IIDuinoX is a simple open-source implementation of a Beatmania IIDX controller based on Arduino and USB HID.
All that is needed is some appropriate arcade buttons, a USB-capable Arduino and an encoder system for the turntable -- I repurposed a DJ Hero turntable.
I've never actually seen a real IIDX arcade machine or controller, so the firmware target is simply "as low latency as possible" and "what feels acceptable", rather than trying to match arcade timings.
(If someone with arcade experience wants to contribute, feel free! :) )
The keys are scanned every 250us. Any faster than that, and the USB HID system starts dropping updates.
Due to software debouncing, the keys have a minimum latency of 2ms. In my tests, I can reduce the debouncing mask to around 1ms before my cheap microswitches start getting uncomfortably jumpy.
Getting the turntable working well is an ongoing struggle. I'm using a repurposed DJ Hero controller as the turntable for my prototype, which has an incredible amount of shutter windows for it's encoder. Most of the turntable handling code is just to slow down the turntable readings into something comprehensible. If you make your own encoder system, you will certainly need to adjust the turntable timing values.
The turntable has a minimum latency of 16ms (1ms debounce * 16 shutters to activate), and has a 200ms cooldown before it can be hit in the same direction consecutively. That means for scratch notes less than 200ms apart, you need to scratch back and forth. This prevents generating multiple scratches when the turntable spins too fast. This seems to work pretty well for me.
I'm playing mostly level 8s and 9s, and the turntable is definitely what's holding up my progression, but I don't know if it's just my inexperience with the turntable or a problem with the firmware. I wasn't that great at precise scratching in DJ Hero either. Comments from more skilled players are welcome.
I basically just put everything in a MDF box.
More detailed construction information might appear here someday, after I build a better unit than my prototype. :)