CAN Bus Monitor: Chorus 2 - Silent Mode

Hello guys,

I have a handful of CAN bus simulators from various sources on the table, and roughly the same number of different Chorus 2 car radios (all of which should be compatible with Chorus 2). The goal is to operate the radio in a car without a CAN bus, while still using all its functions, such as lighting control and other features.

All the car radios behave identically, while some CAN bus simulators offer more features than others – but none of them offer everything together.

My idea was (and still is) to obtain the necessary IDs and data using a CAN monitor, then write some software based on that information and feed it to an AT90CAN128 microcontroller.

However, somehow the CAN monitor only shows data packets that are generated by the CAN bus simulator, and never any data from the car radio itself. With the oscilloscope, I can see on the bus that when the radio is turned on (at the radio itself), these CAN messages are being sent. The radio understands these messages and acts upon them – but, the CAN monitor doesn't show what the radio is sending.

Theoretically, the radio should also be providing data to the FIS (Flexible Information System), but why do I only see certain messages on the CAN monitor?

The only noticeable thing is that I see significant interference on the CAN bus when I plug in the Kaufmann interface, but I do see a CAN participant.

Does anyone have any ideas about what I'm doing wrong, or where my mistake in reasoning might be?

Thank you for your ideas.

Hi,

Is the correct bitrate set in KCANMonitor?

Quote:

What's noticeable is that I see strong interference on the CAN bus when using the oscilloscope.

This suggests that, in your specific case, the 120-ohm termination resistor may not be necessary in the interface.

See:
https://www.canhack.de/en/viewtopic.php?t=2103

A switchable 120-ohm termination is already on the wish list.

Best regards, Rainer.

Hello Rainer,

Perfect - Thank you!

I had already considered the timing aspect, but then dismissed it because the signals didn't appear to be reflections; I thought it was more likely a galvanic isolation from the computer.

Anyway, when I saw in your link that the CAN bus on the adapter was also terminated with 60 ohms, I removed those components – and sure enough, now the second device is communicating with me too!

The signals still don't look quite as I would like them to, perhaps there is still something that needs to be decoupled – I will take a closer look.

Best regards,

You need two 60 Ohm resistors. You must remove both R1 and R2. One resistor is not enough because there is also a capacitor connected to ground.

Best regards, Rainer.

Hello Rainer,

Okay, that makes sense. The (H + L) are connected in series with a mass resistor of 2 * 60 ohms = 120 ohms... but terminating it three times is definitely one time too many. Got it - thanks!

Best regards, Grubi.