I agree completely with Magister.
What we really need to decide is what the goal is here.
Are we making a device that is completely like ScanGauge where it can talk with all protocols in the same unit? If so, we need to bite the bullet with either AT90CAN running the show, or adding on the ELM327.
ELM323 really is a waste -- the Arduino can do ISO alone. Code savings doesn't necessarily equate to cash savings, when you have a large flash in your part. I think space-wise, the Arduino is more than capable of supporting all of the protocols individually (and if it becomes a problem, you can always custom-build things with #IFDEF anyway).
So the question is, which are we aiming for?
- ISO + VPW in the Arduino with minimal HW
- AT90CAN supporting all protocols? (Admittedly, that will be a problem for sharing HW between the pre-OBD-II MPGuino and this, but we're not sharing a code base right now anyway, so...)
- Arduino + ELM327 to handle everything.
EDIT: An AT90CAN board probably won't cost much at all -- it's a super small part. I'd imagine if we could get 20 units made (my friends and I would easily want 5-10 of them anyway), the bare boards would be well below $5 apiece. I'll try laying one out some time this week if I get a chance, and we can determine how we want to move forward from there. Soldering the chip shouldn't be too hard -- just start with pins you don't care too much about
Placing and stuffing is pretty expensive. I think we have some materials here at work though where we might be able to stuff the chips onto the boards relatively quickly, so maybe we can do it as a kit where the board comes with the CPU soldered on?