DIN EN 63035:2018 pdf free.MIDI (Musical Instrument Digital Interface) specification 1.0 (Abridged edition, 2015) (IEC 63035:2017).
4 General
4.1 Hardware
The hardware MIDI interface operates at 31,25 x (1 ± 1 %) kBd asynchronous, with a start bit, 8 data bits (DO to D7), and a stop bit. This makes a total of 10 bits for a period of 320 ts per serial byte. The start bit is a logical 0 (current on) and the stop bit is a logical 1 (current off). Bytes are sent LSB first.
Circuit: (See Figure 1). 5 mA current loop type. Logical 0 is current ON. One output shall drive one and only one input. To avoid ground loops, and subsequent data errors, the transmitter circuitry and receiver circuitry are internally separated by an opto-isolator (a light emitting diode and a photo sensor which share a single, sealed package). The receiver shall require less than 5 mA to turn on. Rise and fall times should be less than 2 jis.
Connectors: DIN 5 pin (180°) female panel mount receptacle which is specified in IEC 60130-9 as type designation IEC-04. The connectors shall be labelled “MIDI IN” and “MIDI OUT”. Note that pins 1 and 3 are not used, and should be left unconnected in the receiver and transmitter. Pin 2 of the MIDI In connector should also be left unconnected.
The grounding shield connector on the MIDI jacks should not be connected to any circuit or chassis ground.
When MIDI Thru information is obtained from a MIDI In signal, transmission may occasionally be performed incorrectly due to signal degradation (caused by the response time of the opto-isolator) between the rising and falling edges of the square wave. These timing errors will tend to add up in the “wrong direction” as more devices are chained between MIDI Thru and MIDI In jacks. The result is that, regardless of circuit quality, there is a limit to the number of devices which can be chained (seriesconnected) in this fashion.DIN EN 63035 pdf download.