I can now plug a controller into my Atari 2600 (VCS?) and make neat 8-bit drum sounds. 

"Hey, that's cool and all, but isn't MIDI a thing?" -some part of my brain

Well I'm glad I asked. 

Boom! The answer to "Why can't I plug my MIDI device into my Atari?" is here.

A little hardware MIDI interface circuit I whipped up takes the weird electrical stuff coming out of the DIN-5 connector, and presents it in a more traditional TTL fashion to an Arduino Nano. A toggle switch on the RX line allows for the Arduino to be reprogrammed without removing it from the circuit (the Arduino uses the same hardware UART for programming the chip and for serial communication, in this case MIDI). Pins D2-D6 serve the button signals to the Atari VCS (2600?), which are active low.

The Arduino sketch is located here on Github.

The code is recycled from a couple other projects that involve doing things in response to MIDI messages, which is why it looks like trash. Only MIDI channel 10 gets recognized by the Arduino and the following MIDI note numbers trigger the drum sounds.

35 - sound 1

42 - sound 2

45 - sound 3

50 - sound 4

38 - sound 5 (which I didn't program in SYNDRUM but will still trigger the Fire button)

Even if you don't end up using this with SYNDRUM, it will at least make for an interesting Atari controller. Imagine playing River Raid or Pitfall! with this and a set of MIDI pads or a keytar!