Pull‐Up Resistor

https://github.com/tmk/tmk_keyboard/wiki/FAQ#pull-up-resistor

Keyboard converter needs pull up resistor on signal lines in most cases. Without the pull-up resistors it may not work correctly.

Place the resistor between VCC and signal line like diagram below. In general 1K-4.7K Ohm is recommended for keyboard converters. Consult with instructions of target converter.

IBMPC converter(for example):

Keyboard                                       MCU(AVR)
   -----.                                      ,-----.
      5V|------------------------------+--+----|VCC  |
        |                              |  |    |     |
        |                              R  R    |     |
        |                              |  |    |     |
    DATA|------------------------------|--+----|PD0  |
   CLOCK|------------------------------+-------|PD1  |
        |                                      |     |
     GND|--------------------------------------|GND  |
   -----'                                      `-----'
R: 1K-4.7K Ohm resistor

https://github.com/tmk/tmk_keyboard/issues/71

Power rating

You can use 1/10W or higher power rating of resistor. See this for resistor power rating.

ADB-USB Converter

https://github.com/tmk/tmk_keyboard/wiki/Apple-Desktop-Bus#pull-up-resistor

You must have an external pull-up resistor between VCC(5V) and DATA(PD0) line. 1k ohm is strongly recommended.(470 ohm is more legitimate? see this.)

Keyboard                                       MCU(AVR)
   -----.                                      ,-----.
      5V|---------------------------------+----|VCC  |
        |                                 |    |     |
        |                                 R    |     |
        |                                 |    |     |
    DATA|---------------------------------+----|PD0  |
        |                                      |     |
     GND|--------------------------------------|GND  |
   -----'                                      `-----'
R: 1K Ohm resistor

4.7k-10k won't work in some cases like longer curled cable or daisy-chained multiple devices. See issues below.

• 4.7k didn't work with daisy-chained multiple devices: https://github.com/tmk/tmk_keyboard/issues/683#issuecomment-850190574
• Some mouses required 4.7k ohm and didn't work with 10k pull-up. https://geekhack.org/index.php?topic=14290.msg3117619#msg3117619
• 10k Ohm didn't work for Turbo Mouse 5 with vintage coiled ADB cable, which is expected to have quite a bit capacitance. 4.7k did work. Note that 4.7k won't work when multiple devices are connected. See above.
• Low value resistor like 100 Ohm doesn't work also: https://geekhack.org/index.php?topic=14290.msg2903590#msg2903590
• Rise time: http://geekhack.org/index.php?topic=14290.msg1070139#msg1070139
• Signal form: http://geekhack.org/index.php?topic=14290.msg296052#msg296052