3.5mm TRS MIDI Type A Wiring and Transmission

This article explains how standard MIDI data is transmitted over a 3.5mm TRS (Tip, Ring, Sleeve) connector using the Type A specification. It details the physical pinout mapping between legacy 5-pin DIN connectors and the 3.5mm jack, the mechanics of the electrical current loop, and how signals are isolated and received by compatible synthesizers, sequencers, and audio interfaces.

The Shift to 3.5mm TRS for MIDI

Traditional MIDI hardware relies on bulky 5-pin DIN connectors. Although these connectors have five physical pins, the original MIDI 1.0 specification only requires three conductors to transmit a signal: a positive current source, a current sink (data line), and a ground shield. As music hardware became more compact, manufacturers adapted the ubiquitous 3.5mm TRS stereo mini-jack to carry these three signals.

To eliminate industry confusion caused by conflicting proprietary pinouts, the MIDI Association officially adopted the "Type A" configuration as the standard specification for TRS MIDI connections.

Type A Pinout Mapping

A standard 3.5mm TRS cable consists of three conductive sections: the Tip, the Ring, and the Sleeve. In the Type A standard, these three conductors correspond directly to the active pins of a standard 5-pin DIN connector:

In this configuration, Pins 1 and 3 of the legacy DIN jack remain unconnected, just as they are in standard 5-pin MIDI implementations.

How the Current Loop Transmits Data

MIDI does not transmit data as raw voltage levels; instead, it utilizes an asynchronous serial current loop operating at a baud rate of 31,250 bits per second. The physical transmission relies on the opening and closing of this circuit:

  1. Current Flow: The transmitter's Ring line supplies current (typically limited to 5 mA via a pull-up resistor) into the circuit.
  2. Serial Modulation: The transmitter’s UART (Universal Asynchronous Receiver-Transmitter) modulates the Tip line. When the transmitting UART pulls the Tip low, current flows from the Ring, through the receiver’s circuit, and down to the Tip.
  3. Binary Representation: An active current flow represents a binary 0 (logic low), while the absence of current flow represents a binary 1 (logic high). In its idle state, the line sits at logic 1, meaning no current flows through the circuit.

Galvanic Isolation and Reception

At the receiving device, the incoming signal from the 3.5mm jack is routed directly through an optocoupler before reaching the receiver's microcontroller.

When current flows from the Ring through the Tip, it powers an internal LED inside the receiver's optocoupler. The light from this LED activates a phototransistor on the opposite side of the component, converting the optical pulse back into an electrical digital signal for the receiving device's UART.

This optical barrier ensures complete galvanic isolation between the two connected devices. Because there is no common electrical ground path shared between the audio processors—the Sleeve acts purely as a radio-frequency shield and is grounded only at the output port—the TRS Type A implementation avoids ground loops, hum, and digital noise interference while preserving serial data integrity.