Official MIDI Specifications

MIDI Transports

The original MIDI 1.0 Specification called for using a 5-Pin DIN cable to connect MIDI compatible devices, but today there are many different "transports" capable of carrying MIDI data, and the specification for 5-Pin DIN has been updated.

Some of the transport specifications for MIDI were developed in conjunction with other organizations (typically those who control the specification for that particular transport). Below are descriptions of all MMA-approved alternate transport specifications and where to get the document.
As computers have become central components in many MIDI systems, USB has become the most widely used protocol for transporting MIDI data. The USB Implementers Forum, including members of the MIDI Manufacturers Association (MMA) and the Association of Musical Electronics Industry (AMEI), have created the USB Class Definitions for MIDI Devices. These specifications defines how USB transports MIDI data.
BLE MIDI 1.0.pdf
This specification defines a method for encoding and decoding MIDI data for transmission over Bluetooth Low Energy (BLE) connections which enables product compatibility across all computing platforms (iOS, MacOS, Windows, and Android).

Bluetooth Low Energy (BLE) is a wireless connection specification supported by the majority of mobile computing devices. BLE (also called Bluetooth SMART) can extend battery life for mobile devices using connected accessories (such as MIDI keyboards and controllers) that don't continuously stream data.

RTP-MIDI
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RTP-MIDI ( Viewable on this webpage -IETF RFC 6295 - is a specification for sending/receiving standard "MIDI 1.0" messages using standard networking protocols ("Real-time Transport Protocol" and "Internet Protocol"). RTP-MIDI includes a data recovery mechanism (MIDI event journaling) to address packet loss that can occur on networks, eliminating the need for packet retransmission (which would increase latency and reduce throughput). A thorough description may be found here:
As an IETF standard, RTP-MIDI is not proprietary technology exclusive to any specific company, and is intended for use by anyone without obtaining a license or paying any royalties.
(CA-033) MIDI 1.0 Electrical Specification Update [2014]

The MIDI 1.0 Specification includes an Electrical Specification which uses a 5-Pin DIN connector and 5 Volt electronics as was common at that time. The Specification was updated in 2014 to reflect current design requirements such as 3.3 Volt circuitry and reduced RF interference.

Electrical Specification Update 1.1 [2014]
This updates the MIDI 1.0 Electrical Specification to include 3.3-volt signaling. This update also adds optional ferrite bead RF filters to the signal pins, and optional grounding provisions for the grounding shield connectors on the MIDI jacks.

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rp54 Specification for Use of TRS Connectors with MIDI Devices.pdf
This document defines how to wire “TRS” (tip-ring-sleeve) connectors for use with MIDI devices, and describes the necessary device circuitry and cable specifications to support MIDI communication over the TRS connection.

TRS
rp27v10-spec(1394).pdf
The "MIDI Media Adaptation Layer for IEEE-1394" (aka 1394-MIDI or Firewire-MIDI) is part of the AM824 Protocol developed in conjunction with the 1394 Trade Association in 1999 (now known as IEC International Standard 61883, Part 6).The MMA Specification for MIDI over IEEE-1394 defines a standard way to place MIDI messages into an IEEE-1394 (aka FireWire) AM824 data packet so that products from various manufacturers can interoperate.

FireWire supports peer-to-peer connections (not requiring a computer) which are also common in MIDI systems, but is lacking a connection management standard, so most FireWire Audio/MIDI devices end up connected directly to computers instead.

While it could be more convenient for users if there was a "class compliant" FireWire-MIDI driver installed on every computer (one driver for all brands of devices) that is not necessary as long as device makers provide the necessary drivers for their own devices. Most FireWire audio and music device makers do provide their own drivers, and so might choose not to follow the MMA format for packetizing MIDI and audio over FireWire.

The IEEE-1394 AM824 Specification is also supported by IEEE-1722 (AVB Transport Protocol).