Link to product https://www.duet3d.com/Duet3Mainboard6HC

https://docs.duet3d.com/Duet3D_hardware/Duet_3_family/Duet_3_Mainboard_6HC_Hardware_Overview

Microswitch

This applies to a bare microswitch, not to a microswitch on a board with a LED.

Duet WiFi, Duet Ethernet and Duet Maestro: connect the switch between GND and STP/IN. These are the outer 2 pins of the 3-pin connector. Note: this is not the same as on RAMPS. Duet 3: connect the switch between the IN and GND pins of your chosen IO_x connector. We recommend you use the normally-closed contacts of the microswitches, which are generally the outside two connections on the microswitch, and set the signal polarity to active high (S1) in the M574 command. If for any reason you use normally-open microswitch contacts, then if you use RepRapFirmware 1.x or 2.x then you will need to set the signal polarity to active low (S0) in the M574 command, and if you are using RepRapFirmware 3 then you will need to use S1 but invert the polarity by putting a ! character at the start of the pin name in the M574 command.