Differences Between MIDI In, MIDI Out, and MIDI Thru

Musical Instrument Digital Interface (MIDI) uses distinct ports to manage the flow of digital performance data between instruments, computers, and hardware. While these standard 5-pin DIN ports look physically identical, they serve completely different directional and functional roles in a setup: MIDI In receives data to control a device, MIDI Out generates and transmits data from that device, and MIDI Thru duplicates incoming data so multiple instruments can be daisy-chained together.

MIDI In

The MIDI In port is an input jack designed exclusively to receive incoming MIDI data from an external source, such as a master keyboard, sequencer, or computer audio interface. When a device receives messages through this port—such as note-on commands, pitch bend, or clock timing—its internal sound engine interprets the data and plays the corresponding sounds or adjusts parameters. The MIDI In port does not generate data; it only listens and responds to external commands.

MIDI Out

The MIDI Out port is an output jack that transmits original data generated by that specific device. For example, when you press a key, turn a knob, or play a sequence on a synthesizer, that performance data is packaged into MIDI messages and sent out through the MIDI Out port to control external hardware or record into a Digital Audio Workstation (DAW). The MIDI Out port only transmits actions initiated directly on the host hardware; it does not pass along data that the device received from elsewhere.

MIDI Thru

The MIDI Thru port (short for "Through") produces an exact, unaltered duplicate of the data arriving at the device's MIDI In port. It does not transmit any performance data generated by the host device itself. Instead, its sole functional purpose is to allow "daisy-chaining"—connecting multiple MIDI instruments in a series from a single master controller. By running a cable from Device A's MIDI Thru to Device B's MIDI In, Device B receives the exact same commands originally sent to Device A, without latency or modification.

The Key Distinction: MIDI Out vs. MIDI Thru

A common point of confusion is the difference between MIDI Out and MIDI Thru, as both send data outward:

• MIDI Out sends only locally created data (what you play or sequence on that unit).
• MIDI Thru echoes only incoming data received via the MIDI In port, ignoring any notes played on the unit itself.

Understanding these distinctions prevents routing errors, signal loops, and communication bottlenecks, ensuring that every synthesizer, drum machine, and module in a hardware setup responds accurately to the intended controller.