VATS ALDL frames

[pman92]

Hi guys,
After having a look at early Ford Smartlock systems it has got me interested in how the VATS system works in commodores.
I know the BCM sends an "ok to start" message to the PCM over ALDL, and I know later models (VT onwards?) have the PCM and BCM linked as well.
I recall noticing in $12P there are options to select VR or VS type VATS.

My thinking is that in VR/VS it would occur something like this:
-PCM sends request to BCM.
-BCM responds with either a generic "ok to start" or "not ok" aldl frame.
-Engine shuts down if no BCM response or a "not ok" is received.

I'm guessing these are pretty similar VR to VS. Is the only difference because of different device ID's for the PCM/BCM between VR/VS?
What do the data frames actually consist of? What is an "Ok to start" data frame? Does the PCM re-request an "ok to keep running" after a while or is it only at start up?

The VT onwards system I assume is very similar. Since the BCM is bus master how does it know when to send an "ok to start" message to the PCM? Does the BCM request the PCM to tell it if it wants an "ok to start" frame? How does it all unfold? What models are BCM/PCM linked (is it from VT or from VX?)

The reason I ask is because I have been looking to experiment with ALDL with an Arduino for a while, and as an experiment I was planning to try and keep a VR/VS PCM with vats enabled keep running by using my Arduino to spoof the VATS system.

Thanks in advance

[antus]

Im not sure of all the details and im inclined to think we shouldnt document it all publicly, but I can give some general information. The systems you describe have some data in the ecm/pcm eeprom and in the bcm and they do need to be linked. It isnt just a static string like the old ford system you've mentioned. The pcm asks the bcm for the ok unless vats is disabled. It asks when its started and when it gets the right and matching response from the bcm it sets a flag for vats ok. without that flag being set, and after a couple of seconds, the injectors will stop firing. Thats the general idea, but I dont want to go in to the IDs and the linking process which is the heart of the protocol.

If you want to spoof the handshake, record a real one from a matched pair and remove the bcm and handle the response in the arduino. It'll work, but wont be portable to another car unless you have some kind of learn function.

[immortality]

I agree with Antus, something like this makes a great tool to otherwise gain access to a vehicle you maybe shouldn't have. Not good to be out in the public domain.