From: dam@dcs.glasgow.ac.uk (David Morning)
Newsgroups: comp.sys.ibm.pc.soundcard
Subject: Re: Gravis external midi port cable question
Date: 7 Jan 93 11:52:12 GMT
Organization: Glasgow University Computing Science Dept.

dp@hydra.carleton.CA (Dave Perry VE3IFB) writes:

>In article <1993Jan5.032247.22188@nuscc.nus.sg> matcsp@nuscc.nus.sg (Chan Shih-Ping) writes:
>>Does anyone have a definitive answer on 
>>   Whether the SB midi box or the SB Pro midi box would work
>>   with the GUS game port cum midi port? I vaguely  remember 
>>   reading someone's comment that OUT will work but IN won't 
>>   because the 'level' is too high. Is this correct??
>>
>>Does anyone have a circuit diagram on how to make a midi box
>>that will work? How are you connecting your external synths to

>I'm trying to look into this too, and I have a couple of observations.
>In poking around the DB15 connector I believe what they have done is
>to redefine the function of pins 12 and 15. Pin 12, which is one of 3
>grounds on a "normal" game connector is, I believe, the midi output
>pin. You can see it move with a scope when you play a song out the
>port with cakewalk, so I'm pretty sure of this. Pin 15, which is one
>of 3 +5 pins on a "normal" game connector, has been claimed for use
>as the midi input. I say this because it has a high impedance and 
>can be pulled low with a 200 ohm resistor.

>Note that I *don't* mean midi in and midi out in a literal sense -
>I suspect you *STILL HAVE TO USE AN OPTO ISOLATOR CIRCUIT* similar
>to the one on archive.epas.utoronto.ca.  Now - about that circuit -
>has anyone successfully built and used it? I ask because I think it
>may contain an error. Pin 15 is shown connected to the midi output
>circuit, and if my assumptions are correct, it shouldn't be. 

>I may be trying all of this out myself soon - if I aquire a midi
>keyboard :-)

Try this:-

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
              |   |
              |   +------+
              |   5K6    |                       220R
              +--/\/\/\--+    +------+----------/\/\/\--------\
              |B         |C   |A     |                         \  4
            +-|----------|----|-+    |                                MIDI IN
            |  8        6     2 |  -----                       /  5
            |                   |   / \ IN914               +-/
            |      6N135        |   ---                     |
            |                   |    |                      |
            |           5     3 |    |                      |
            +------------|----|-+    |                      |
                         |    |K     |                      |
pin-5 Gnd  --------------+    +------+----------------------+


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

Dave (another one)

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