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DIY-MIDI
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DIY_Amiga_Midi
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D.I.Y. Amiga MIDI
-----------------
............by Steve Wright.
Ed. note: Steve's come up with another very useful contribution
for the hobbyist/musician. Follow directions and you'll have a
very cheap MIDI interface. If you find the graphics flashing when
you double-click on the icon, just press the TAB key to turn it
off.
I don't know about you but it's frustrating to pick up and read mags like
"Amiga World" and "Amazing Computing" and see the advertized prices for
articles a third of the price asked for here in OZ. We all know the reasons
for it, Duties, exchange rates, etc, but this doesn't seem to ease the pain
much.
Whats the answer? Locally produced or manufactured items certainly have a
more attractive price tag. Distribution under licence of copied software
where only royalties to the OS writers leave the country can also lower the
price. Or, Do-It-Yourself (D.I.Y.).
D.I.Y. Midi
-----------
This article we'll show you how to make your own Midi interface for around
$30.00 which works as well as any you can buy with a nice added feature to
boot.
The design of the circuitry can be the basis of a larger unit, eg., Matrix
box, Thru box, etc, but of course this is totally optional as this unit
described here works well as is. For cost reasons I haven't included a serial
pass-through, although it can be added if wished. I've included 2 LED
indicators to visually show the signal coming to and from the interface for
those times when patching troubles between Midi units become frustrating and
not knowing where the problem lies.
Operation
---------
The Midi-In signal is received via pin 5 of the Midi-In DIN socket and
passed to the opto-coupler (IC1) which electricaly isolates the input signal
from the rest of the circuit. Notice that no ground is connected on Midi-In.
This eliminates current loops and other sources of hum. D1 guards against
reverse polarity.
The output from the opto-coupler is sent to two inverter buffers (IC2)
connected in parallel. Buffer IC2c inverts the signal and sends it to pin 3
(Rx) on the Amiga's serial port. Buffer IC2e receives the same signal as
IC2c and is used to iluminate the Midi-In LED.
Midi-Thru is simply passed through buffer IC2d who's signal was taken from
the output of IC2c.
Midi-Out's signal is passed from pin 2 (Tx) on the Amiga's serial port to
buffer IC2a, resister R4 and then on to pin 5 of the Midi-Out DIN socket.
Buffer IC2f is run in series to IC2a for the Midi-Out LED.
Midi-Thru & Out's power on pin 4 of their DIN sockets is run to the Amiga's
+5v rail via the R3 resister.
The Amiga's serial pins 21 (+5v) and 7 (Gnd) are also used to power the
IC's. So in all only four wires connect the Midi interface and the serial
port.
Construction
------------
You will note from one of the pics I used a Versa Strip board for the
circuit (DSE cat # H-5616). It ended up quite compact and neat. You can use
a standard strip board if you wish. Cut the Versa Strip board down to suit.
One Versa Strip board will make about 5 or 6 Midi boards.
Its best to install the small items first, such as resistors, diode and IC
sockets. I always use IC sockets or Molex pins, they can be eliminated if
you're brave.
Double check your soldering before inserting the IC's. The 6N138 IC can be
damaged by static electricity. So, fingers off the pins and keep it in the
foiled foam until you're ready to plug it into its socket.
Now solder the LEDs in by extending the legs with short pieces of hook-up
wire (about 50mm), not forgetting about polarities.
The wiring up is now fairly straightforward. I used line DIN sockets,
although panel DIN sockets may make the hook-up easier, but the housing box
may need to be enlarged to accommodate them. The smallest jiffy box I could
find worked and looked fine (28x54x83mm). The 25 pin DP plug should be on a
cable length of at least 400mm so as to allow the unit to sit beside and to
the front of your Amiga unless you omit the LEDs and don't require the
visual indication of the signal flow, then of course the cable length can
be as short as you wish.
When the board is cut to its correct size, it fits very neatly into the
ribbed sides of the jiffy box, negating the need for any fixing reqirements.
With this in place insert the LEDs into the holes at the front of the unit;
mounting bezels look good but can be left out.
Screw the lid on, plug it in and voila!
A few hours, a few dollars and you're plugged into the Amiga Midi world.
Enjoy.
Options
-------
As I said earlier there could be many options, but one easy one is to add
an extra Midi-Out for the cost of only a resistor and a DIN socket. Connect
pin 1 to pin 3 of IC2, and to pin 4 connect a 220R resistor, then from this
resistor run to pin 5 of the second Midi-Out DIN socket. Pin 4 of the DIN is
as per the Midi-Thru & Out. PIN 2 to earth. Easy.
Parts List
----------
Resistors (all ¼ watt 5%)
R1-4 .......... 220R (DSE $0.07 ea)
R5-6 .......... 100R " "
R7 .......... 1k " "
Semiconductors
D1 .......... 1N914 (DSE $0.05)
IC1 .......... 6N138 (Geoff Woods $8.96)
IC2 .......... 74LS04 (DSE $0.70)
LED1 .......... red 5mm (DSE $0.45)
LED2 .......... green 5mm (DSE $0.50)
Miscellaneous
3×5 pin line DIN sockets (DSE $1.15 ea)
25 pin DP plug incl back shell (Jaycar $6.15)
Jiffy box (DSE $2.30)
8 pin IC socket (DSE $0.20)
14 pin IC socket (DSE $0.25)
2×LED mounting bezels (DSE $1.35 ea)
Versa strip board (DSE $4.95)
1m twin shielded flex (DSE $0.85)
1m 4-core flex (DSE $0.95)
-----------
Total $32.95
-----------
Note :- All items listed were available at time of writing and prices
indicated are supplied as a guide only.
____________________________________________________________________________
Till next time ..............................................Steve Wright.