The i-MOD connectors are standard connectors defined by Keith & Koep Seco to allow customers to easily add functions to a baseboard.
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This is the pinning of the connector on the baseboard carrier board.
Extension boards use the opposite numbering scheme.
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Check the baseboard datasheet for button IO-voltage and usage. Typically the 3 button-signals are to be used with an open-collector Output / switch to GND.