MIDI 1.0

There are two sides to MIDI 1.0: the hardware transport specification describing the electrical and mechanical connection, and the message format specification.

The MIDI standard consists of a communications messaging designed for use with musical instruments, as well as a physical interface standard. It consists physically of a one-way (simplex) digital current loop electrical connection sending asynchronous serial communication data at 31,250 bits per second. 8-N-1 format, i.e. one start bit (must be 0), eight data bits, no parity bit and one stop bit (must be 1), is used, so up to 3,125 bytes per second can be sent.

Only one end of the loop is referenced to ground, with the other end "floating", to prevent ground loops which may otherwise cause interference and hum in analog audio signals. The current loop on the transmitter side drives the LED of an opto-isolator on the receiver side. The current loop is specified as 5 mA. The opto-isolator must be a high-speed type, with less than 2 μs risetime. As most opto-isolators have asymmetrical positive-going and negative-going slew rates, they slightly alter the signal's duty cycle. If several MIDI devices are connected in series by daisy-chaining the MIDI THRU to the next device's MIDI-IN, the signal gets more and more distorted, until receive errors occur due to pulse narrowing.

At the physical layer (MIDI cable), a pair of wires carry the MIDI signal. The voltage difference is normally 0 volts (both at positive potential referenced to ground) in the idle state, which is seen as a '1' at the MIDI receiver due to logic inversion by the Opto-isolator. A MIDI message start bit (0) causes a voltage differential on the wire pair (current loop) which is seen at the MIDI receiver as a '0'. The 8 data bits can be either '0' (low) or '1' (high) with the stop bit (1) seen at the MIDI receiver as a '1'. To summarize:

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