Hot-swap up to 64 TOS images
About 15 MB of ROM space holds every TOS revision you care about. Pick one from the SWITCHER menu and reboot.
Kits start at €45 plus taxes. Six kits cover the ST, STF, STFM, STE, Mega ST and Mega STE motherboards, and ship from Spain.
NEW
Every kit now ships with the ROM Emulator board revision 4 (RP2350B) and firmware v4.0.0. A TOS that won't boot? Hold RESET at power-on to get back.
What's new ↗
Hot-swap TOS versions on your Atari ST
The SidecarTridge TOS Emulator replaces the ROM chips inside your Atari ST, STE, Mega ST or Mega STE with a programmable board. Store up to 64 TOS images on the board. Switch between TOS 1.04, 2.06, EmuTOS and custom ROMs from the SWITCHER menu without opening the case again. Designed and supported on stock machines.
Each kit is two boards stacked together. The carrier board plugs into your motherboard’s ROM sockets and is made for your Atari model. The ROM Emulator board sits on top, holds your TOS images and has the USB-C connector. Every kit ships with ROM Emulator board revision 4 (RP2350B) running firmware v4.0.0.
Key features
Custom ROMs welcome
EmuTOS ships out of the box. Drop in any TOS revision, MagiC 6.30, Jackintosh or any custom build that fits 192 KB (ST) or 256 KB (STE).
Fits inside the case
The board sits in the original ROM sockets. Close the lid and your Atari looks exactly as Sam Tramiel left it.
USB or in-Atari uploads
Plug the board into a computer and it shows up as a USB drive. Or upload new ROMs directly from the Atari using SWITCHER.
ST to Mega STE, stock
Kits for the ST, STF, STFM, STE, Mega ST and Mega STE. The standard Atari Blitter patch on the Mega ST is supported.
Rescue with the RESET button
A TOS image that won't boot? Hold the Atari's own RESET button while you switch it on, and the board starts the rescue image. No extra cable needed.
How it works
The TOS Emulator uses the PIO units of the Raspberry Pi RP2350 to bit-bang the address and data signals that a stock 27CXXXX ROM chip would put out. From the Atari’s point of view, nothing changed: it sees the ROM bus respond as expected and boots normally. From your point of view, the SWITCHER menu lets you pick which 192 KB / 256 KB image the bus serves on the next boot. If a motherboard needs it, the bus timing can be tuned in a settings file on the board.
The board’s flash holds the full image catalog. Plug it into a USB port and it mounts as a drive on Windows, macOS or Linux, so you can drag-and-drop ROM files. Since firmware v2.x you can also upload and delete ROMs from the Atari itself, without opening the case.
Board revision 4 (RP2350B)
The ROM Emulator board in every kit shipped from 1 November 2026 is board revision 4, built on the RP2350B. Its 48 GPIO pins give every address and data line of the ROM a pin of its own, so the board no longer multiplexes them as board revision 3 (RP2350A) does. It is a short card with a gold edge connector that slots into the carrier board and is held with a screw, with the same red, green and orange status LEDs as before.
What you can do with it
- Compare EmuTOS against TOS 2.06 without flashing or burning chips. Useful when you are deciding which one to leave installed.
- Run an older TOS for old games that misbehave on later revisions, while keeping TOS 2.06 available for everything else.
- Test your software across TOS revisions without leaving your real Atari for an emulator.
- Bring back an Atari with a dead or pulled ROM by dropping in a known-good image as a permanent replacement.
- Boot your Atari as a Macintosh Plus with Jackintosh, and get back to TOS with the rescue mode whenever you like.
More than TOS: MagiC and a Mac Plus
The TOS Emulator serves any ROM image, not only Atari’s. Two projects build one for you in the browser, ready to copy to the board:
Jackintosh: your Atari as a Mac Plus
Switch on your Atari and a Macintosh Plus comes up: no TOS, no emulator program, the genuine Mac Plus ROM running on the Atari's own 68000 with System 6.0.7 or 6.0.8. The Jackintosh builder turns your own Mac Plus ROM into a 192 KB image (ST, STF, STFM, Mega ST) or a 256 KB one (STE, Mega STE), in your browser. You also need a monochrome monitor and the Multi-device, which serves the Mac's floppy and hard disk images from microSD.
