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I have two MIDI output devices: an Axiom49 keyboard and a Korg SQ-1 step sequencer. The Axiom49 drives a MIDI input circuit that I've built using an Arduino just fine. The Korg SQ-1, however, does not work with it at all.

The Axiom49's output is simply 31.25k baud RS-232, which conforms with the MIDI specification. The output on the Korg SQ-1, however, is totally different than I was expecting and does not work with my MIDI input circuit at all.

So, I hooked both of them up to an oscilloscope (across pins 4 and 5 of the MIDI 5-pin DIN connector, coming from the MIDI OUT plug on both devices) to see what their output looked like (screenshots below).

Also worth noting is that I was able to hook the Korg SQ-1 up to a friend's computer using a MIDI-to-USB cable and drive a software synth.

So, does anyone know why the waveforms between two MIDI devices could be so different?

Axiom49 oscilloscope screenshot Korg SQ-1 oscilloscope screenshot

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  • \$\begingroup\$ Can the Korg drive anything successfully with that disaster of a serial waveform? \$\endgroup\$
    – ThreePhaseEel
    Commented Dec 2, 2016 at 3:37
  • \$\begingroup\$ @ThreePhaseEel It drove a software synth on my friend's laptop last night using a Roland MIDI-to-USB converter. \$\endgroup\$
    – Dan
    Commented Dec 2, 2016 at 3:44
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    \$\begingroup\$ that's one tolerant MIDI-USB converter man. I wonder if there's anything you can do to the Korg to get it to clean up its act? \$\endgroup\$
    – ThreePhaseEel
    Commented Dec 2, 2016 at 3:45
  • \$\begingroup\$ Looks broken, or you're on the wrong pins. \$\endgroup\$
    – Daniel
    Commented Dec 2, 2016 at 6:29
  • \$\begingroup\$ Show your MIDI input circuit. \$\endgroup\$
    – CL.
    Commented Dec 2, 2016 at 7:53

1 Answer 1

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RS-232 and MIDI are completely different. In particular, MIDI uses a current loop; the "space" state (logic 0) is defined as a current of (at least) 5 mA.

Depending on the MIDI output circuit, you cannot simply measure the voltage at a bare pin (or between two bare pins) of the MIDI connector. Your Korg waveform looks as if you tried to measure a floating voltage, and got noise from something else. (And a sample rate of 80 kHz is much too low to capture the actual MIDI signal.)

You have to measure the current by, e.g., connecting a resistor between pins 4 and 5 and measuring the voltage drop over that.

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  • 1
    \$\begingroup\$ I'll try measuring a resistor across pins 4 and 5 tonight when I get home from work. I'm curious though -- my Arduino MIDI IN circuit is reading the Axiom49 device just fine as a 31.25k baud RS-232 signal (I'm assuming that's what I'm reading when I'm using the RX pin on the Arduino with the baud rate set to 31250). \$\endgroup\$
    – Dan
    Commented Dec 2, 2016 at 15:42
  • \$\begingroup\$ The RX pin would not work with RS-232 signals. \$\endgroup\$
    – CL.
    Commented Dec 2, 2016 at 16:08
  • \$\begingroup\$ I bought a PC900 optocoupler last night to build the official input circuit as provided by the original MIDI specification: Official MIDI input circuit. I'm going to try this tonight. \$\endgroup\$
    – Dan
    Commented Dec 2, 2016 at 18:51
  • \$\begingroup\$ Also, I see what you mean about it not being RS-232 now. RS-232 is 12VDC serial communication, whereas the Arduino TX/RX pins use 5VDC serial communication. Thanks for pointing that out. \$\endgroup\$
    – Dan
    Commented Dec 2, 2016 at 19:00
  • \$\begingroup\$ The MIDI interface itself does not use a specific voltage; the output/input circuits convert between 5 V logic and the 5 mA current loop. The official circuit can be improved by making Rd 1kΩ and adding a bypass capacitor (see the PC900 datsheet). \$\endgroup\$
    – CL.
    Commented Dec 2, 2016 at 19:43

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