MIDI interface V2.0 for the amiga 2000 by Antti Ketola, Finland

MIDI2.description
-----------------
This circuit is a midi interface for the Amiga 2000. The interface plugs in
the Serial Port, and as a matter of fact it is only a signal converter.
It contains no intelligence, no buffers, it only acts as a bridge beetween
the serial port and Midi cables.

It consists of a RS232 line transmitter (1488), a RS232 line receiver (1489),
an opto-isolator (6N139) and a +5V voltage regulator, and a few passive
components. 
Detailed description of these components are found in databooks, for example:

 1488,1489: Motorola telecommunications device data (DL136)
 6N139:     HP Optoelectronics designers' guide
 7805       Motorola linear/switchmode voltage regulator handbook (HB206)

The circuit should be in accordance with MIDI 1.0 specification, which I
have refernced.

IC1 functions as receiver, it isolates input as speceified. Internally,
MIDI in drives a led in IC1, and R5 limits the current. The diode V1 is
for protection against wrong connections. R1 and R2 bias the phototransistor
which leads the signal to pin 2 on IC2.
IC2 is a RS232 line transmitter(driver). It needs supply voltages of -12V 
(pin 1) and +12V (pin 14), which it takes from the connector pins 10 and 9.
Not forgetting the zero volts ground, it connects from pin 7 to pin 7 on 
the RS connector.
IC3 is a RS232 line receiver, that is it takes ±12V in and gives 0-5V out.
My connection makes a trick (almost). +5V is fixed to pin 4 of MIDI out 
connector and the pin 5 goes up and down. It inverts,as well as IC1! 
Don't worry, see the datasheets and the specification, and you'll see that it 
should be like this. Not elegant? Nah, it's cheap and it works. In Midi, 
"Current ON" means "0".
IC4 takes some +12V and gives out +5V supply for the Opto, the 1489 and the Midi
out 4. In the picture MIDI2.pic.320 it is the PLASTIC SIDE UP. 
Left pin= Voltage IN
Middle pin= GROUND 0 Volts
Right pin= +5V out (from ground)

Capacitors are polyester, at least 30 V rated (I use EVOX 100V).
Resistors are 1/4 W.

Parts list (it is in the drawing, too)
----------

IC1      6N139
IC2      MC1488
IC3      MC1489
IC4      MC7805
       (there are other manufacturers than Motorola)
V1       1N914   or almost any small diode
R1       270 Ohm
R2       1.2 kOhm
R3,R4,R5 220 Ohm

C1       0,33µF
C2       0,1 µF

2 pc     5p/180° female DIN connectors for MIDI IN and MIDI OUT
1 pc     RS-232 25-pin D-type female

connection board (I fitted mine to 75mm x 50mm)   
case             (I fitted mine to a 80mm x 55mm plastic box)
screws 

Use IC sockets: 1 pc 8pin DIL for the opto and 2pc 14pin DIL for RS ICs.

You might build it on a printed circuit board. I do not include a layout
picture, but lay-outing is very simple from the drawing, because there is no 
wire crossings. One-sided pcb is sufficient.
Alternatively, use VERO -board, a test connection board with copper stripes 
and drilled holes. Or just wire it up on a holeboard. 

Be careful with your Amiga. 

       DO NOT SHORT THE OUTPUT VOLTAGE PINS ON THE SERIAL PORT!

They are so damn near to each other. I shorted mine for a while and then
I had to replace the RS line drivers on the Amiga. It's a hell of a
job to replace something with no sockets on 4-layer Amiga motherboard. 
I almost cut the foils!