Consumer info
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If a daughterboard breaks, it can be replaced with any of the C series.
It is not recommended to go backwards in numbering; i.e. do not downgrade from a C3 to C1 if the board came with a C3. However, it will still function normally (just be careful not to zap the board with too much static or whatnot).
Always use a one-to-one cable. Cables which invert the pinout will cause reverse voltage damage to your keyboard.
One good option for testing and low quantity boards is the Sparkfun Qwiic cables; these are confirmed functional and compatible with the Unified USB Daughterboard series.
For higher order quantities, Digikey carries both the "correct" and "incorrect" variants of the JST cables in various lengths; before ordering, check the specification/drawing PDF for the item and verify that the cable is of the correct type.
The correct type is the 4-wire variant of the "Socket to Socket, Reversed" type cables listed here.