I'm working on reverse engineering the remote control interface on my TM-241a. When it was a new radio you could buy options to use it: RC-10, RC-20, IF-20. The RC-20 looks like any other remote head but with the IF-20 you could hook it up to 4 radios. You could end up with one control and microphone to drive a 2m radio, 440, 220 or 1.2ghz. It also worked across different revisions of the radios although I think the x41 were the last ones.
I emailed Kenwood looking for any information on this interface awhile back, they couldn't help me with the protocol but they were kind enough to send me PDF documents with schematics on the RC-20 and IF-20. Between those, and a PDF service manual for the TM-241a I found.. I have figured out how to do an electrical interface. I'm using a Bus Pirate v3 from Sparkfun to do it. Not entirely sure if I'm interfaced correctly, but I am getting repeatable data. It's quite noisy because I am just using the unshielded probe cables I got with my Bus Pirate.
I did accidentally overwrite a couple of the memories already with junk information. Somehow one wound up with 444.900 and the other had 109.490 in it. Obviously didn't try transmitting there. The radio probably wouldn't have let me, it gave me the error beep when I pushed the REV button to see where the offset ended up. I'm mostly concentrating on receiving the display data that the radio constantly sends out right now. I'm making a bit of progress on the puzzle but it would probably go a lot quicker if I had a RC-10 or RC-20 to experiment with.
All well, I love a puzzle.
My next step may be to collect a lot of information and make Visual Basic programs to help me analyse it. Both for errors, and also for changes between samples (ie, data for one frequency in a memory channel, and then data for another frequency in the same channel)