I've put together a circuit I believe works.
The baud rate is proving difficult though, I have proven 160-baud is not correct, 8192 baud is incorrect as well.
But, 1920 and 1953 both give replies.
I've been testing with RPM today, and can get varied results from both 1920-baud and 1953-baud, the data seems mostly correct when using both of the baud rates (as I'm taking the byte returned and reading it's hex value and then taking that to read out RPM).
I'm going to need to find something a bit more reliable to measure, something, that perhaps is more accurately measured, to see which of the above two baud rates are correct. Admittedly they are very similar, so the data could be identical at both 1953 baud and 1920 baud.
The list here is where I've tried RPM and battery voltage -
http://mmcdlogger.sourceforge.net/#Protocol
I'm looking for something I can test using the readings via the ALDL interface, and, independently - RPM is too hard to gauge accuracy - I had 2125 and 2180 - something that is hard to read on the RPM gauge. On the other hand, both of those numbers could have been perfectly correct, even at the different baud rates. I'd like to test and confirm it. Any ideas?