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This is part of the schematic of the Roland A49 MIDI controller: enter image description here

This is the connector that makes the connection between the keyboard and the main board. PR0-7 and SC0-7 are scan signals, the program iterates through them (applies a voltage to one pin at the time) and reads T0-7 to see which keys are being pressed. On the keyboard side, its a simple switch network.

I drew the complete circuit for one key: enter image description here

The 100 Ohm resistors I can understand as overcurrent protection in case the GPIO pins are configured both as outputs.

But what is the case with the caps? What do they do?

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The capacitors provide some ESD protection in addition to removing RF and slowing down the signal transitions for controlling EMI.

Also I am sure that the 100 ohm resistors are not there in a ready made product if an IO pin is accidentally set as output, the product would be faulty if that happens.

The resistors also act as RC filters with the caps to limit EMI by slowing down signal edges and limiting bandwidth, acting as series termination resistors to prevent overshoots/undershoots, ringing and signal reflections in the random wiring/cabling inside the device.

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  • \$\begingroup\$ I was also thinking about the resistors being a part of an RC filter, but in this configuration it is not an RC filter. The resistor should come first in the signal chain, and then the capacitor to ground. In this configuration the cap charges instantly. \$\endgroup\$
    – Márton Gáspár
    Commented Oct 7 at 8:53
  • \$\begingroup\$ @MártonGáspár Incorrect. Both the SC and PR pins have resistors right at the MCU. Even if they didn't, the MCU innards have a finite output impedance, likely in the 20-50 ohms range. Oh neat they use a STM32F103VB as the MCU, you can open the data sheet. \$\endgroup\$
    – Justme
    Commented Oct 7 at 9:00
  • \$\begingroup\$ The SC and PR are outputs from the MCU. Doesn't this filter the direct output of the MCU and not the noise added in the cables? \$\endgroup\$
    – Márton Gáspár
    Commented Oct 7 at 9:16
  • \$\begingroup\$ @MártonGáspár The cables can act as receiving and transmitting antennas. The caps and resistors are there to remove high frequencies so that the MCU scanning the keys by toggling the outputs does not radiate electromagnetic interference. \$\endgroup\$
    – Justme
    Commented Oct 7 at 9:21

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