The Duet 3 Mainboard 6XD is designed to allow interfacing with the most common external stepper and servo motor drivers using step and direction signals, with support for 6 external drivers. In addition it provides opto-isolated Input and Output connections for some of the GPIO functions to allow easier interfacing with PLC control systems.
Product page is here: https://www.duet3d.com/duet3mainboard6xd
Documentation is here: https://docs.duet3d.com/Duet3D_hardware/Duet_3_family/Duet_3_Mainboard_6XD_Hardware_Overview
As always please discuss this on the Duet3d forum: https://forum.duet3d.com/