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Help please- in making a midi to d-sub cable

Discussion in 'General Electronics Discussion' started by diensth, Apr 27, 2012.

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  1. diensth


    Apr 27, 2012
    Hello folks,

    I have been searching high and low on the net and.. no luck.

    I need to make a 9 pin D-Sub to 5 pin MIDI cable.
    I have a new midi cable. I went out and bought the required d-sub jack with casing. I have a soldering iron. And the guy at the electronic store told me that It can be done (with confidence).. it seems that I just need to find out which of the 5 midi wires connect to the particular d-sub pins.

    I patiently await your help-suggestions-and advice.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You need to find what the correct pinouts are for the two connectors and ensure the correct pins are connected together.

    I have no experience with MIDI which would inform me as to whether the pinouts for DB9 are standard or not.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Johnny.
    It may be possible to do what you want.
    You need to understand that MIDI and RS-232 are not exactly the same, electrically, but can be made compatible.
    RS-232 uses a common ground and signals are bipolar (positive and negative) with respect to this common ground rail.
    MIDI uses current loops for data in both directions. Two pins on the connector are used for data in each direction. With MIDI, idle line is zero current, and active ("0" data state) is current present.
    With RS-232, idle line is negative voltage, and active is positive voltage present.
    So to convert an RS-232 output (on pin 3 of a D9 connector) to MIDI, you need a diode (to block the negative voltage) and probably a current limiting resistor.
    To convert a MIDI signal to be compatible with an RS-232 input (on pin 2 of a D9 connector), you may need a resistor to pull the signal to a negative state (when no current is being supplied by the MIDI device), and probably a diode to feed the positive voltage from the MIDI device to the RS-232 input when the MIDI device is transmitting data "0".
    Do you have pinout information for the MIDI interface on the device you want to connect to?
    Do you need to RECEIVE MIDI information from the device, or is transmission from the D9 port to the MIDI device all you need?
    If you can give me this information, we might be able to work out how to wire a cable to allow a D9 port to transmit data to, and possibly receive data from, a MIDI device.
    The final problem is the data rate. MIDI uses a data rate of 31.25 kilobits per second, i.e. each data bit is 32 microseconds wide. This data rate is not achievable with a standard serial port (e.g. an RS-232 port on a PC).
    This is why it's normal to use a converter between a PC serial port and a MIDI device. The converter handles the electrical differences between RS-232 and MIDI, but also handles the data rate difference.
    I hope this has been of some help. Please reply with more details and I may be able to help further.
  4. diensth


    Apr 27, 2012
    First of all, thank you for such a great response Kris! I really appreciate it :) My next watching of Flight of the Concords shall be devoted to you. ahah

    First, I'd like to say- this (SPDIF Midi breakout cable), as i have just found out, is the exact cable that I am missing. I bought my device used, and it did not come with it:

    I have a 9 pinout on the back of my presonus firebox recording interface. It is labeled "MIDI - S/PDIF".

    I am wanting to run pianos, electronic drums.. and such into it- which are all 5 pin MIDI. So yes, I need to receive information into my interface device.

    I understand what you are saying, though I get lost in the terms you speak.
    Hoping that I have given you enough Info..

  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Oh! I assumed that the 9-pin D-sub you were connecting to was a standard RS-232 serial port, but obviously it's not. It's a combined MIDI and SPDIF connector so it will have a proprietary pinout. The cable most likely contains no components; it's just a breakout cable with the appropriate type of connectors.
    I've done a web search for "presonus firebox pinout breakout cable" but found only other people complaining that the pinout is not available.
    If you can borrow one from a friend, you can do continuity measurements to figure out the pinout. There's a possibility there may be active components in the cable (perhaps optocouplers for MIDI receive), in which case you would need to open up the cable, but nine pins is plenty for a simple point-to-point cable for MIDI in, MIDI out and SPDIF in and out, so it's probably just a "dumb" cable.
    Otherwise you'll need to buy one. If you do, work out the connections and post it on an audio forum so that other people will be able to find it!
    One other possibility: email presonus and ask them nicely. If you explain that you bought it second hand and the cable wasn't with it, and that you are experienced enough to make a new one, they might play nice.
    Last edited: Apr 28, 2012
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