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AN-1 Simple MIDI Interface

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Introduction

Some existing and future Kilpatrick Audio synthesizer modules support additional functions through MIDI. The MIDI standard features capabilities that are difficult or impossible to implement solely through analog connections, and therefore some modules are designed to make use of both MIDI and analog connections, with the MIDI often being available only on an internal connector to save panel space. These modules have software that may be updated at a later time to add capabilities or fix bugs. Users can upgrade the software via MIDI, so even if you don't plan to use the MIDI connection for musical purposes, you might want a MIDI interface to perform software updates.

Use for DIY Projects

For DIY modular synthesizer designers, having a simple and convenient MIDI connection on microprocessor-based projects is handy, even just for debugging. Therefore we are proposing a simple pinout which uses a 10 pin (5x2) male header that provides +5V power, MIDI in and out, a test pin to be used to force modules to enter a testing or setup mode, and a spare pin that you can use for some other purpose. These small connectors are easy to include in a design, even if it's built on protoboard. And the ability to power the MIDI interface from the module means that it's simple to connect with only a single cable.

MIDI Interface

The pinout of the module to MIDI interface cable is shown below:


Fig. 1 - Module MIDI Interface Connector Pinout

The MIDI interface converts to and from the standard MIDI current loop and provides electrical isolation on the input to prevent ground loops. This is a very standard circuit which is easy to build from commonly available components. Some MIDI interface schematics show the 4N25 opto-isolator which is a single transistor type, but the 6N138 part shown here produces a much better output signal. It is highly recommended that you use this type. The entire circuit can be built on a protoboard using through-hole components. The LEDs are optional. Use high brightness LEDs for maximum visibility since they will not be very bright when driven directly from the MIDI signal.

Notes for DIY Designers

  • You should make sure to always pull up your MIDI RX line on your own circuit. Use a 10K resistor to +5V to prevent false reception when the interface is disconnected.
  • If you want to use this circuit on +3.3V, it can be adapted as follows:
    • Change R1 and R2 to 47 ohms.
    • Change R6 and R7 to 120 ohms (or similar, based on your LEDs - 10mA max current)
    • Optional: change R4 to 470 ohms for harder pullup at lower voltage.
    • Optional: change R5 to 2.7K for higher speed - required for some models of 6N138

MIDI Interface Schematic

Click the image for a high-res PDF version


Fig. 2 - MIDI Interface Schematic

The information presented here is offered in the public domain and may be used for hobby and commercial designs. Kilpatrick Audio makes no claims as to the accuracy of the information or fitness for a particular purpose.

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