The control of our 9 faders will require some careful design. In a nutshell, a motorized fader is made of a DC motor, a linear potentiometer, and a touch sensor. Therefore, to control it, you need to be able to drive a DC motor, which has to happen in the digital domain, you need to read a potentiometer setting, which requires an ADC converter, and you need a digital input for the touch sensor.
The simplest solution for driving a DC motor is to use an h-bridge. But when you add the circuitry required for the ADC conversion, you quickly realize that a more integrated package would be much nicer to deal with. Here comes the MM908E625, an integrated single package solution including a high performance HC08 microcontroller with a SMARTMOS analog control IC. For less than $5, it packs everything we would need to drive a motorized fader and its associated pair of bargraphs.
By using this component, we will be able to reduce the number of GPIO pins required on the Parallella side, while confining the DC currents used to drive the motorized fader to its associated bargraph PCB, instead of carrying it through the enclosure. And with 13 I/O pins to play with, we should be able to drive the 3 LED switches that surround every dual bargraph as well.
Each of the 9 bargraph boards will have its own microcontroller, and the 9 microcontrollers will be connected in a cascade to submodule 1 over SPI. This will allow all 9 motorized faders, 18 bargraphs, and 27 LED switches to be controlled from a single submodule, using only 4 GPIO pins. Furthermore, it will require a single cable from the backplane to one of the bargraph boards, while the remaning 8 bargraph boards will be daisy-chained together with short cables.