After a brief examination of your last (very useful) information, I've discovered that the SM89516 derives from the 8052 family and since the RDX pin of this last is connected to the internal baud rate generator, I have reached the conclusion that it is a simply RS232 signal.
Indeed analyzing the signals shown in
this post, you can see very well that in reality are 5 bytes (8n1 format) and with a baudrate of 14400.
For example, the first signal (flip up), in hexadecimal is: 54 54 89 1C 4D
Moreover, if you pay attention to the 6 signals, you'll see that the first two bytes (54& 54&) are common to all signals (I think a sort of preamble), the 3rd byte seems to be a sort of addressing, as it is the same for the flip commands and the same for the input selection commands (VGA, TV, etc..), but different between the two grups.
The last two bytes could be the command to execute, or alternatively, the 4th byte could be the command and the last byte a kind of CRC error.
Plese feel free to ask me if you need more informations about it.