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SidecarTridge® 256KB Decoder + TOS Emulator for MegaST

SidecarTridge® 256KB Decoder + TOS Emulator for MegaST

€69,00 EUR
€69,00 EUR
Sale Sold out
Taxes included for EU only (see banner). Shipping calculated at checkout.

Run 256 KB STe-class TOS on your Atari Mega ST

The SidecarTridge 256KB Decoder + TOS Emulator Combo lets a Mega ST transparently boot 192 KB and 256 KB TOS images, including STe and Mega STe builds, without touching the original ROM sockets and without soldering on the 68000. The decoder seats in the MEGABUS connector and links to the SidecarTridge TOS Emulator carrier (revision 4 or newer) through a 6-pin flat cable. From the Atari's point of view, it just sees the ROM bus respond as expected.

Assembled combo kit, ships from Spain. Mega ST only.

Key features

  • Solderless MEGABUS install: the decoder seats in the Mega ST bus connector. No 68000 desolder work and no permanent modifications.
  • Transparent ROM switching: generates the missing address and CE signals so the emulator maps the $E00000 range exactly like a Mega STe.
  • Full 192 KB and 256 KB coverage: loads TOS 1.00 to 1.04, KAOS, TOS 2.06, SuperTOS 2.06, custom TOS 2.16 and EmuTOS in both 192 KB and 256 KB builds. Please note that TOS 1.06, 1.62 and 2.05 decode correctly but still do not run on Mega ST hardware, so they are not usable on this machine.
  • Carrier board rev4 or newer 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 Atari Blitter mezzanine that most Mega ST units carry. No desoldering required.
  • Built for modern firmware: designed around the RP2350-powered SidecarTridge TOS Emulator and its USB and SWITCHER workflow for loading and switching ROM sets.

What you can do with it

The Mega ST never officially received the later 256 KB TOS builds. With this combo you can:

  • Flip between multiple TOS images and EmuTOS without swapping ROMs.
  • Keep the factory ROMs untouched. Everything lives on the add-on stack.
  • Run STe and Mega STe titles that require the 256 KB memory map.
  • Test custom ROM builds before committing them to physical chips.

Installation overview

  1. Open the Mega ST and remove the RF shield to expose the MEGABUS slot.
  2. Seat the decoder straight into the MEGABUS connector. Components face up, arrows point toward the 68000.
  3. Install the carrier board in the ROM sockets and plug in the 6-pin flat cable.
  4. Route cabling and shielding so nothing pinches the decoder board.
  5. Power on and follow the SidecarTridge TOS Emulator wizard to pick your ROM images.

Step-by-step photos and the troubleshooting checklist live in the hardware installation guide and the quickstart.

Compatibility and requirements

Supported exclusively on Atari Mega ST computers fitted with a SidecarTridge Mega ST carrier board revision 4 or newer.

Mega ST motherboard Carrier board Status
C100167-001 Rev. 4.0 Rev 4.0 SidecarTridge carrier Supported
C100167-001 Rev. B Rev 4.0 SidecarTridge carrier Supported
C100167-001 Rev. 5.0 Rev 4.0 SidecarTridge carrier Supported
C100501-001 (2 ROMs) Rev 4.0 SidecarTridge carrier Supported
C103277 Rev 2.2 Rev 4.0 SidecarTridge carrier Supported
C103544-001 (2 ROMs) Rev 4.0 SidecarTridge carrier Supported

Not sure which board you have? Open the case and check the silkscreen on the centre-bottom of the motherboard. The full breakdown lives in the SidecarTridge TOS buying guide.

What's in the kit

This is the complete assembled combo. The kit contains:

  • SidecarTridge TOS Emulator board (RP2350), prepped for Mega ST use.
  • SidecarTridge Mega ST carrier board revision 4, with multiplexer and sockets.
  • SidecarTridge 256KB Decoder board with MEGABUS edge connector.
  • 6-pin flat cable linking the decoder and the carrier.
  • RESCUE cable and push button.

You will also need:

  • A USB-C cable to connect the TOS Emulator to a computer for firmware updates and TOS transfers. You can buy a USB-C to USB-C or USB-A to USB-C cable in our store if you do not own one.
  • The SWITCHER.TOS application, downloadable from our downloads page. It also ships on a 3.5 inch floppy disk and lives inside the device volume.

Good to know

  • Mega ST only. The ST, STM, STF and STFM versions that required soldering onto the 68000 are discontinued. If you have a non-Mega ST, look at the SidecarTridge TOS Emulator instead.
  • Designed, tested and supported on Atari Mega ST machines in their original stock configuration. The standard Atari Blitter patch is the only supported modification. Memory expansions, CPU accelerators, custom logic replacements, bodge wires and aftermarket ROM adapters are out of scope and not covered by warranty. If your Mega ST is modified, contact us before ordering.
  • This is a homebrew product for hobbyists who are comfortable opening the case once to seat the decoder and route a flat cable. It is not a sealed appliance.
  • Already own a SidecarTridge TOS Emulator with a rev4 or newer Mega ST carrier? You do not need the full combo. Add only the 256KB Decoder for Mega ST and reuse your carrier.
  • TOS is copyrighted material. You should own a legal copy before using it with the board. EmuTOS is free.

New to the device? Follow the quickstart guide.

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