How 5-Pin DIN MIDI Cables Transmit Data

A traditional 5-pin DIN Musical Instrument Digital Interface (MIDI) cable transmits digital event messages—not audio signals—between electronic musical instruments and audio hardware. This article details the physical and electrical architecture of the 5-pin DIN connection, explaining how it uses an asynchronous serial protocol, a 5-milliamp current loop, and opto-isolation to deliver reliable, latency-free musical performance data between devices without introducing electrical interference.

Digital Performance Data vs. Audio Signals

Traditional MIDI does not transfer sound. Instead, it transmits binary instructions representing musical actions. When a musician plays a key on a synthesizer, the keyboard generates commands such as "Note On," which specific pitch was pressed, and the velocity (force) of the strike. These instructions are packaged into small binary packets and sent serially across the cable to trigger sounds or automate controls on a receiving device.

The 5-Pin DIN Layout and Pinout

Despite the connector having five physical pins arranged in a 180-degree semicircle, the classic MIDI standard only utilizes three of them.

• Pin 1: Unconnected (reserved for proprietary or future expansions).
• Pin 2: Ground / Shield. Connected to the cable shield at the MIDI Out and MIDI Thru ports to prevent radio-frequency interference (RFI).
• Pin 3: Unconnected.
• Pin 4: Current source (+5V supply fed through a 220-ohm current-limiting resistor).
• Pin 5: Current sink / Data line (connected to the transmitting UART output through a 220-ohm resistor).

Pins 1 and 3 are intentionally left disconnected to avoid ground loops and preserve compatibility across diverse hardware.

The 5mA Current Loop

Unlike modern computer interfaces (such as USB or Ethernet) that rely on high-frequency voltage fluctuations, traditional MIDI relies on a robust 5-milliamp (mA) current loop.

Data is communicated by switching the current on and off rather than measuring voltage levels at the receiver:

• Logic 0 (Active / "Space"): Current flows through the loop. The transmitting device pulls Pin 5 low, completing the electrical circuit from Pin 4 (+5V) through the cable and into the receiving device.
• Logic 1 (Idle / "Mark"): No current flows. Pin 5 is held high, turning the current off.

Because the system measures the presence of current rather than a fragile voltage reference, it exhibits high noise immunity against electromagnetic interference across cable runs of up to 50 feet (15 meters).

Opto-Isolation: Eliminating Ground Loops

A critical design feature of the 5-pin DIN specification is hardware-level galvanic isolation via an optocoupler located at the MIDI In port.

1. The current arriving from Pin 4 and Pin 5 passes directly through an internal Light Emitting Diode (LED) inside the receiving instrument's optocoupler.
2. The illuminated LED activates a light-sensitive phototransistor directly opposite it inside the chip.
3. The phototransistor switches the receiving instrument's internal logic circuits, reading the incoming binary message.

Because the signal is transferred as light inside a sealed chip, there is no direct electrical connection between the transmitter and receiver. Pin 2 (the shield) is specifically left floating (unconnected) at the MIDI In port. This prevents ground loops—a common source of 50/60 Hz hum and audio buzz in studio and stage environments—and shields sensitive musical equipment from damaging voltage spikes.

Serial Protocol and Transmission Speed

MIDI data moves in a unidirectional (simplex) fashion over an asynchronous serial stream managed by a Universal Asynchronous Receiver-Transmitter (UART) chip:

• Baud Rate: 31,250 bits per second (31.25 kbaud), with a tolerance of ±1%. This specific rate was chosen in the early 1980s because it was an exact division of the standard 1 MHz and 2 MHz clock speeds of microprocessors at the time.
• Framing: Each transmitted byte uses standard 10-bit asynchronous framing consisting of:
  • 1 Start bit (Logic 0)
  • 8 Data bits (transmitted least significant bit first)
  • 1 Stop bit (Logic 1)

At 31,250 baud, each individual bit lasts 32 microseconds, meaning a single 10-bit character frame takes 320 microseconds to transmit. A standard three-byte "Note On" message requires roughly 1 millisecond to travel across the wire, ensuring instantaneous response times that feel imperceptible to human performers.