Serial MIDI Converter: Quick Guide to Turning Serial Data into MIDI Signals

How to Build a Serial MIDI Converter: Step-by-Step Tutorial

Overview

A serial MIDI converter translates serial data from microcontrollers, sensors, or legacy devices into standard MIDI messages (note on/off, CC, program change, etc.) so they can control MIDI instruments, DAWs, or synth modules. This guide assumes a microcontroller (Arduino-compatible) that outputs TTL serial and converts it to MIDI DIN-5 (5-pin) and USB-MIDI.

Parts and tools

• Microcontroller: Arduino Uno, Pro Micro, or any with hardware serial (1x)
• MIDI OUT jack: 5-pin DIN female and socket or MIDI breakout module
• MIDI DIN cable (if testing with hardware synth)
• 220–2200 Ω resistor (typical 220 Ω)
• 5–10 mA current-limiting resistor for optocoupler if used
• Optocoupler (e.g., 6N138) — optional for isolation on input from device
• USB-MIDI capability: use a board with native USB (Pro Micro, Leonardo) or add USB-MIDI via USB host/device chip
• Breadboard, jumper wires, soldering iron, small enclosure, perfboard/PCB
• Power supply: 5V (USB or regulated)
• Computer with Arduino IDE and a DAW or MIDI monitor

MIDI basics (concise)

• MIDI uses 31.25 kbps serial, 8-N-1 framing.
• MIDI messages start with a status byte (≥ 0x80) followed by 1–2 data bytes (< 0x80).
• Common messages: Note On (0x9n, note, velocity), Note Off (0x8n, note, velocity), Control Change (0xBn, controller, value).

Step 1 — Choose message mapping

Decide how incoming serial data maps to MIDI:

• Simple text commands: “N60 V127” → Note On, note 60, velocity 127
• Binary packet: [0xAA, cmd, data1, data2, checksum]
• Sensor-stream mapping: map sensor range to CC 0–127

Assume for this tutorial: ASCII commands like “NOTE,60,127 ” and “CC,1,64 “.

Step 2 — Wire MIDI OUT (DIN)

Use the standard passive circuit:

• Connect Arduino TX (if using hardware serial at 31250 bps) through a 220 Ω resistor to MIDI pin 5.
• Connect MIDI pin 4 to +5V through a 220 Ω resistor.
• Connect MIDI pin 2 to ground.
  (Or implement opto-isolation if receiving MIDI IN; for MIDI OUT this simple circuit is common.)

Pinout (DIN-5):

• Pin 2 = GND
• Pin 4 = +5V via resistor
• Pin 5 = TX via resistor

Step 3 — Configure serial baud and parse input

• MIDI baud: 31250 — for outputting raw MIDI bytes over DIN.
• Use a second serial port (SoftwareSerial or Serial1 on boards with multiple UARTs) if you also need USB serial for debugging. Example for Arduino with Serial1:

Code sketch (Arduino C++):

cpp
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// Assumes board with Serial1 (e.g., Pro Micro). Serial for USB debug.
void setup() {
Serial.begin(115200);      // USB debug
  Serial1.begin(31250);      // MIDI DIN out
}

void loop() {
  if (Serial.available()) {
    String line = Serial.readStringUntil(’ ‘);
    line.trim();
    if (line.length() == 0) return;
    // Expected: “NOTE,60,127” or “CC,1,64”
    int comma1 = line.indexOf(’,’);
    int comma2 = line.indexOf(’,’, comma1 + 1);
    String cmd = line.substring(0, comma1);
    int a = line.substring(comma1 + 1, comma2).toInt();
    int b = line.substring(comma2 + 1).toInt();

    if (cmd == “NOTE”) {
      byte status = 0x90; // channel 1 Note On
      Serial1.write(status);
      Serial1.write((byte)a); // note
      Serial1.write((byte)b); // velocity
    } else if (cmd == “CC”) {
      byte status = 0xB0; // channel 1 CC
      Serial1.write(status);
      Serial1.write((byte)a); // controller
      Serial1.write((byte)b); // value
    }
  }
}

Step 4 — USB-MIDI (optional)

• Use a microcontroller with native USB (Pro Micro/Leonardo/MicroMIDI) and implement the USB-MIDI library, or use the Arduino MIDIUSB library to send MIDI over USB to the host.
• For boards without native USB, use a USB-to-MIDI converter chip/module or send serial to a computer and run a small bridge program that converts to virtual