PS/2 MIDI Eurorack module edition

 

I made a Eurorack version of the PS/2 MIDI project I did a while ago. It features a PS/2 input (which can accept USB keyboards that comply with the PS/2 protocol as well, which my most recent mechanical keyboard does), a MIDI output (now with 5 channels: 1 direct play, 3 recorded and 1 for drums), a 1v/oct output of the main notes and a gate belonging to that 1v/oct. There is also an LED that will show that the gate output works when there is no plug in the jack (this is done to prevent unnecessary energy use by the LED, given that my own power supplies can only supply 300 mA).

The source code is available, and I will try to keep it updated with future improvements. The source code can run both on a 20 MHz PIC16F690 or a 8 MHz one, depending on the presence of the MODULE definition.

F1-F4 can be used to record the different tracks. It can play 4 different drum sounds at the same time, each with a selection of 3 different drums for a total of 12 options. A future module will expose the recorded channels as 1v/oct as well.

Originally I used a Sallen-Key filter for the PWM, but it turned out this created some spiky behavior that especially my Chipz module didn't like on the input, so I switched it to a plain two pole low pass filter.

This is the first module for which I made by own panel out of aluminum, which was an exciting and slightly scary thing to do, involving sawing, drilling, dremeling, sanding and painting. The painting was the biggest struggle, as you can see the bottom part has a slightly different hue.

PS/2 Keyboard Output (sending data to a keyboard)

As you have seen on this blog I have made multiple implementations that allow reading data from a PS/2 keyboard. However, in order to control the LED you have to also be able to write to the keyboard. This is far trickier than it seems, and the documentation on the Internet is limited. Basically the best document is only available using the wayback machine. There is also a working piece of code for the Arduino which isn't pretty and uses interrupts, which can be something people would shy away from (although I might try to implement a version with it).

The most important aspect is the timing schedule. There are many diagrams on the Internet, and most of them are wrong. This is the correct diagram, made by Craig Peacock and copied from the above linked website.

And here is a simplified C code implementation for PicMicro processors (suspendKeyboard() will take the clock line and move it to 0, resumeKeyboard() will release the clock line, keyboardClockHigh() waits for the clock line to go high, keyboardClockLow() waits for the clock line to go low, RB4 is considered to be the data line):

void sendKeyboard(unsigned char value, unsigned char parity) {
    suspendKeyboard();
    __delay_us(60);
    TRISBbits.TRISB4 = 0;
    PORTBbits.RB4 = 0;
    resumeKeyboard();
    keyboardClockHigh();
    for (int i = 0; i < 8; i++) {
        keyboardClockLow();
        if (value & 1) {
            PORTBbits.RB4 = 1;
        } else {
            PORTBbits.RB4 = 0;
        }
        value = value >> 1;
        keyboardClockHigh();
    }
    keyboardClockLow();
    if (parity) {
        PORTBbits.RB4 = 1;
    } else {
        PORTBbits.RB4 = 0;
    }
    keyboardClockHigh();
    keyboardClockLow();
    TRISBbits.TRISB4 = 1;
    while (PORTBbits.RB4) {};
    while (!PORTBbits.RB4) {};
    keyboardClockHigh();
    suspendKeyboard();
}

The full version of this code I will add later. Note that in order to actually change the LED on the keyboard, you need to send two bytes, as follows: first you need to send 0xED, then wait for the keyboard to respond with 0xFA, after which you send 0-7 based on which LED you wish to turn on. 

PS/2 keyboard to MIDI translation

In 2011 I made a PS/2 keyboard to MIDI converter. It came with a sample source code, but this source code had some issues as it was originally meant for a regular keyboard read, not one that is MIDI specific. So at the encouragement of Oscar I wrote a better version of the source code that has the following additional features:

• It allows mapping of every key on the keyboard using two tables, one for the regular keys (BaseKeyboard) and one for the keys whose scan codes start with 0xE0 (SuperfluousKeyboard). Setting a key is as simple as giving it a note (1-127).
• It removes the auto repeat that some keyboards have that could create notes with an annoying echo.
• It allows assigning of MIDI CC messages. This is done by adding 128 to the channel you wish the CC message to be sent over. By default it will send key presses with 127 and releases with 0. This can be changed globally, but not per CC.
• It has various fixes that make the code run smoother and be less error prone.

The source code is available here. Remember to keep the spacing intact, as otherwise the file may not compile. It is intended for a PIC16F690. The wiring is described at the top, and in the original article.

The biggest flaw so far is that upon startup it sends a few random MIDI sequences out, because the keyboard sends its start up sequence to the PICmicro. This can be avoided by mapping those specific keys to 0, or by adding a startup delay.

PS2 to MIDI converter

Recently I purchased an EMT-10E (Yamaha sound extender) which was meant for the Clavinova to increase the different sounds you could make. I bought it for the project I mentioned earlier on this blog, so I would have something to hook the toy piano with MIDI to once I was done. However, due to issues with the switches the toy piano still doesn't have MIDI, and I still wanted to play with the EMT-10E. So I decided to make a PS/2 keyboard to MIDI converter. Since a PS/2 keyboard already sends key press and release commands it would be just a matter of converting the protocol and assign a note value to each key.

As this didn't seem to difficult, I decided to give myself another requirement, which was that it had to look nice. As you can see on the picture, I think I succeeded. Initially I decided to use a wood box, but the PS/2 connector that I got from Jameco was too narrow for wood, so I needed something thinner, but still strong. After walking around in Lowes for a while I discovered that an light switch box with a blank plate would work very well. I also bought a 1/2" drill bit for the holes, and it turned out it was quite easy to put everything together.

I once made a servo controller for Brian Asman, and I already had converted that board for MIDI output. It turned out it had the right size for the box, and I therefore only had to add the PS/2 connector (I connected the data line to PORTB bit 4, and the clock line to PORTA bit 2. You also should connect +5V and ground. Wikipedia has a very clear diagram of the connector, but make sure you have the right mirror image (initially I connected it wrong, reversing the voltages. My keyboard was quite lenient, though). You'll know when you have the right connections if you see the keyboard's LED blink. To hook up the MIDI you need two 220 Ohm resistors for use in the data line and the +5V line. The data line should be hooked up to the TX pin of the PIC16F690 (you can also use a PIC16F688, if you want something smaller). The EMT-10E conveniently has a 9-12V output, which I use to power the box.

You can see and hear the box at work here:

The EMT-10E has a "split keyboard" mode and allows transposing up and down for several octaves, and it has a number of different instruments (no drums, unfortunately). The only problem remaining is that a PS/2 keyboard has auto-repeat on keys, which is undesirable if you want to play notes for a longer period of time (it also causes some notes to become "stuck", possibly because more press events are received than release events).

Here is the first version of the source code, which basically reads a key from the keyboard, converts it into a note value, and either plays it or stops playing it, depending on whether it is a key press or a key release.

If you're interested in this project you can always contact me and I can provide more details if needed.