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Decoders and matrices.

To read the key contacts, we use a decoder circuit and a diode matrix. The decoder circuit forms the link between the MIDI mainboard (which may be the main board of the Small-Midi or the main board of the BIGMIDI) and the diode matrix.
A diode matrix is ​​a circuit in which diodes are arranged in groups of eight diodes whose cathode is connected to eight key contacts. This group goes up to 64 positions: so 8 groups of 8 diodes, each diode with its cathode connected to a key contact.
The opposite of the key contacts is the key contact rail, which is now divided into pieces, so that now the split pieces include a range of 8 keys.

The microcontroller of main board  each time gives a signal to each decoder circuit in the MIDI system with the message, "Now it's your turn to pass the keystrokes". The identification of each decoder circuit for the microcontroller is accomplished by giving each decoder circuit  a unique "address".

 Section diagram of a matrix


The decoder circuit now gives each group of 8 switches, a clock signal. At the moment there is a clock signal given from the decoder circuit the microcontroller read  the state of the switches via the data bus D0 ... D7.
Each matrix of 64 switches (and diodes) is done 8 times: So 8 clock signals,  and each clock signal confirmes eight switches gives 64 positions.

Note: Read instead of key contacts also the contacts of the switch register.

Decoder and matrix on one PCB.
Ordercode: DECMATPR-B Assembly kit
Ordercode: DECMATPR-G Assembled and tested

 The combination of a decoder and a matrix on one PCB makes things very simple. The picture on the right shows a decoder circuit and a matrix circuit for 64 keys on one PCB. This PCB can be connected directly to the GINO-BUS.
Key contacts (or the stop switches) are connected by a 16 pole ribbon cable directly on this matrix board. The ribbon cable on the PCB is connected by the well-known IDC connectors.
Far left is a 10 pin IDC male header placed where the eight clock signals of the matrix are connected. On the download page you can download the scheme, print drawings and building instructions.

The size of this PCB are: 3.5 cm x 21 cm.
The print is quite small so easy to place somewhere behind a keyboard.


This PCB can also be used for a tone pedal 32 (shown on the picture) and stops for connecting switches. The maximum number of connection switches is 64.

The prices of the 10 - and 16-pole ribbon cable can be found on the  ORDER page. For a 5 octave keyboard you need  1 meter 10 pole flatcable and 1.5 meters 16 pole flatcable.

This picture shows the decoder_matrix PCB used as a matrix for a small stop panel. The GINO BUS is indicated and the two 16 pin ribbon cables running to separate stop switches. The far right we see the 10 pin ribbon cable to the clock signals of the matrix.


Decoder PCB's
Ordercode: DECPR-B Assembly kit
Ordercode: Assembled and tested

The link between the matrices of the keyboards, pedal and/or stop switches is formed by the decoder. This decoder PCB's are mounted close to the matrices and through the cable of the GINO BUS connected to the main board of the Small-Midi 2 or the BIGMIDI.
Each matrix has its own address. This private address is set via a jumper at JP1(Jumper). See picture.

This GINO BUS runs like a ribbon through the organ and picks up the signals from the decoder PCB's. The wiring is thus minimized and gives it all a clear look.

The widely used FATAR keyboards already have a matrix and can be directly connected to a decoder PCB. Diagram and construction details are available at the download page.

Picture with a decoder PCB connected to the GINO BUS.

Decoder PCB stabbed on the first PCB of a keyboard matrix.


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Last update: 05 mei 2015.