You will need a 24v power supply if you wish to use the inputs or output on that side of the board. A 12V power supply will work if you only need to use the inputs.
The inputs on that side of the board are typically used for limit switches, emergency stop, or homing switches. The outputs on that side of the controller are typically used to turn on and off machine devices such as spindle, coolant valve, etc. There is also a 0-10V PWM output on that side of the board.
Another solution would be to purchase an expensive 36VDC-->24VDC convert..just connected to one of the DC outputs on your 36VDC PS and you don't have to purchase another large 24DC PS.. The power requirement is pretty low so it won't/shouldn't affect the 36VDC used to drive motors/etc.
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Hello purchased a 3 axis elect. combo (heavy gantry). Printed a wiring dia. for the mach3 USB board from the page with the picture of the Mach3 USB board. Question; wiring schm. showes USB board wierd to a 24v/12v power supply. Power that came with combo kit is 36v. This work ok? Thanks for info