Mmm, possibly it isn't async which would explain why you only ever see 4 bytes. Have you got a scope trace of a packet? The bus I looked at was MBus, definitely not AiNet.

For checksums, I figured it out by capturing packets identical except for one bit, eg track 2->track 3 (bit 0 being set in the status packet) and compare the checksums then. It *might* have been XOR of the 4-bit values, maybe. It was about 6 months ago I last looked at it...

Hugo