A bus is an electronics term that means a collection of wires all running together, taking information from one part of a circuit to another. The importance of a bus here is that it is used to connect the keyboard of the synth to it’s brain. There are 12 wires in this bus. I have prepared the length of rainbow cable by trimming the insulation and tinning the wires, to make them easier to solder onto the circuit board.
Above you can see the soldered connections. The white ribbon with the orange stripe is connecting to the other side of the board. The other end of this ribbon connects to a separate board that has the pads where a circuit is made when you press a key on the keyboard.
Now you might be asking: 12 wires? That doesn’t sound like enough to make a connection for each key on the keyboard. Yes that is right, there are 32 keys in two and a half octaves on the keyboard. The solution is that the keyboard is scanned rather than having a wire for each key.
The 32 keys are divided into 4 columns and 8 rows (4 x 8 = 32). When you press a key one column is connected to one row. The synth pulses each column in turn (very quickly) and as it does so it checks to see if there is a voltage on each row. When it finds a voltage it knows that key is currently pressed.
When you connect the oscilloscope to one of the column pins and connect the reference to the earth pin of the IC you see this. Time is measured left to right and voltage up and down. The base line represents 0V and the peeks are 5V. This is typical in a digital circuit.
This image shows how each column pin has a turn at pulsing. When one has finished, the next one starts and then back to the beginning.
I passed the rainbow cable out through the back so I can do further testing without taking it apart all the time.