Polysix - Replacing the 8049 Key Assigner

I've finally resumed work on my earlier plan to replace the key assigner in my Korg Polysix.  My purpose is not actually related to the key assigner -- my purpose is actually related to adding aftertouch and velocity sensitivity to the Polysix.  In order to achieve these goals using my new Fatar 61-note keybed, I felt that the best course would be to replace the key assigner so that I had full knowledge and full control over all the signals.  To jump to the end of today's story, here's a picture of MIDI control of my Polysix using an Arduino!

Arduino Mega (with MIDI Shield) Inserted in Place of the Key Assigner's 8049 Microprocessor

OK, stepping back to the proper place in the story, the last work that I had reported was on extracting the timing of the control signals generated by the 8049 microprocessor that is at the heart of the Polysix's key assigner.  My plan is to replace the 8049 with an Arduino, so I needed full knowledge of all the control signals.  Since that post, I've begun developing software for an Arduino Mega 2560 to recreate all of the control signals generated by the Polysix's 8049.  That work has been going well, so I felt that it was time to actually start plugging into the Polysix...it was time to make the rubber hit the road.

To physically get the control signals from the key assigner PCB to the Arduino (and back), my plan is pop the 40-pin 8049 out of its DIP socket in the Polysix and to connect a bunch of wires between the Arduino and the now-empty socket in the Polysix.  Basically, this is a less elegant version of same approach used by these folks, who are replacing the microprocessor in a Prophet 600 with another type hobbyist microprocessor, the Teensy++.  Whereas the Teensy already comes in a nice 40-pin DIP form and can be dropped directly into the Prophet 600 board (with a minor mod), the pin-out for the Polysix is radically different -- too different to enable easy use of the Teensy.  So, unfortunately, I'm stuck using the much-larger Arduino Mega.

To get the wires from the empty 8049 socket on the Polysix PCB to the Arduino, I've chosen to take a new 40-in DIP socket, to solder a bunch of wires to it, and to insert the newly-wired socket into the empty 8049 socket.  Unfortunately, as you can see in the picture below, a plastic DIP socket isn't really meant to be soldered to -- you can see in the picture that the plastic around each soldered pin got quite melted.  It would be really easy to push a pin right on through the socket's frame.  Be careful!

Soldering a Wires to a Soft-Plastic 40-Pin DIP Socket

Of course, you just can solder wires all willy-nilly -- you have to have a plan of which pin from the 8049 socket is needed for what purpose.  So, after looking at the Polysix schematic, I came found that I needed 27 wires to attach to 27 of the 8049's connections (see my hand-drawn pin-out below).  Add another wire for power and another for ground and that's a total of 29 connections.  With that number of connections, you can see why I needed an Arduino Mega instead one of the more-common (and cheaper) Arduino variants.

Signals from the Polysix 8049 that I Need to Bring to the Arduino Mega

So, after soldering all of the wires to my DISP socket, I pulled the Polysix's 8049 out of its socket and pushed my new socket into its place.  Pushing it into the socket was actually pretty difficult because my socket had round pins and the existing socket was expecting flat pins.  I was worried about breaking the socket or over-stressing the PCB.  So, I unscrewed the PCB and pushed some material underneath the PCB to give it plenty of support.  Eventually, I was able to push hard enough that it finally and decidedly dropped into place.

My Colorfully-Wired DIP Socket Inserted in Place of the Polysix 8049 Processor.

With the socket wired-up and finally in-place, I brought the wires back to the Arduino Mega.  I dropped the software onto the Mega and fired it all up.  There was no magic smoke released, so I was pleased.  Of course, it also couldn't do anything because I had no keybed attached yet.  So, I attached a Sparkfun MIDI Shield to the Arduino, which is the picture shown at the top of this post.  Using an external MIDI keyboard (along with MIDI parsing software routines that I wrote for the Arduino), I could now tell the Arduino what notes to play and, theoretically, the Arduino would command the Polysix to play them.

Did it work?  Well, no.  Software always has bugs.  Did it work eventually?  Yes.  Totally.

As of right now, I've got the following Polysix functions working correctly: Poly, Unison, Chord, Hold, and the Octave knob.  The only Key Assigner mode that is not yet implemented is the Arpeggiator.  I even have a few new functions already implemented: (1) a sustain pedal, and (2) user toggling between retrigger every note (like a normal Polysix) and no-retrigger when playing legato (like a Moog).   I'm particularly enjoying the sustain pedal functionality.

Now that I've demonstrated that it works, the next step is to begin to install it more permanently into the Polysix.  Then, I can install the new keybed (along with its controller).  At that point, I can finally start implementing the aftertouch and velocity sensitivity.  What a long road!

Update: Here's the next step...the New Keybed with my Arduino Key Assigner
Update: I finally shared my Arduino code here.