[MTCB Android Head Units Q&A] lucky-jon:
Any one who can add CanBus and SWC info?

I agree that CanBus is a huge help hooking up these multi-functional head units. I am finding that there is a serious lack of dependable info. One reason is that vehicle manufacturers need new features to make us want new cars. Sending controls over CanBus is one way they can make it impractical (or at least difficult) to link aftermarket gear into their vehicles.

Someone can edit this link out if it's not appropriate, and I have no connection with the business concerned, but they have made a business of making CanBus hookups:

http://www.connects2.co.uk/ProductRange.aspx

As far as I have worked out by my testing, previous experience and digging, this is CanBus:

CAN is Control Area Network I think (or something like that) and is a data bus (IE: a medium over which data is sent) that is suitable for device control applications where devices on the 'bus' can talk to each other. Exactly what devices are present in a particular environment, or indeed what they have to 'say' to each other is going to be totally dependent on each application. Indeed, car manufacturers would absolutely want to avoid liability for their devices misbehaving when used in another manufacture's CAN environment, so each manufacturer will want to keep control of what is going on between their CanBus devices in each type of vehicle.

As new uses are found for allowing devices in a vehicle to talk to each other around the vehicle, the command structure is likely to become inadequate and need updating / extending. CanBus could doubtless be used for all kinds of purposes - much like what happened with Bluetooth.

I have tried to work out what the Key1 and Key2 wires are on these head units. I keep being told that they are for SWC / Steering Wheel Controls. Steering Wheel Controls are buttons, therefore they are digital - ON or OFF. However, to get several control functions on a steering wheel would really be tricky if it needed loads of wires connected. So, the button states are converted into data that can be sent down a single pair of wires. I am not sure if this is digital, encoded, or just a resistance change.....

The next part I don't get at all. Is there a cross-vehicle standard set of SWC commands? From the way people are talking, SWC in some raw form is not CanBus - although it is similar in as much as it is serial data representing the state of the steering wheel buttons. There would be no reason why these button-commands could not be sent over CanBus, and read directly by the head unit as CanBus commands - using a protocol convertor from resistance or whatever SWC is - to CanBus.

In the case of VW Factory head units (RCD200 / 300 / 500 etc) the head unit is connected directly to CanBus through the Quadlock connector, and commands on CanBus control the head unit. With a CanBus analyser, you could see these control messages on the CanBus and find out all the devices in each vehicle that are communicating and controlling each other in this way. The RCD200 factory head unit in my VW Caravelle has no ACC. power, but just perm 12v. Inserting the key in the ignition barrel does nothing. Moving the switch to Ign On turns the head unit on - but only if it was already left on last time the engine was run. The command to turn off the head unit is sent only once the key is fully removed from the barrel. These commands are sent to the head unit using CanBus.

So back to Key1 and Key2 wires. These wires are a serial digital or other encoded / resistive communications bus intended for receiving Steering Wheel Commands. To some extent, whatever protocol is on this bus, it is probably not universal between SWC systems, hence the setup option on the head unit to allow steering wheel buttons to be mapped to specific head unit functions. Just for fun (and hopefully not to stupidly!) I tried connecting K1 + K2 to the Can/H and Can/L CanBus wires on the Quadlock connector. Initially nothing happened. I then tried using the Setup SWC Programming menu options, and managed to get the head unit to at least respond to turning / removing the ignition key. Somewhat amusingly, I also ended up with the head unit turning on and off when I unlocked the vehicle doors. I believe that the head unit was seeing the CanBus data and trying to see it as SWC data. Any way, this was not really any use.

The factory sent me their CanBus adapter to try out. What it appears to do is look for CanBus commands and use them to turn individual control wires on or off.... effectively synthesizing standard 'analogue' signals following CanBus instructions. This did not work at all on my vehicle. I had it confirmed that this CanBus adapter supplied was almost certainly designed for most recent VW CanBus implementation. I spoke to a company that specialises in aftermarket upgrades for all kinds of gear that communicates using CanBus. They told me that VW updated their CanBus protocol from v1.6 for my generation of Caravelle / Transporter (2007) to v2.0 from around 2010, so that could explain it.

I would like to hear from anyone who has hooked up Key1 and Key2 to somewhere and got SWC working.

The Connects2 CanBus interface that I am going to try next is supposed to specifically work with ALL VW's and Skodas. I guess it depends on what control info each vehicle puts onto CanBus as to what you can decode / read off CanBus, but in theory, their CanBus interface should create regular 'analogue outputs' (ie single wires) that are exactly as if you had run them from an ACC connection, Lights On (for dimming), Reverse (for rear camera) and Park Brake (for inhibit of certain head unit functions). If it works, using CanBus instead of running wires all over the place is better in every way.

With Chinese head units sold for a specific application, they should include this level of CanBus function even if a vehicle has no SWC. On many current vehicles, CanBus is the only way that the head unit can be turned on and off linked to the ignition key. Clearly, there is no such thing as Universal CanBus, but a good way to specify compatibility would be by referring to the factory head units that an after market head unit is plug-swappable with retaining most basic functions..... like turning on and off the same way as the factory unit did.

Sorry for the long post. I'll update on what (if anything) actually works when I get the CanBus interface.