mac-midi.txt (revc)

(Note: this file is a companion to the macbasic.gif, and the macdelux.gif files.
 These 3 files must always be distributed together, and in an unmodified form - 
other than this restriction, you are free to use the files as you wish)

I designed these adapters as a lark some time ago. being rather dis-satisfied 
with the self powered adapters that would not work on the mac portable. (This 
machine is the worst of the lot as far as available voltage goes)

The power supply in these units is derived from the drive signals on the mac 
connected to the adapter.  The trick is to use schottky diodes - in the basic
model I used MBR330 diodes, as these were what were available to me.  These are
rather oversized for the application, but work fine.  In the revised unit, 1N5817
diodes are used.  These are interchangable, and the only requirement for the 
schottky diodes is a 100mA/25PIV rating.  If you are using these units on a 
desktop mac,  you can use ordinary 1N4000 diodes.

As for the 6N137, these can be substituted - there are suitable _active_
opto-isolators made by several companies. For example, the 6N138, and
the Sharp PC900 will work.  Note that the device should trigger
at 4 mA, as the 'space' current is 5 mA. As the 6N137 is a open collector
device, the pullup resistor may not be required with other devices.

The 1MHz crystal oscillator is of the ordinary metal, rectangular variety.

Pin numbers on the plugs are the standard numbers on the connectors - these
may not be marked on the sockets, so the pinouts are reproduced here-
 
      8     7     6                               
                                      4    2    5 
    5         4     3         1                    3        -  this is looking into sockets
                                                                     from the outside  
        2       1

     MiniDIN-8                    DIN-5

The MIDI spec says that the shield must be connected directly to ground on the
MIDI OUT end, and connected through a 0.1uF, 200V cap on the MIDI IN end.  I 
consider these connections to be unneeded, but you may wish to install them.

In the Delux version, the THRU button is added, and TX-RX lights with pulse 
stretchers to make the signals more visible are added.  You can, of course, leave 
all the lights and associated stuff disconnected.  Be sure to wire any unused 
inputs of the '04 to the -2.5V supply.

If you have any questions, feel free to e-mail me:
Michael O'Hara
cetacean@netcom.com

----------------Release history---------------------
10/28/93  First release.
10/29/93  Noticed that the released "deluxe" schematic didnot include a 
		correction in the wiring for the "thru" switch.
11/4/93    Correction to the indicator lamps (moral - dont lose the redline :) )
		Also clarified the documentation somewhat.
11/6/93		Typo in the revised text file.  Added DIN-5 pinout, opto subs.