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Help with MIDI hardware — Parallax Forums

Help with MIDI hardware

PyrotomPyrotom Posts: 84
edited 2007-10-12 07:31 in Propeller 1
I've been working with MIDI on my Propeller, and need a little bit of help with a hardware issue. I've attached the circuit for the MIDI in implementation that I'm currently using. Looking at the data sheet for a 6N138, it appears that I could use 3.3V intead of 5V on the output side of the 6N138 and eliminate the 1000 ohm current limiting resistor. My question is - would I also need to change the value of the 270 ohm resistor, and if so what value should I use?
midi.jpg
47K

Comments

  • AribaAriba Posts: 2,690
    edited 2007-06-05 05:25
    I guess you don't have to change the 270 Ohm resistor. The value is already lower than it has to be.

    If you also plan to make a MIDI Output with 3.3V then you should change the 2 resistors, that are normally 220 Ohms in a standard circuit, to 100 Ohms.

    Andy
  • computer guycomputer guy Posts: 1,113
    edited 2007-10-12 02:49
    What is the component displayed as a dotted oval.

    Thank you smile.gif

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Check out my robot using the propeller RECONAUTOR
  • hippyhippy Posts: 1,981
    edited 2007-10-12 03:25
    @ computer guy : The dotted oval indicates the screen of the two-core cable, which is connected to 0V at the circuit end, but note it is not connected to the 5-pin DIN.
  • AribaAriba Posts: 2,690
    edited 2007-10-12 03:27
    It's the shield of the MIDI cable.
    This shield is normally connected only at a MIDI out, but not on a MIDI in.
  • computer guycomputer guy Posts: 1,113
    edited 2007-10-12 03:37
    So on the output the shielding connect to pin2 of 5-pin DIN and GND.
    On input it connects to only GND.

    Thank you smile.gif

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Check out my robot using the propeller RECONAUTOR
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-10-12 04:47
    Whenever there's a separate ground wire, the shielding in shielded cables should always be grounded at one end only. This is to prevent currents in the shield itself, which would render the shielding less effective.

    -Phil
  • KeithEKeithE Posts: 957
    edited 2007-10-12 06:02
    My undertanding is that the point of the optoisolator is to prevent ground loops - if you grounded the shield at both ends then you would form a ground loop. Anyone that's setup a reasonably sized studio has probably had problems with ground loops at some point.
  • deSilvadeSilva Posts: 2,967
    edited 2007-10-12 07:31
    It generally helps to look at the interiors of a chip; Toshiba also has a nice datasheet..
    www.toshiba.com/taec/components2/Datasheet_Sync//212/4241.pdf
    the 270 R is the (current limiting) collector resistor, so the series R had been of not much use anyhow. But the 270R should be increased, as no current to talk of is drawn from the darlington..

    What had to be taken care of with 5V was the current between 5V and 3.9V at the Prop through the ESD diode.. 270 is marginally for that.

    Post Edited (deSilva) : 10/12/2007 7:37:22 AM GMT
    1N138.JPG
    240 x 240 - 9K
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