MagiC 6.30: multitasking from ROM
MagiC, the multitasking operating system by Andreas Kromke, is now open source. Build a MagiC 6.30 ROM image in your browser with the MagiC ROM Builder and boot straight into it.
Jackintosh requires your own Macintosh Plus ROM; we do not supply it. It is open source under GPLv3, and the Jackintosh site lists which Mac programs run. Macintosh and Mac are trademarks of Apple Inc.
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Find your kit
Six kits cover every supported Atari motherboard. Answer a couple of questions and we point you to the right one.
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1
Which Atari do you have?
Don't know which? It's printed on the bottom of the case and on the motherboard. Open the buying guide ↗
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2
What's the motherboard number?
Open the case, look at the centre-bottom of the motherboard. The number is silkscreened in white.
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3
How many ROM chips are on the motherboard?
Same family, two layouts. Count the large rectangular ICs labelled with a TOS revision (e.g. TOS 1.04).
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Prefer the full table?
| Computer | Motherboard | ROM chips | SidecarTridge TOS Emulator kit |
|---|---|---|---|
| Atari 520ST+ and 520STM | C070243 | 6 | ST+ and STM Kit C070243 |
| Atari 520STFM, 1040STF and 1040STFM | C070789, C070859, C103175, C103414 | 2 | STF and STFM Kit C070789 C070859 C103175 C103414, 2 ROMs |
| Atari 520STFM, 1040STF and 1040STFM | C070789, C070859, C103175, C103414 | 6 | STF and STFM Kit C070789 C070859 C103175 C103414, 6 ROMs |
| Atari 1040STF and 520STFM | C070523 | 6 | No kit of its own. The ST+ and STM Kit C070243 fits, very tight: read before ordering |
| Atari Mega ST1, Mega ST2 and Mega ST4 | C100167, C100501, C103277, C103544 | 2 | Mega ST Kit |
| Atari 520STE and 1040STE | CA4003290 | 2 | STE Kit |
| Atari Mega STE | CA400677 | 2 | Mega STE Kit |
Not sure which motherboard you have? Open the case and check the silkscreen at the centre-bottom of the board, or read the detailed buying guide before ordering.
Frequently Asked Questions
Hardware & compatibility
What is the difference between the TOS emulator and the Multi-device?
They are different beasts. The TOS emulator replaces the internal ROM TOS with a custom board, making it as easy to change TOS versions as it is to change files in RAM. It’s a replacement for an existing computer part. The Multi-device connects to the expansion port and can emulate different devices like external ROM (but not the TOS ROM), floppy, RTC, hard disk, etc. It works as an expansion device, not a replacement for an existing computer part.
What’s new in board revision 4?
Every kit shipped from 1 November 2026 comes with the ROM Emulator board revision 4, built on the RP2350B. Its 48 GPIO pins give every address and data line of the ROM its own pin, so the board no longer multiplexes them as board revision 3 (RP2350A) does. That leaves room to grow, and makes it a general-purpose microcontroller board we can also use for other machines, which is how we keep it at the same cost as before.
It runs firmware v4.0.0, which brings rescue mode from the Atari’s own RESET button: hold RESET while you switch the Atari on and the board starts the rescue image. The RESCUE cable and button are now optional; you can still add them to your order.
Which board do I have, and which firmware does it take?
Look at the ROM Emulator board, the one with the USB-C connector. Or open INFO.TXT on the drive it shows on your computer: Board variant: RP2350B is board revision 4, RP2350A is board revision 3.
- Board revision 4 (RP2350B): firmware v4.0.0, files with
rp2350bin the name. Never install a firmware v3.x file on it. - Board revision 3 (RP2350A): shipped from mid June 2025 to October 2026. It also runs firmware v4.0.0, with the files that have
rp2350ain the name. Coming from firmware v3.x? Follow Upgrading to v4.0.0: you also need the new SWITCHER and rescue switcher. - Board revisions 1 and 2 (RP2040): shipped until mid June 2025. Firmware v2.1.0 is their last firmware.
The Which board do I have? guide has photos of each board. Firmware files are on the download page.
