ISOTP is a protocol to transfer messages larger than the maximum CANbus frame size of 8 bytes. While it’s purpose is only this, it is usually associated with diagnostic, request-reply type of traffic, as opposed to the broadcast “free frames” type of chatter.
When we started with CanZE we had more information about the free frames and while it is a real pain the xxx to implement and requires commands not available in cheap clone dongles, we also prefered it as it did not require us to inject any traffic whatsoever on the car’s CANbus.
Things have significantly changed since. Using diagnostic commands exclusively has a couple of advantages
- it’s a given on the Ph2 vehicles. No way to get to it’s free frames;
- it’s commands are fairly well documented from more or less public sources;
- some data is simply not available on free frames;
- more dongles will work without using free frames.
So, the setting for ISOTP are as follows, though they may seem a bit counter intuitive.
- For the Ph2 cars, it has no meaning. We should probably disable it.
- For Ph1 cars, if using a CanSee dongle, we nudge ISOTP off, as it’s the fastest option and works fine. If using a commercial dongle, we nudge it on.
- Nudging means: it will save as set, but every time you open the settings, it will switch it again to the preferred option.
All in all, we might decide to get rid of the entire option as it’s confusing. If there are strong opinions either way, let us know.