AN-1 Simple MIDI Interface
Expand your Kilpatrick Audio Modules for Low-cost!
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.
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. The TEST MODE jumper
is also optional. Kilpatrick Audio modules pull up this pin to +5V internally.
Shorting it to ground causes the module to enter a setup or testing mode such
as tuning the CV outputs, programming MIDI channels, and so on.
Notes for DIY Designers
- TEST MODE can be implemented on your own modules by pulling high a pin
on your microcontroller and polling to see if it is brought low. (by it being
shorted to ground with a jumper, screwdriver, other circuit) Use a 10K resistor
or internal pull-ups in your micro.
- 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 R7 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 for larger 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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