MIDI began in 1983 with the release of the first MIDI compatible instrument the Sequential Prophet-600. Soon more instruments followed that could communicate through the common language of MIDI to accomplish basic functions such as playing notes, adjusting volume, changing velocity and changing patches.
USB handles both MIDI IN and MIDI OUT data through a single port while also providing an up to 5v DC power supply to a MIDI instrument. For most basic MIDI keyboards the DC power, within a distance of up to 5 meters, is sufficient to power the device.
MIDI cables have unidirectional data flow meaning separate MIDI IN and MIDI OUT cords are necessary for average use. Computers and laptops have no dedicated MIDI port. MIDI cables do not power MIDI devices.
Since the proliferation of USB, the most common MIDI conversion is MIDI to USB conversion. USB handles multiple protocols over a single connection and since MIDI data packets are small by modern standards, a single USB port can process up to 16 individual MIDI connections.
Modern MIDI controller devices offer both USB and MIDI ports while most MIDI compatible synthesizers offer only MIDI ports. As laptops and computers have no discreet MIDI port all MIDI compatible synthesizers require a USB conversion and for MIDI controllers the USB interface offers lower latency than using a separate intermediary.