DMX USB Interfaces are usually a small box that connects to a computer via a USB cable. The box will contain an electronic circuit board that receive signals from a computer and then convert them to that of the DMX-512 protocol and send them out via a DMX connector to control DMX lighting fixtures.
Some DMX USB Interfaces have to be powered by an external power supply, most are self powered via the USB cable.
Some DMX USB Interfaces have a Stand-Alone mode which allows storage of a lighting cues, scenes and can be replayed without a computer connection.
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