the USB cable powers the two 74HC245 octal buffer IC's that connects the printer port outputs to your stepper drivers

the 12 to 24v supply is required to power the five input opto-isolators
& the PWM to 10V analogue converter that's controlled by pin 1

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when a switch grounds an input current flows through the opto-isolators LED so the NPN photo transistor is turned ON
so the corresponding printer port input is grounded

when the switch is tripped and opens
the printer port pin goes high
(set pin as active high in mach3)

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John