Allen MOS II Organ Keyboard Matrix to USB MIDI Adapter

This is a Teensy 4.1 sketch for connecting an Allen MOS II Organ's keyboards to a computer over USB MIDI. It's designed to read the 11x18 combined keyboard matrix, and export them as a MIDI device suitable to connect to any virtual pipe organ.

Operation

The keyboards all share a common set of read wires. Each keyboard uses 11 read wires and the pedal board uses 7 of the 11. Each keyboard has its own set of six signal wires. To read the keyboards a microcontroller (the organ itself or a Teensy) raises each signal wire in turn to a high state and then reads the key pressed signals from the read wires. This is done in a sequential cycle, reading many times per second. In this way a relatively few wires can read the entire keyboard quickly, and multiple keys can be pressed at once.

The MOS II Organ, at least our vintage, has its keyboard matrices wired in a peculiar way. The first key has it's own output wire, and subsequent groups of six keys have their own output wire. Diodes prevent back feeding.

The input wires are done differently. There are six input wires per keyboard and each wire feeds the input of every sixth key, with the 61st key having input wire 1 into it.

This asymmetry between the input wires and the output wire groups means the mapping is a little strange. See the end of this document for a chart. You can also see the mapping in the ino file.

Here's a diagram of how an individual keyboard is wired. Both keyboards are wired like this, but each has its own set of six input wires. The pedal board also has six input wires, but only uses seven output wires. The OUT wires in the diagram are the eleven black wires going to the keyboard matrix board edge connector.

You cannot connect the Teensy to the keyboard at the same time the normal organ circuitry is connected. A circuit board is needed to use relays to galvanically switch the keyboard between the stock organ and the Allen internal circuitry.

The sketch turns keyboard presses and releases into MIDI NOTE ON and NOTE OFF events, with a velocity of 127. The first keyboard events are on channel 1, the second on channel 2, and the pedal board on channel 3. If you want to change it, you can set it in the sketch.

Black common wires

The 11 black read wires connect to Teensy pins 20-30. Looking at the keyboard from the top, the left-most key is connected to its own black wire to pin 20. Every group of 6 keys after that connects via diodes to a common black wire and those go in order to pins 21 and higher.

Looking at the 44-pin MOS Matrix board connector, the connections are:

Teensy	MOS Matrix pin
20	11
21	13
22	9
23	5
24	7
25	3
26	1
27	8
28	10
29	12
30	14

Keyboard 1 Read Wires

The top keyboard matrix is read by sending high signals down six wires, each wire connected to 11 keys. These are connected left to right to Teensy to pins 2,3,4,5,6,7. These wires can also be accessed via the 44-pin keyboard matrix edge connector on 22,23,26,25,24, and 21. On our organ these wires are white.

Teensy	MOS Matrix pin
2	22
3	23
4	26
5	25
6	24
7	21

Keyboard 2 Read Wires

The second keyboard matrix is also read by sending high signals through six wires that connect to Teensy's pins 8,9,10,11,12,and 13. On our organ these are orange wires and are connected to the edge connector to pins 28, 29, 32, 31, 30, and 27.

Teensy	MOS Matrix pin
8	28
9	29
10	32
11	31
12	30
13	27

Pedal Board Read Wires

The pedal board is read by sending high signals to six wires that connect to Teensy pins 14, 15, 16, 17, 18, and 19. These wires are blue on our organ and connect to pins 40, 41, 44, 43, 42, and 39 on the 44 pin matrix edge connector. Note that only the first seven of the black read wires are used on the pedal board

Teensy	MOS Matrix pin
14	40
15	41
16	44
17	43
18	42
19	39

Mapping of keyboard wires to key/note

Key	Note	Input	Output	Combination
1	C2	IN1	OUT1	IN1-OUT1
2	C#2	IN2	OUT2	IN2-OUT2
3	D2	IN3	OUT2	IN3-OUT2
4	D#2	IN4	OUT2	IN4-OUT2
5	E2	IN5	OUT2	IN5-OUT2
6	F2	IN6	OUT2	IN6-OUT2
7	F#2	IN1	OUT2	IN1-OUT2
8	G2	IN2	OUT3	IN2-OUT3
9	G#2	IN3	OUT3	IN3-OUT3
10	A2	IN4	OUT3	IN4-OUT3
11	A#2	IN5	OUT3	IN5-OUT3
12	B2	IN6	OUT3	IN6-OUT3
13	C3	IN1	OUT3	IN1-OUT3
14	C#3	IN2	OUT4	IN2-OUT4
15	D3	IN3	OUT4	IN3-OUT4
16	D#3	IN4	OUT4	IN4-OUT4
17	E3	IN5	OUT4	IN5-OUT4
18	F3	IN6	OUT4	IN6-OUT4
19	F#3	IN1	OUT4	IN1-OUT4
20	G3	IN2	OUT5	IN2-OUT5
21	G#3	IN3	OUT5	IN3-OUT5
22	A3	IN4	OUT5	IN4-OUT5
23	A#3	IN5	OUT5	IN5-OUT5
24	B3	IN6	OUT5	IN6-OUT5
25	C4	IN1	OUT5	IN1-OUT5
26	C#4	IN2	OUT6	IN2-OUT6
27	D4	IN3	OUT6	IN3-OUT6
28	D#4	IN4	OUT6	IN4-OUT6
29	E4	IN5	OUT6	IN5-OUT6
30	F4	IN6	OUT6	IN6-OUT6
31	F#4	IN1	OUT6	IN1-OUT6
32	G4	IN2	OUT7	IN2-OUT7
33	G#4	IN3	OUT7	IN3-OUT7
34	A4	IN4	OUT7	IN4-OUT7
35	A#4	IN5	OUT7	IN5-OUT7
36	B4	IN6	OUT7	IN6-OUT7
37	C5	IN1	OUT7	IN1-OUT7
38	C#5	IN2	OUT8	IN2-OUT8
39	D5	IN3	OUT8	IN3-OUT8
40	D#5	IN4	OUT8	IN4-OUT8
41	E5	IN5	OUT8	IN5-OUT8
42	F5	IN6	OUT8	IN6-OUT8
43	F#5	IN1	OUT8	IN1-OUT8
44	G5	IN2	OUT9	IN2-OUT9
45	G#5	IN3	OUT9	IN3-OUT9
46	A5	IN4	OUT9	IN4-OUT9
47	A#5	IN5	OUT9	IN5-OUT9
48	B5	IN6	OUT9	IN6-OUT9
49	C6	IN1	OUT9	IN1-OUT9
50	C#6	IN2	OUT10	IN2-OUT10
51	D6	IN3	OUT10	IN3-OUT10
52	D#6	IN4	OUT10	IN4-OUT10
53	E6	IN5	OUT10	IN5-OUT10
54	F6	IN6	OUT10	IN6-OUT10
55	F#6	IN1	OUT10	IN1-OUT10
56	G6	IN2	OUT11	IN2-OUT11
57	G#6	IN3	OUT11	IN3-OUT11
58	A6	IN4	OUT11	IN4-OUT11
59	A#6	IN5	OUT11	IN5-OUT11
60	B6	IN6	OUT11	IN6-OUT11
61	C7	IN1	OUT11	IN1-OUT11

Issues

Currently the Teensy occasionally reboots. This could be do to unstable voltage provided by a USB hub. Possibly dedicated a 5v power supply to the Teensy is needed (A trace on the Teensy 4.1 has to be cut to safely do this), or a powered hub, or plug the Teensy directly into the computer.