Sorry for the strange question. I was wondering if anyone knew anything about the internals of the RCX mindstorms brick. Specifically, does it have a pull-down resistor (if no sensor is connected, will it read 0)?

1 Answer 1


The RCX input port has a pull-up resistor, not a pull-down (so it reads high: 1024 in raw mode). You can see reverse engineered I/O port electronics here: http://www.brickshelf.com/cgi-bin/gallery.cgi?f=109570

RCX sensor circuit

  • Thank you for the response! Just want to double-check: if I connect a button to the RCX, it would read that it is on when it is off and vice-versa?
    – dts
    Feb 13, 2017 at 12:02
  • 1
    Depends on how you read it! in raw mode, you would get 0 (or a low value if your switch has some internal resistance as does the RCX touch sensor) with switch pressed, 1024 if released. But if you configure your sensor as a touch sensor, you'll get 0 if released, 1 if pressed.
    – Philo
    Feb 13, 2017 at 12:55

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