Casio SK-1 Rosetta OS: the Casio SK-1 mod (custom sample ROM + SK-1 Studio)

Casio SK-1 / SK1 mod, also for the Radio Shack Realistic Concertmate-500. Website: yuyoi.github.io/Casio-Sk1-Rosetta-OS

Own sounds in the Casio SK-1, the 1985 sampling keyboard: replace its preset ROM with an EEPROM and fill it with your own sounds from WAVs, Roland S-770 CDs or Akai S1000 CDs.

Works on hardware:

• a Winbond W27C512 in the ROM socket, no other mods
• one 3.2 s sound filling the whole ROM, played at full length (the stock presets are 0.4-0.9 s)
• 5 custom presets (guitar, voices, synth, FX) built and burned
• custom loop points: a held key sustains forever on our own loop

SK-1 Studio

• 5 preset slots, one per SK-1 PCM preset. Fill each from:

  • a WAV
  • a Roland S-760/S-770 CD image
  • an Akai S1000/S3000 CD image (.iso / .nrg)

• Pitched to the slot: type the sample's note (A4, C#3...) and it lands in tune on the same keys as the stock sound it replaces. own keeps its speed (FX, chords).

• Reverse any slot.

• Loop one slot, with a waveform loop editor (EDIT LOOP):

  • drag LOOP START and END on the waveform
  • SNAP TO ZERO puts both on zero crossings
  • set the crossfade (0-300 ms)
  • PLAY LOOP to audition it before burning

  The looping slot always ends exactly at the end of the ROM, where the SK-1 jumps back to the loop start.

• 1 long sound mode: one ~2.9 s sound on every preset.
• PLAY / PREVIEW ALL: hear the slots exactly as the SK-1 will (8-bit, 9387 Hz) before burning.
• SAVE a 64 KB image for a W27C512, or 512 KB for an SST39SF040 (Hex Wizard).
• BURN W27C512 straight to an XGecu T48 through minipro.
• The stock drum kit is kept, so the rhythms still work.

Run it

pip install numpy scipy
pythonw tools/sk1studio.pyw

Windows (Python 3.10+). Set the paths once at the top: your own ROM dump, your Roland / Akai CD folders, minipro.exe. They're remembered in ~/.sk1studio.json.

The hardware mod

• Desolder LSI2 (the NEC D23C256EAC-011, right next to the two D4168C RAMs) and fit a 28-pin socket.
• W27C512: jumper its pin 1 to GND or 5 V. The images hold the same ROM twice, so either works.
• W27C256-style 32 KB chip: jumper pin 1 (VPP) to pin 28 (5 V).
• Pin 22 (OE) is already wired to ground on the board (a wire link), so it's a drop-in.
• Never program the W27C512 in the socket: it needs 12-14 V on pin 22, and that pin is grounded on the board.

SST39SF040 (DIP-32, Hex Wizard) in the 28-pin socket

Seat it bottom-aligned: SST pins 3-30 go into socket pins 1-28, and pins 1, 2, 31, 32 hang out past the socket.

• SST pin 32 (VCC) → pin 30. Pin 30 sits on socket pin 28 (5 V), so A17 is high, which is fine.
• SST pin 31 (WE#) → 5 V.
• SST pins 1, 2, 3 (A18, A16, A15) → GND, or a switch for banks.

Dump your own ROM

The ROM is Casio's code and isn't included. Read your own chip:

minipro -p "AM27C256@DIP28" -x -r sk1_lsi2.bin

A good dump has CRC32 d615963c (SHA1 0dbf2d1c4c776f1a1c35dd2be4d6ca03882afd4c). A plain 27C256 read works; no adapter needed.

What we know about the SK-1

• CPU: MSM6283-01GS, an OKI-made Casio custom. Its program lives inside the chip. The external ROM is only data: tables + sounds, no CPU code (checked byte by byte, see ROM_MAP.md).
• ROM layout:
  • 0x0000-0x0AFF: tone records, envelope curves, the 13 envelopes, rhythms, demo song, the sample table (0x085C)
  • 0x0B00-0x0FFF: the drum kit
  • 0x1000-0x7FFF: the 5 PCM presets
• Sounds: 8-bit unsigned, 9387 Hz. A sound plays until an FF FF marker on an even address, so lengths are free.
• Loops (confirmed on hardware): a sound without an end marker runs to 0x7FFF, and the CPU jumps back to the address in table entry 8 (stock 0x7EB2) for as long as the key is held. There's one loop for the whole ROM, so one looping preset.
• Slot order: which preset button reads which table entry is a best guess from the stock sounds, not confirmed yet.
• Sampling: the SK-1's own sampler stays at 1.4 s. That limit is in the CPU. A slower clock gives longer, lo-fi samples; see CLOCK_MOD.md.
• Bus timing: the sister model's service manual shows the CPU waits about 410 ns for memory data at the SK-1's clock, so any 200 ns or faster ROM/RAM is fine. See CPU_RESEARCH.md.

Command-line tools

Tool	Does
tools/sk1studio.pyw	the GUI
tools/sk1rom.py	WAV → ROM from the command line: one-long, slots, slots-switch, rev: prefix to reverse
tools/roland_presets.py	the 5-preset Roland/WAV build used on hardware
tools/probe1.py, probe5.py	probe ROMs: button map + loop test, sample-pointer test
tools/roland.py, tools/akai.py	Roland S-7xx / Akai S1000 CD readers (from U110 RomHex Studio)

Docs

• ROM_MAP.md: the ROM, byte by byte
• CPU_RESEARCH.md: what's known about the MSM6283, from both service manuals
• CLOCK_MOD.md: faster / slower clock
• CONTEXT.md: memory map, pinouts, early notes
• SK-1 service manual + user manual: polynominal.com/casio-sk-1

Credits

• yuyoi: hardware, dumping, testing, sound picks
• MAME sk1.cpp (Vas Crabb): ROM hashes
• circuitbenders.co.uk forum: MSM6283 pin list
• Jonas Karlsson: SK-1 octave clock mod
• Roland / Akai CD readers from U110 RomHex Studio
• Built with Claude Code