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MIDI - the physical interface

Electrical specification and interfaces

  • MIDI 1.0 DIN Electrical Specification, straight from the horse's mouth for 3.3V and 5V. The full spec reveals that the receiver must require less than 5 mA to turn on and that rise and fall times should be less than 2 microseconds, limiting the choice of opto couplers.
  • Kevin's MIDI Cheatsheet is even better with an RX/TX schematic and pinnings

Opto-Couplers at the RX side

MIDI interfaces always use opto couplers at the receive side - at least I haven't seen a MIDI interface without one yet. There a several good reasons for the galvanic isolation achieved with an opto coupler:

  • MIDI equipment running at different earth potentials together with ground loops may create potentially destructive equalizing currents that are better kept away from the precious electronics of your vintage synth.
  • The same ground loops may also result in injection of 50 Hz hum into your audio path. The German term "Brummschleife", translated hum loop, reflects this frequent result of a ground loop.
  • Ground loops may also introduce noise creating transmission errors.

Notes and Volts MIDI for the Arduino - Circuit Analysis describes the MIDI In circuit in detail for a 6N138 opto coupler in detail, including the biasing resistor for faster turn-off. Even more details on opto coupler behaviour can be found in Opto-couple confusion – 6N137 or 6N138?. Some common opto couplers are:

  • 4N37 / 4N38: Built with a single photo transistor,turn-on and turn-off times are specified around 10 us. Don't use these.
  • 6N137 / VO2601 / VO2611: Specified for 5 mA LED current, featuring a fast photo diode receiver (t_r typ. 23 ns, t_f typ. 7 ns) and a Shottky clamped open collector output. Min. VCC is specified with 5 V. VO2630 / VO2631 / VO4661 have two channels.
  • 6N138 / 139: Specified for LED currents in the range 0.5 ... 12 mA and min. current transfer ratios of 300% (6N138) resp. 500% (6N139) with a photo diode / darlington receiver. Min. VCC is specified as 5V, but the collector of the output transistor (Pin 6) can be connected to 3.3V via a pullup resistor in the range 270 ... 470 Ohm. The base of the output transistor (Pin 7) should be tied to ground with 4k7 resistor to speed up turn-off times. Propagation delays are specified with wide variations but 2 us are achievable.
  • HC11L1: Receiver is built with a foto transistor, still non-saturated turn-on and turn-off times around 3 us are specified. A non-saturated current transfer ratio of around 100 % is specified. I've obtained good results with this.
6N138 at 3V3 / 5V with R_LED = 750 Ohm, R_RX = 470 Ohm and R_Base = 4k7 6N138 at 3V3 /5V with R_LED = 750 Ohm, R_RX = 470 Ohm and R_Base = 10k

6N138 with split 3V3 / 5V supply, R_LED = 750 Ohm (5 mA), R_RX = 470 Ohm and R_Base = 4k7 resp. 10k

6N138 at 3V3 with R_LED = 750 Ohm, R_RX = 470 Ohm and R_Base = 4k7 6N138 at 3V3 with R_LED = 750 Ohm, R_RX = 470 Ohm and R_Base = 10k

6N138 with 3V3 supply, R_LED = 750 Ohm (5 mA), R_RX = 470 Ohm and R_Base = 4k7 resp. 10k

6N138 at 3V3 with R_LED = 750 Ohm, R_RX = 470 Ohm and R_Base = 10k (rise) 6N138 at 3V3 with R_LED = 750 Ohm, R_RX = 470 Ohm and R_Base = 10k (fall)

6N138 with 3V3 supply, R_LED = 750 Ohm (5 mA), R_RX = 470 Ohm and R_Base = 10k (details)

6N138 at 3V3 with R_LED = 750 Ohm, R_RX = 470 Ohm and R_Base = infinity

6N138 with split 5V / 3V3 supply, R_LED = 750 Ohm (5 mA), R_RX = 470 Ohm and R_Base = infinity

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