After a misstep where I had faulty data, I am getting valid data out of it now. I can tell because I have figured out how the frequency is encoded and I can predict the results of changing to a different one. There's a lot of work still to be done because so far all I can do is read. I also don't know how everything is encoded.
So far I can read the frequency, if there's an offset and which direction, if I'm in VFO mode, CALL mode or Memory channel mode and which channel. I can also see if there's a signal but I haven't figured out how the S-meter is transmitted yet.
There are several bytes where I'm sure data is encoded at the bit level, I've already seen it with offset, CALL and there's a bit that is set if I'm receiving a signal.
There are quite a few LCD elements that I don't know where they map yet. Much of my experimentation so far has been to set something on the radio, turn it off, configure the Bus Pirate, turn the radio on and read it. If I had a RC-10 or especially RC-20 I could probably rig it up to drive it from the Bus Pirate. Then I could configure the packet of bytes to feed it and see what it does. I could also use it to send control packets to the radio to see what they are. I'm guessing the remote control interface is like a "dumb terminal".. if you push a key, it send that key to the radio instead of doing processing itself. That explains why the radio isn't sending a lot of information out such as what the offset is, or if there's a CTCSS tone and what it is. All the radio is sending is what should be displayed on the LCD. I can probably expect a different packet if I'm in a screen to change the tone, for example. Already I have noticed that when I'm on the CALL channel, it omits data for the VFO and MEM channel.
I'm also a little curious as to how the IF-20 sends information to the RC-20 when there's a second (at least) radio connected. The TM-241a only sends out information for itself. The IF-20 would connect to multiple radios and format the information from all of them into a form that can drive the main and sub display on the RC-20. I'm guessing it's an addressing change.
I know Kenwood killed off the remote control displays sometime after the x41 series. I wonder if the protocol survived in the x51 and future radios? Probably not, but you never know. There's a lot of potential power here, this could have been a major selling feature even now.