If you'd like to retrofit MIDI to a DR-110, here's how to do it using a Highly Liquid MIDI-to-trigger kit.

The Boss DR-110 is pretty famous for its great sounds and cheap price. I picked this one up on the internet for not much, in perfect condition, complete with carry case. But in it's original state it's not much good to me, since it doesn't have MIDI.

The kit I got was the MD-24, a 24-channel unit. Since the DR-110 only has eight trigger inputs, I decided to mount the whole thing in a rack unit so there would be room to add more percussion circuit boards at a later date.

After poring over the schematics, it was apparent that the Voice Board didn't actually need to have the control board with the CPU etc plugged into it at all. I cut the 18-wire flat cable connecting the two and threw the control board away:

The MD-24 defaults to running in 'note trigger' mode, where the unit puts out 0V, and MIDI 'note on' messages tell it to output a 5V signal, going back to 0V with a MIDI 'note off' message. The DR-110 wants inverted 6V signals, where it puts out 6V, and the trigger voltage falling to 0V triggers the sound. I programmed the MD-24 using linux command-line program amidi:

amidi -l
(Lists the names of all connected MIDI devices)
amidi -p hw:1,0,0 -S "F0 00 01 5D 03 01 00 02 00 3C 01 02 00 3D 02 02 00 3E 03 02 00 3F 04 02 00 40 05 02 00 41 06 02 00 42 07 02 00 43 F7" 

You would replace hw:1,0,0 with whichever MIDI interface you wanted to use from the amidi -l command. This command sends sysex to the MD-24, telling the first eight channels to go into 'inverted trigger mode'. This gave me a steady 5V output on all channels, dropping to 0V with note on mesages - nearly what we need for the DR-110. Sadly, the triggers did not work, with all instruments continuously triggered - the white noise and cymbal oscillators sounded continuously.

After a bit of thought, I cut traces on the DR-110 PCB to eliminate the power regulator, which puts out the 6V the voice board runs on. I then tapped 5V from the regulator on the MD-24, and ran the voice board on that instead. Hey presto, it works perfectly. :-) 

You can see where to connect triggers, and where I cut the power traces at bottom right of this photo:

The hand clap is the only bit that requires a bit more work - the CPU board outputs a burst of three 10ms pulses on one trigger line (CP I) for the multiple 'clap' impacts, then a single 10ms pulse on a second trigger line (CP II) to get the 'reverb tail'. The TR-808 uses a charging capacitor and series of comparators to do the same job, but I think this would be a good place for a microcontroller to translate the triggers from the MD-24 into the proper series of pulses.

In the meantime, I can trigger them manually from a sequencer as I have both trigger lines hooked up to the MD-24.

Plans for the future include modifying the DR-110 voice board and covering the front panel of the rack case in knobs; as mentioned, there is also room for more drum module circuit boards in the case. I like the percussion sounds from the Roland TR-77 very much, and these are passive designs which only have a few parts, ideal for this box.

Useful links:

• Dominic's modded DR-110
• vidiotsquad have modded their DR-110
• cykong.com have also MIDIfied their DR-110
• The NIN Hotline DR-110 page has lots of info on the DR-110
• Schematics for the DR-110
• Highly Liquid MD-24 is great for MIDIfying drum machines