|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
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
Each matrix of 64 switches (and diodes) is done 8 times: So 8 clock
signals, and each clock signal confirmes eight switches gives 64
Note: Read instead of key contacts also the contacts of the switch