Other very useful tooling for debugging serial stuff is a DSO.

Since everything is unknown, I guess you don't even know baud rate, parity etc. A storage 'scope will allow you too measure bit times etc letting you determine baud rate etc. This is how I figured out Alpines Mbus. Then it's a matter of looking for patterns etc in the received data. Although you would be surprised how often things use plain ASCII with no CRC etc to talk.

As a note when doing this Alpine stuff, I initially discovered things looked like 8E1 (I think) at about 1500 baud. It then seemed that one byte was sent for a 2 bit sequence. Thus 4 bytes were sent for each byte transferred.

What in fact it seems to be is that a binary 1 was represented by 2.5ms on period and 1.5ms off period and a 0 was represented by a 2ms on period and 2ms off period. It just so happened to match certain characters at 1500 baud. This would require an custom serial interface making it a lot harder to interface to the empeg. So instead for the moment I am using the 1500 baud 8E1 method.