Solderless 256KB ROM Decoder + TOS Emulator for Mega ST
The newest SidecarTridge TOS 256KB Decoder is purpose-built for the Atari Mega ST line and now installs 100% solderless. The decoder plugs directly into the MEGABUS connector and mates with the SidecarTridge TOS Emulator carrier board (revision 4 or newer) through the bundled 6-pin flat cable. This combo lets any Mega ST transparently run both 192KB and 256KB TOS ROMs—including STe and Mega STe images—without touching the original ROM sockets.
This hardware revision supports Atari Mega ST only.
Highlights
- Solderless MEGABUS install. The decoder board seats in the Mega ST bus connector—no more 68000 solder work.
- Mega ST carrier board rev4+ included. The kit ships with the matching SidecarTridge TOS Emulator carrier for Mega ST plus the 6-pin interconnect cable.
- Blitter-patch friendly. Works alongside the stock Blitter mezzanine that most Mega ST units carry—no desoldering required.
- Full 192KB + 256KB coverage. Instantly load TOS 1.00–1.04, KAOS, TOS/SuperTOS 2.06 and EmuTOS (192KB/256KB) from the same hardware.
- Transparent ROM switching. Generate the missing address & CE signals so the emulator can map the $E00000 range exactly like a Mega STe.
- Built for modern firmware. Designed around the RP2350-powered SidecarTridge TOS Emulator plus its USB/SWITCHER workflow for loading and switching ROM sets.
What’s in the kit
- SidecarTridge TOS Emulator board (RP2350) prepped for Mega ST use
- Mega ST carrier board revision 4 (multiplexer + sockets)
- SidecarTridge 256KB Decoder board with MEGABUS edge connector
- 6-pin flat cable to link the decoder and carrier
Why Mega ST owners love it
The Mega ST never officially received the later 256KB TOS builds. With this decoder + emulator stack you can:
- Flip between multiple TOS images (and EmuTOS) without swapping ROMs
- Keep factory ROMs untouched—everything lives on the add-on stack
- Run STe/Mega STe-only titles that require the 256KB memory map
- Test custom ROM builds before committing them to physical chips
Installation overview
- Open the Mega ST and remove the RF shield to expose the MEGABUS slot.
- Seat the decoder straight into the MEGABUS connector (components facing up, arrows pointing toward the 68000).
- Install the carrier board in the ROM sockets and plug in the 6-pin flat cable.
- Route cabling + shielding so nothing pinches the decoder board.
- Power on and follow the SidecarTridge TOS Emulator wizard to pick your ROM images.
Full illustrated steps live in the hardware installation guide.
Compatibility & requirements
| Mega ST motherboard | Carrier board | Status |
|---|---|---|
| C100167-001 Rev. 4.02 | Rev 4.0 SidecarTridge carrier | âś… Supported |
| C100167-001 Rev. B2 | Rev 4.0 SidecarTridge carrier | âś… Supported |
| C100167-001 Rev. 5.02 | Rev 4.0 SidecarTridge carrier | âś… Supported |
| C100501-001 (2 ROMs) | Rev 4.0 SidecarTridge carrier | âś… Supported |
| C103277 Rev 2.22 | Rev 4.0 SidecarTridge carrier | âś… Supported |
| C103544-001 (2 ROMs) | Rev 4.0 SidecarTridge carrier | âś… Supported |
Frequently asked questions
Can I still use this on a 520/1040 ST?
No. This revision is Mega ST-only. ST/STM/STF/STFM versions that required soldering onto the 68000 are discontinued.
Do I need to solder anything?
No. The decoder board plugs into MEGABUS and the carrier sits in the ROM sockets. The flat cable handles the rest.
I already own the SidecarTridge TOS Emulator—can I buy just the decoder?
If you have the latest Mega ST carrier (rev4+) we can offer an upgrade bundle. Get in touch with your order number and we’ll sort it out.
Does it work with the stock Blitter mezzanine on Mega ST boards?
Yes. The solderless decoder leaves the CPU area untouched, so the OEM Blitter daughterboard can stay in place.
Which TOS versions are tested?
TOS 1.00, 1.02, 1.04, KAOS 1.4, TOS 2.06 / SuperTOS 2.06, custom TOS 2.16 plus EmuTOS 192KB & 256KB. TOS 1.06, 1.62 and 2.05 still don’t run on Mega ST hardware, even though they decode correctly.
