Generic Midi interface

1 Specifications
A Midi  interface is the hardware  connection between two Midi devices. Only
digital information  describing which note is to be played is transferred by
Midi. The actual audio signal is carried by separate cables. To avoid ground
loops, the Midi  interfaces  of the  devices  are  electrically isolated. An
electronic  device called  an  optocoupler is used to achieve the isolation.
The optocoupler  translates the incomming  electrical-signal  into light and
then it translates the light-signal back to an electrical-signal.

1.1 Serial protocol
The Midi interfacing standard uses a 31.25kHz baudrate. For Midi, the parity
must be  set to NONE,  the  number of stop bits must be set to 1 and finally
the number of  data-bits must be set  to 8. For  Intel-like devices like the
8252 UART or the 8031  micro-controller  it is difficult to combine the Midi
baudrate with 'normal' baudrates like 9600 baud.

1.2 Hardware requirements
A Midi  interface is a  current-loop  type interface. The current is approx.
5mA. The advantage  of useing a current loop lies in the fact that a current
flow is less sensitive to noise than a voltage-level signal, like RS232.

2 How to make a Midi interface for your project
The UC_MIDI.GIF contains the schematics of a receive and a send circuit. For
an  Midi-thru  option  I refer  to the Amiga interface which is explained in
another  place  on  this WEB-site. The numbers  placed near the wires in the
schematics are the pin-number of the DIN plugs.

2.1 Send circuit
The top circuit shows the schematic of a typical send circuit. The output of
the UART (=Universal Asynchronous Receiver Transmitter=serial interface) can
be either  a TTL totem  pole  output or an open  collector output. In either
case the  TXD output  will sink the current  floating through the interface.
Sinking the 5mA is no problem for any TTL-compatible output.

2.2 Receive circuit
The receive  circuit consist of 2  resistors  and  an  optocoupler.  In some
circuits, which I've seen floating around on the net, a 6N138 optocoupler is
used.  Personally I prefer to use the 6N136 (when working with standard TTL)
over the 6N138  because the configuration  with the  6N136 consumes nearly 9
times as  less power.  The reverse  biased  diode  across  the  LED  of  the
optocoupler  is not really  necessary, but  highly  recommended.  This diode
blocks  reverse  voltages  which  may  be higher than the  reverse  blocking
 voltage (=30V at max!) of the LED in the optocoupler. 

3 Comments
If you have any questions or suggestions please feel free to email me at:

nctnico@cistron.nl