OmniSCC — MSX MultiCart

OmniSCC is an RP2040-based MSX cartridge that loads ROM and floppy disk images from the Pico's 16 MB flash. It emulates plain and mapped ROM cartridges, the Konami SCC sound chip, and floppy drives backed by .dsk files.

When powered from USB without the MSX, it appears as a removable drive for copying images by drag-and-drop. On the MSX, the default Screen 5 menu browses files and folders and remembers the last booted image. An alternative menu provides ten configurable favourites.

OmniSCC — a 16 MB "purple" Pico clone soldered directly onto the carrier board.

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Contents of this repository

Path	What it is
firmware/	Prebuilt firmware images.
tools/	Menu ROMs, and MSX-DOS utilities.
src/	RP2040 firmware (C11 / C++17) + the Z80 side (msx/preloader.asm, msx/loader.asm, msx/loader_fav.asm). See src/README.md.
Board/OmniSCC/	KiCad project for the base board: schematic, PCB, 3D STEP, fabrication ZIP.
Board/OmniSCC+/	KiCad project for the level-shifted board.
enclosure/	SolidWorks parts and printable 3MF files for the case.
images/	Board photos used by this file.

Two git submodules are used, so clone with:

git clone --recursive https://github.com/<your-fork>/omniscc.git
# or, in an existing clone:
git submodule update --init --recursive

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Hardware variants

Both boards run the same firmware sources; they differ only in the board header selected at build time (-DPICO_BOARD=omniscc or omniscc_plus).

OmniSCC (base)

The Pico module is soldered flat against the carrier board along both pin rows.

The Pico drives the MSX bus directly at 3.3 V. Fewest parts, lowest cost. Three 3-pole solder jumpers pick what reaches the Pico on GP26–GP28:

Jumper	Choice	Required setting for this firmware
IORQ / AUDIO	GP26	AUDIO — GP26 is the SCC PDM audio output.
BUSDIR / M1	GP27	BUSDIR — the firmware never reads /M1.
WAIT / INT	GP28	WAIT — the loader stretches every bus cycle with /WAIT.

OmniSCC+

OmniSCC+ — same Pico module, plus SN74LVC8T245 bus transceivers for proper 5 V level translation.

Address, data and control lines run through level shifters, so the MSX sees real 5 V logic. There are no option jumpers: GP26 is AUDIO, GP27 is the data transceiver direction (DATA_DIR), GP28 is /WAIT. Build with -DPICO_BOARD=omniscc_plus, which defines WITH_SHIFTERS.

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Quick start

1. Flash the firmware. Hold BOOTSEL, plug into USB, release; copy a .uf2 to the RPI-RP2 drive. Pre-built images live in firmware/ — OmniSCC.uf2 for the base board, OmniSCC+.uf2 for OmniSCC+. To build your own, see src/README.md.

2. Fill the drive. Plug the cart into USB with the MSX powered off. A removable drive named OmniSCC appears (the first boot formats the flash and writes WELCOME.TXT). Copy tools/loader.rom and your .rom files to its root. To build the menu yourself, see src/README.md.

3. Nothing to configure. The menu lists what is on the drive and writes /loader.ini itself when you boot something. Write it by hand only to set a countdown or a title before the first boot:

   delay=3000
   scc_on_exit=1
   name=gradius.rom,Gradius

   For the ten-favourites menu, copy tools/loader_fav.rom to the drive as loader.rom and use favN= entries in loader.ini; see src/README.md.

4. Eject, plug into the MSX, power on. The menu appears; it auto-boots whatever you booted last when the countdown runs out.

Menu keys

Key	Action
Cursor up / down	Move the highlight; the list scrolls at the edges
Cursor left / right	Page up / page down
RETURN	Enter a directory, or boot a file
BS	Up one directory
SPACE	Put the highlighted .dsk in drive B, or take it out again
ESC	Leave the menu for BASIC (maps a bare SCC cartridge first if scc_on_exit is non-zero)

Any keypress cancels the auto-boot countdown for the rest of the session.

Floppy drive emulation

The cartridge serves .dsk files on its USB volume as floppy drives to the MSX. Select an image in the browser menu to mount it as drive A; the MSX then resets and boots through the emulated Disk ROM. Press SPACE on another .dsk first to put it in drive B. The browser saves that choice as disk2= in loader.ini. You can also set disk2= by hand. Whether B: appears to the MSX depends on the Disk ROM's own drive count; some dumps advertise only A:.

