Interfacing a Kenwood KR-8090 with a PC

I had the same problem, so the "KR-Device-Project" was born. Target was a combination of hard- and software to control my favourite MP3-player with the Kenwood's RC.

So the first thing was to choose the hardware. There are enough projects on the net on how to use an IR-receiver and connecting it to the PC. But this is the hard way, I think, because the Kenwood-devices got a "system-control"-bus plug. The normal use is to connect Kenwood CD-Players and stuff to it. But why not plugging a PC into it? :)

• A PC using Windows 9x, or, if you want to write your own driver, something with a printerport.
• A Kenwood-receiver, which got this "System-Control"-bus. I only tried my own receiver (KR-8090), but I'm sure that it's a bit more compatible. Any "XS8"- or "SL16"- (whatever this means) devices should do it.

the HARDWARE

Because I've got no information about the bus, I connected the plug to an oscilloscope (ok, it was my old soundcard, but who cares ;) and pressed a key on my remote control. It looked like this:

OK, this is a reconstructered output, as my old soundcard doesn't sample so sharp edges, it's a SoundBlaster, so what? ;)
Anyway. We'll ignore the "left" channel, because there are no information in it we desperately need. To get the right channel, we use a very small hardware. You could connect the right channel and GND just to some signal on the printerport, but we don't know which voltage they got (and I still got no real scope ;).
But it can't be so much, so we will use a simple TTL to protect the printerport.
Because I was to lazy to buy a new one from the electronic-shop, I searched for some old TTLs. After a lot of useless crap I got a CD74HCT32E, which seems to be more or less compatible to a 74LS32, a simple "Quad 2-Input OR Gates".
The IC costs less than $1, so if it gots broken because you killed a pin, lost it somewhere etc., you can simply buy a new one. It got 14 pins, with the following layout:

          VCC B4 A4 Y4 B3 A3 Y3
           14 13 12 11 10  9  8
            |  |  |  |  |  |  |
          +-+--+--+--+--+--+--+-+
          |                     |
          |)                    |
          |*                    |
          +-+--+--+--+--+--+--+-+
            |  |  |  |  |  |  |
            1  2  3  4  5  6  7
           A1 B1 Y1 A2 B2 Y2 GND
         

A  B | Y
-----+--
L  L | L
L  H | H
H  L | H
H  H | H

Using the A1 and B1 together with Y1 connected to /BSY, we can check if the device is actually plugged into the printerport, and if everything else is correct.

The reason why I used ACK is easy. Everytime the level toggles at ACK, an interrupt is generated. We don't have to poll the signal, since it's got a frequency of about 200Hz (we'll come to this later). So it's nice to have an IRQ, in combination with the timer we could have an accuracy of more than 0.8 microseconds with no cpu power used. Nice, isn't it?