone knows of a easy to get IC that does exactly this, inhibiting CAN or RS485 signals.
Anyone has any input on any of these possible solutions?



Edit: I see 4066 has ~150 ohm on state resistance. that is rather high in relation to the official 120ohn termination. Maybe making it a little unreliable.

[rteodor]

I would not even use CAN interfaces but only a MCU that would monitor the bus for the frame that contains the problematic signal and corrupt the frame. Its simple in hardware but there might be quite some effort in investigating the bus (to find the frame number) and implementing it.

[cybermaus]

Yea, I considered the man-in-the-middle approach
But I don't want this to be a "project". Just a little gluing together of some components.

a pushbutton, a c005 timer and a PNP FET on a breadboard was what I had in mind. And ideally thus also a CAN bus disable chip.

[neillnz]

If you want to break the connection of a balanced signal you could could use a double pole small signal relay e.g. TQ2-5V. 

As for CAN transceivers you should consider that everything at the Tx input will appear on the Rx output this is because in order for CAN to operate it must listen to the bus all the time, because of this I'm not sure connecting two transceivers "back to back" is going to do what you expect. 

[langwadt]

is any of that better than a dual pole switch?

[cybermaus]

@neilnz Transceivers have an enable signal. Implying they also have a !disable signal. One that consumes far less energy then a relay.

@langwadt
Apart from switching off the CAN, I would also switch on the power to the radio. And a physical switch can be forgotten, leaving the radio on the entire day/night/week, killing the battery.
So I want it on a 1 or 2 hour timer with auto-off
So a pushbutton, and that pushbutton then also should control the canbus

[HwAoRrDk]

Are you sure it's something on the CAN telling the radio to shut down? And not something simpler like the radio deciding to shut off after it doesn't see any CAN traffic for a certain period?

If it's the latter, then you'd want to do the opposite of what you're thinking, and keep some artificial CAN traffic going while the vehicle is off and the radio is on battery.

[cybermaus]

Good suggestion. But no, I am quite sure. I tested by cutting the CAN. I next crimped some nice JST connectors to reconnect, in anticipation of above mod.

[jwet]

Just a note- can pushes real current, dual 120 ohm terminations often, give 60 ohms, with 2 or so volt drive levels, can have > 20 mA levels.  Short circuit currents get near 100 mA.  Keep in mind for the analog switch type solutions.  Little hidden toggle switch sound pretty clean to me.

[cybermaus]

Another good point. It made me reconsider my first thought of just using a 7805.





Edit: I also suddenly realise, if I do this, the radio side will be cut-off from the normal bus 120 ohm termination resistors. I would have to add my own, and I can only add both of them on the same side as a single 60 ohm. Transmission line reflections would be all messed up. Oh well, lucky the media bus is a low-speed bus.

Edit2: Maybe I should go with a small relay. Originally I though it's 5~10mA I can save on, but driving a 2nd CAN bus is going to be more expensive. @jwet and @neillnz might be correct, a simple switch may be the thing.

[rvalente]

Are you sure its a CAN message? A few cars have a KL15R for accessory. Did you check the radio pinout?

[cybermaus]

As stated, I checked by cutting the CAN.

But yes, the pinout is also relevant. Incoming into the car is ACC, ILL, IGN
ACC in your terminology would be 15R and IGN would be 15. ILL would be 58, in Bosch codes.
ILL is of course used for radio backlight.

In order to keep the radio on, it needs to have +12V on ACC and IGN as well as have the CAN bus cut.
When IGN disappears, it will turn off, but you can turn it back on with the power button on the radio itself.
Next up after about 2 minutes, is a CAN signal, which turns if off again, even if ACC stays at 12V. However, you can turn it back on.
And finally after about 5 minutes ACC dissappears, turning off the radio yet again.
At this point, it is possible to turn it back on, and this time it will stay on (with ACC and IGN gone)

HOWEVER, when you open/close a door, you wake up the ECU, and ACC goes high again, and the cycle starts again for CAN and ACC.
Basically the car has 3 separate off signals, and you need to prevent then all 3: ACC (keep high) IGN (keep high) and CAN (disconnect)
And this was tested in all permutations, so yes, I am quite sure.



My previous car, a VW, had *everything* over CAN. There was no ACC, ILL, IGN. The radio just had B+ and CAN. nothing else. Which makes sense from a production cost perspective.
Why this Renault has a mix of old and new controls is weird. I am sure all those signals come over CAN too, why even bother with the hard wiring.
As a tinkerer, I prefer the old hardwires of course, but darn it, because they also have CAN, I can't rely on that, I have to do both. Silly frogs.

[rvalente]

Well, If you wanna can hack, get savvyCan + cheap USB can adaptor, reverse engineer the network, make a dbc and ad a man in the middle to remove certain messages that are not desirable. It's very rime consuming, check gits for dbcs, often someone already started something similar.

[Smokey]

https://www.ti.com/lit/ug/tidudb5a/tidudb5a.pdf
Ignore the isolation if you don't care about that.  The rest of the circuit is a CAN network bridge with arbitration logic.

But this seems like a lot of work to not use a switch and be done.