From: remek@mamura.ee.mu.oz.au (Remek Lipinski)
Subject: GUS: Working MIDI connection circuit.
Organization: Electrical and Electronic - University of Melbourne 
Date: Sun, 10 Jan 1993 10:35:47 GMT

I have used the followin circuit to connect to the Ultrasound to get the 
MIDI port working. This has been posted before but to get it to work I
had to change one of the resistors, namely the pullup resistor connected
between pins 6 and 8 of the optoisolator I also used 6N138 insted of 6N135
(I couldn't get it at the time). Also the inverters have to be able to work
at a high speed (i think midi is around 19000 but I am not sure), one of the
inverter chips I used didn't work even though the chip was fine, I gues
it could not handle the speed.
I then tested the port by playing something on the keyboard and listening
back to the seqence, the timing seemed ok. to make sure I installed a
Roland MPU 401 card into the pc and connected its midi out to the midi in
of the Gravis. I then used one sequencer to play something out of the Roland
port and another sequencer to record with the Gravis port. Then I listened
to the recorded sequence by playing it first through the Gravis midi out
and the through the Roland midi out, both sounded exacly like the original.

Anyway I hope this will be useful to someone, I for one am happy to have
the Gravis midi port working (for the fraction of the cost of what the
SB connector box costs).


15 pin D connector
                                          220R
pin-1 +5v ----+--------------------------/\/\/\---------------\
              |                                                \ 4
              |                                          Gnd--2      MIDI OUT
              |      |\      |\            220R                / 5
pin-12 tr >---|------| o-----| o----------/\/\/\--------------/
              |    14|/ 13 12|/ 10
              |                            220R
              +---------------------------/\/\/\-------------\
              |                                               \ 4
pin-15 rx <---|--------------------+                    Gnd--2      MIDI THRU
              |      |\      |\    |        220R              / 5
              |   +--| o-----| o---+-------/\/\/\------------/
              |   | 1|/ 2   3|/ 4
              |   |
              |   +------+
              |   270R*  |                       220R
              +--/\/\/\--+    +------+----------/\/\/\--------\
              |B         |C   |A     |                         \  4
            +-|----------|----|-+    |                                MIDI IN
            |  8        6     2 |  -----                       /  5
            |                   |   / \ IN914 or IN4148     +-/
            |      6N138**      |   ---                     |
            |                   |    |                      |
            |           5     3 |    |                      |
            +------------|----|-+    |                      |
                         |    |K     |                      |
pin-5 Gnd  --------------+    +------+----------------------+


Inverters are LS7404
Leave pin 2 of the MIDI IN unconnected (Don't connect to ground)

* this resitor was 5.6K in the original post, I used a the 6N138 optoisolator
  and found that this value was too high as not enough current was allowed
  to sink into the collector of the output transistor.

**In the original post this was 6N135, I could not readily find this here
  (Australia) so I used 6N138 which I think is funcionally the same,
  one of the catalogues stated that this is a "high speed MIDI standard"
  device.


Happy GUSing.
===============================================================================
** Remek Lipinski   :  remek@mullian.ee.mu.OZ.AU                             **
**                                              "I'll be back!"  - T800      **
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Newsgroups: comp.sys.ibm.pc.soundcard