You must supply your own 16 KB MSX Disk ROM dump; none is distributed with this project. Copy it to the USB volume as disk.rom, or set diskrom=<path> in loader.ini to use a different file. The firmware patches a copy of the dump in RAM when mounting a disk and leaves the file untouched. A dump must pass the firmware's validation, so an arbitrary 16 KB ROM will not work. Please not at the developing stage two kind of roms are tested and proved to work:

• yis805-128r2_disk.rom (MD5: 814aad415b58cc3f78c7b571512b2006)
• NATIONALDISK.rom (MD5: 86269da485e852d9f581ac27f4ba32ff)

Images are writable: MSX-DOS and Disk BASIC can save files directly into the .dsk on the cartridge's flash. Set an image file's read-only attribute on the USB volume to make the emulated disk write protected. CALL FORMAT initializes a 360 KB or 720 KB image on either drive. SCC audio remains available while a disk is mounted.

The firmware accepts any non-empty image whose size is a multiple of 512 bytes; it does not validate the disk's filesystem or geometry when mounting. Standard 360 KB and 720 KB images work with the supported Disk ROMs. Other sizes may mount, but Disk BASIC or MSX-DOS may fail when reading them, and CALL FORMAT supports only the two standard sizes.

After booting a disk, you can change images without rebooting. For example, CALL MOUNT("B:SIDE-B.DSK") inserts an image into drive B and CALL EJECT("B:") removes it. CALL DISK reports both drives; CALL DIR("GAMES") lists disk images on the cartridge's USB volume.

The same outer-volume also supports file management from BASIC:

CALL ODEL("OLD.DSK")
CALL OCOPY("SOURCE.DSK","BACKUP.DSK")
CALL ONEW("EMPTY.DSK")           ' 720 KiB formatted FAT12 image
CALL ONEW("SINGLE.DSK",360)      ' 360 KiB formatted FAT12 image

ODEL refuses an image mounted in drive A or B. OCOPY and ONEW never overwrite an existing file, and copy transfers preserve every source sector. New images are empty FAT12 images and are not mounted automatically. Paths are literal .dsk paths on the cartridge volume; subdirectories and spaces are supported.

The matching MSX-DOS programs are MOUNT.COM, EJECT.COM, DISK.COM, ODIR.COM, ODEL.COM, OCOPY.COM and ONEW.COM; copy them into a DOS image before use. See the Disk BASIC command guide and MSX-DOS command guide for syntax, protocol limits and error handling.

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Supported mappers

Detected automatically from the ROM contents: plain/linear, Konami (8 KB pages), Konami SCC, ASCII8, ASCII16. Detection can be overridden by putting [Plain], [Konami], [KonamiSCC], [ASCII8] or [ASCII16] in the filename before the extension.

The SCC is emulated on core 1 and rendered as a PDM stream through PIO + DMA on GPIO 26. A bare SCC cartridge with no ROM can also be mapped from the menu (just exit to basic), for software that expects an SCC to be present but supplies its own code.

ROMs up to 128 KB are cached in RAM; larger ones are staged into a 1 MB region of the Pico flash and read from there.

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Enclosure

enclosure/ contains ready-to-print 3MF file: carts.3mf. Fill free to remove non required parts in the slicer.

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Documentation

• src/README.md — building, flashing, loader.ini reference, mapper tags, troubleshooting.
• src/docs/omniscc_api.md — the Z80↔Pico command protocol, for writing your own MSX-side software.

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IMPORTANT!

The RBSC provides all the files and information for free, without any liability. The provided information, software or hardware must not be used for commercial purposes unless permitted by the RBSC. Producing a small amount of bare boards for personal projects and selling the rest of the batch is allowed without the permission of RBSC. When the sources of the tools are used to create alternative projects, please always mention the original source and the copyright!

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Contact information

The members of RBSC group Tnt23, Wierzbowsky, Pyhesty, Ptero, GreyWolf, SuperMax, VWarlock, ALSP and DJS3000 can be contacted via the group's e-mail address:

info@rbsc.su

The group's coordinator could be reached via this e-mail address:

admin@rbsc.su

The group's website can be found here:

https://rbsc.su/ https://rbsc.su/ru

The RBSC's hardware repository can be found here:

https://github.com/rbsc

The RBSC's 3D model repository can be found here:

https://www.thingiverse.com/groups/rbsc/things

-= ! MSX FOREVER ! =-