A 24V connection to the USB motion card is only needed if you are using limit switches (inputs), powering the spindle/VFD speed and direction, and for the outputs, for powering other devices (mist, coolant, etc.).
If you are using a VFD (Variable Frequency Drive, also called Inverter) for a spindle, they typically have a 24V out terminal that can be used with our USB motion card. If you are not using a spindle/VFD (we highly recommend one), then you will need to purchase a 24v power supply to power that side of the board.
The outputs mentioned above are to connect to external devices and will require a relay (a mechanical switch that is controlled by a digital signal). This mechanical switch can turn on a device like coolant and mist, or even a router, if you are using one. A relay is required to turn on these devices because the devices are typically higher voltage, like mains (110v or 220v). So, a relay can turn on or off a high voltage device with a lower voltage (24v) signal.
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What is the relay coil on the USB motion card for?