Correct. MIDI gets decoded by the UART and then sent directly to the CPU. I believe the Ion doesn't have aftertouch at all, so I agree with the conclusion that it is a problem local to the SD-1's KPC board.madtheory wrote:It's highly unlikely, I think, that external MIDI input gets routed through the keyboard control circuitry. That's for converting mechanical movement in the Ensoniq's keyboard into control signal. MIDI already has the AT encoded.
Looking back at your original description of the issue, it looks like the actual pressure sensors are working fine, but the behaviour with the pedal would suggest some kind of bad ground or inconsistent loading or interaction of CV signals on the analogue side, or maybe at whatever point those signals are mixed, or multi-plexed (don't know if they are or not). So I'd look at everything before the ADC (I assume there is one taking in keyboard and pedal voltages) input.
As far as things likely to blow, Q1 and Q2 on the KPC PCB are the most vulnerable items dealing directly with the footswitches prior to them being read by the main keyboard handling processor [Motorola 68HC11A1P] which--as could be guessed--also handles the final steps of keyboard processing before passing the CPU tidy packages of footswitch, keypad, and keyboard data.
KPC BOARD OPERATION
Motorola 68HC11A1P is the main brain in the keyboard processing board:
Port A selects which group of music keys are currently to be scanned and also reads in the state of the footswitches
Port B is addressing (exclusively)
Port C is address/data lines which gets data to/from the EPROM and KPC chip
Port D is serial transfer to/from the mainboard and the display/keypad PCBs
74LS373 is a latching IC that demultiplexes Port C of 68HC11 into separate data and address streams.
The EPROM (27C256) contains the instructions for the operation of 68HC11.
The KPC CHIP [U3] handles the reading of the actual music keyboard:
F0~F3 are output lines coming from the keyboard, to be decoded by the KPC chip and the result is forwarded to the 68HC11; the result of F0~F3 is dependant upon which keys are currently being driven by 68HC11 Port A (CS0~CS3).
68HC11 processes both the display/keypad button and music key data and then sends it to the mainboard as serial data.
As far as the actual inductance functionality goes, this thread explains the theory of operation:
http://electronics.stackexchange.com/qu ... d-function