Pushpin MIDI interface update (Still no success)

December 1st, 2007

To make a long story short:
The bad news is I still haven’t come up with working design. The good news is that I think my problem is with my MIDI interface (A Radium49 keyboard) I tried several opto-couplers, and I couldn’t get the 6n13x’s to give the slightest hint of a signal on the other side. Admittedly, I didn’t find a 6n138 at the lab, so all I tried was 6n136, 6n137 and 6n139, which should all be good for MIDI. This was something I double checked with an oscilloscope, but no reaction.
I also tried a CNY17-3, which did work, but only barely (Sometimes copied the data corrupted)
I think my Radium has too much resistance, so that the current it can gives cannot drive the LED in the opto-coupler. (The alternative is that I just suck and should give up :p ) Some day maybe I’ll check inside. But my next attempt will be to find another MIDI device to check if I was correct.
Until then, stay tuned.

6 Responses to “Pushpin MIDI interface update (Still no success)”

  1. Clay says:

    Pesky little bugger! Don’t give up Nitro, you can do it!

  2. george lazenbleep says:

    dude, you need the level converter between your opto circuit and the gbc,
    quoting noah ;
    ‘GB uses 5V, but MIDI does not.
    MIDI actually uses a *current loop.*
    That means the voltage almost doesn’t matter, there just has
    to be some current flowing versus not to mark the diff. b/t 0 and
    gonna build it next week

  3. nitro2k01 says:

    george: MIDI is communicating with a current loop, yes. But you don’t need a level shifter before the opto-coupler. The MIDI protocol is specifically designed in such a way that the current should be able to drive an opto-coupler. Internally most MIDI circuits are actually using 5 V logic. And the MIDI protocol, even using a current loop, is centerred around 5 V. (The two pins that are being used are 5 V connected through a 220 Ohm resistor and some sort of CMOS driver also padded with 220 Ohm)
    When Noah says “GB uses 5V, but MIDI does not.” he doesn’t mean that most MIDI circuits don’t use 5 V (They do) but that you can use any voltage you wish (3.3 V or 9 V for example, altough I’ve personally seen neither) so long you pad it with proper resistors to drive the opto-coupler on the other side of the cable without trashing it. (A quick calculation gives me that a MIDI transmitter using 9V logic should use around 560 Ohm resistors to give a suitable current (Min 5 mA) for the opto-coupler)

    The level shifter mentioned was actually meant to be placed after the opto-coupler, to shift the voltage up to RS-232 levels. This, however, has little relevance in the case of Gameboys. I think the confusion comes from the fact that the Pushpin interface was based on a MIDI < -> palm Pilot interface or something like that. (The Palm Pilot uses RS-232 to communicate)

    If you look at Noah’s original interface, you see that ther’s no level shifter. (A level shifter would’ve used two capacitors) What you see is ont opto-coupler, and something which is probably a schmitt trigger (Like 74hc04) to clean up the signal.

    Do your homework instead if quoting something you don’t fully understand.

  4. george lazenbleep says:

    ok man , its all cool, just want to build it now

  5. zombectro says:

    I’m so impatient that your interface works !

    I tried to understand the hardware construction on the Pushpin page, but it’s too hard for me, I need pictures to learn, like on your second pushpin post !
    I really don’t understand electronics things, I have only circuit bending skills, so you’re my only hope to help me build this !
    You explains things so well !

    (damn, I’m sucking your balls, it’s bad)

  6. LameBoy says:

    try a 4n33

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