Does it work with all Atari ST models?
At this time, SidecarTridge TOS is compatible with the Atari ST, STE, Mega ST and Mega STE with two or six socketed ROMs. Read the Before you buy section of the documentation for the full compatibility matrix.
Can the device harm my computer?
SidecarTridge TOS has undergone extensive testing on Atari ST, STE, Mega ST and Mega STE computers. It is designed to be safe. As with any electronic device, especially vintage computers, there is always some risk of damage. Follow the installation instructions carefully and double-check the device is correctly seated before powering on.
Does the SidecarTridge TOS support the Mega STE?
Yes. The Mega STE (motherboard CA400677) is supported in its stock configuration and has a kit of its own. Support for the Mega STE’s 16 MHz bus goes back to firmware v2.1.0.
Can I use the SidecarTridge TOS Emulator with modified computers or CPU accelerators?
The TOS Emulator is designed, tested and supported on Atari ST, STE, Mega ST and Mega STE computers in their original stock configuration. Modified machines, including memory expansions, CPU accelerators, custom logic replacements, bodge wires and aftermarket ROM adapters, are out of scope and not covered by warranty. The only exception is the standard Atari Blitter patch on the Mega ST, which remains supported.
If your machine is modified, contact us before ordering. Some users report that certain CPU accelerator setups work alongside the TOS Emulator, but such configurations are unsupported and used entirely at your own risk.
Installation
Is it difficult to install the SidecarTridge TOS?
If your ROMs are socketed, install is straightforward and requires no soldering or permanent modifications. If your ROMs are soldered, you will need to desolder the originals and fit sockets first. That step requires soldering skill and basic electronics experience. The hardware installation guide has photos for each motherboard.
Do I need the RESCUE cable and button?
No, not with board revision 4 and firmware v4.0.0. If a TOS image does not boot, hold the Atari’s RESET button while you switch it on and the board starts the rescue image instead. Set RESCUE_TIMEOUT once, as the user guide explains. The RESCUE cable and button are still available if you prefer a dedicated button.
Is this product for me?
Yes, if you enjoy tinkering with your Atari and you are comfortable opening the case to swap ROM chips. It is a homebrew product, built for hobbyists who want to compare TOS revisions, run EmuTOS or rescue a broken ROM. It is not a sealed appliance, and it is not the right fit if you only want to plug something in and never look inside.
TOS images & firmware
What is the maximum size of the ROM image that can be used?
The SidecarTridge TOS supports ROM images up to 256 KB. The Atari ST and Mega ST need TOS of 192 KB; the STE and Mega STE need TOS of 256 KB.
How many ROM images can be stored on the SidecarTridge TOS?
Up to 64 ROM images, the rescue image included. Board revision 4 has about 15 MB of space for them, plenty for every TOS revision and custom ROM you might want.
Can I use the SidecarTridge TOS with EmuTOS?
Yes. EmuTOS ships as the default image on the device. You can also build a MagiC 6.30 ROM in your browser with the MagiC ROM Builder, or a Jackintosh image that boots your Atari as a Macintosh Plus.
Can I use the SidecarTridge TOS with other custom ROMs?
Yes. The board accepts any ROM image file that fits the size requirements. We have tested it with EmuTOS and several other community builds.
Can I use the SidecarTridge TOS with any existing TOS version?
Yes, any TOS revision from 1.0 to 2.06 that fits the size requirements. TOS is copyrighted material, so you should own a legal copy before using it with the SidecarTridge TOS.
Can I upload and delete TOS ROM images from the Atari ST computer?
Yes. Since firmware v2.x you can upload and delete TOS ROM images directly from any Atari ST, Mega ST, STE or Mega STE, without a PC and without opening the case.
What is the SWITCHER menu?
SWITCHER is a small app you run from the Atari to pick which TOS image the board will serve on the next boot. It also handles uploads and deletes. Firmware v4.0.0 needs SWITCHER 4.0.0. The full user guide lives in the SidecarTridge TOS documentation.
Community & policy
Links and Resources
Certifications & Compliance
This product has been assessed for European (CE) and UK (UKCA) conformity. Official Declarations of Conformity and EMC Assessment reports for each kit variant:
