MIDI With Two 4021s and Arduino

Here is an example of how to read two 4021s and send the state out as MIDI data.
A transition from a low to a high state on the 4021s will result in a note on message.
A transition from a high to a low state on the 4021s will result in a note off message.

Arduino Code:
The Arduino code can be downloaded here:
http://milkcrate.com.au/_other/downloads/projects/MIDI_With_Two_4021s/

Here are some important parts of the code.

byte switchVar1 = 0;
byte switchVar2 = 0;
These are the bytes that contain the information for the 4021 states. Each 4021 has a byte. If more 4021s are to be added, make some extra variable bytes such as switchVar3, switchVar4 and so on.

byte MIDI_channel = 0;
This sets the channel minus one. So for example, setting this to 15 results in sending data on channel 16.

byte MIDI_pitches_offset = 60;
This sets the lowest pitch (so, the note that corresponds to button 0 from the first 4021). The each subsequent button will be one semitone higher.

byte MIDI_on_velocity = 127;
This sets the note on velocity. A high on any of the inputs of a 4021 will trigger a note on event. A low will trigger a note off event.

switchVar1 = shiftIn(dataPin, clockPin);
switchVar2 = shiftIn(dataPin, clockPin);
midiShadows(switchVar1, 0);
midiShadows(switchVar2, 8);
This is where the 4021s are read in and the MIDI data is sent out. To add additional 4021s, simply add additional lines of code like this:

switchVar1 = shiftIn(dataPin, clockPin);
switchVar2 = shiftIn(dataPin, clockPin);
switchVar3 = shiftIn(dataPin, clockPin);
switchVar4 = shiftIn(dataPin, clockPin);
midiShadows(switchVar1, 0);
midiShadows(switchVar2, 8);
midiShadows(switchVar3, 16);
midiShadows(switchVar4, 24);

and so on.

And that's pretty much it. You can connect up to 128 buttons using 16 4021s using this method.

Breadboard

Max/MSP Patch