Hot-swap up to 64 TOS images
16 MB of flash memory holds every TOS revision you care about. Pick one from the SWITCHER menu and reboot.
NEW
SidecarTridge TOS Emulator v3 is here, powered by the Raspberry RP2350 with fine-tuned bit-banging for better compatibility.
What's new in v3 ↗
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 in 16 MB of flash. 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. Based on the SidecarTridge ROM Emulator board.
Kits start at €45 plus taxes. Seven variants ship from Spain depending on your Atari model and motherboard.
Key features
Custom ROMs welcome
EmuTOS ships out of the box. Drop in any TOS revision or 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 into a PC and the board mounts as a FAT16 drive. Or upload new ROMs directly from the Atari using SWITCHER.
MegaSTE 16 MHz, cache, blitter
Works in both 8 MHz and 16 MHz on MegaSTE, with cache enabled. Blitter patch on Mega ST is supported.
Tunable bit-banging
The RP2350 lets you fine-tune the bit-banging timings, so the board behaves on a wider range of vintage motherboards.
How it works
The TOS Emulator uses the PIO units of the Raspberry 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.
The 16 MB flash holds the full image catalog. Plug the board into a USB port and it mounts as a FAT16 drive on Windows, macOS or Linux, so you can drag-and-drop ROM files. From Firmware V2 onwards you can also upload and delete ROMs from the Atari itself, without opening the case.
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.
Watch Our Video Gallery
Find your kit
Seven kits cover seven Atari motherboard layouts. 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?
| SidecarTridge TOS Emulator Kit | Supported Motherboards | Number of ROMs |
|---|---|---|
| ST+ and STM Kit | C070243 | 6 |
| STF and STFM Kit C070523 | 070523 | 6 |
| STF and STFM Kit | 070789-001, C103175-001, C103414-001 | 2 |
| STF and STFM Kit | 070789-001, C103175-001, C103414-001 | 6 |
| STE Kit | CA4003290 | 2 |
| Mega ST Kit | C103544-001, C103277, C100167, C100501 | 2 |
| Mega STE Kit | CA400677 | 2 |
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 is the difference between the SidecarTridge TOS Emulator boards v1 and v2 and the new v3?
The SidecarTridge TOS Emulator v3 is a significant upgrade over v1 and v2, featuring the new Raspberry RP2350 chip. The upgrade improves performance, compatibility and reliability. The v3 also allows fine-tuning of the bit-banging emulation, which makes the board easier to dial in on a wider range of stock Atari ST, STE, Mega ST and Mega STE motherboards.
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 16 MHz mode?
Yes. It works on the Mega STE in both 8 MHz and 16 MHz modes.
Does the SidecarTridge TOS support the Mega STE cache?
Yes. The device is compatible with the Mega STE cache enabled.
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.
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?
The current firmware supports up to 64 ROM images. The board has 16 MB of flash, plenty for every TOS revision and custom ROM you might want. Future firmware may raise this limit.
Can I use the SidecarTridge TOS with EmuTOS?
Yes. EmuTOS ships as the default image on the device.
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. Starting from Firmware V2, 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. Older firmware needs a one-time update first.
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 from Firmware V2. The full user guide lives in the SidecarTridge TOS documentation.
Community & policy
Why can’t I access the store even though you ship to my country?
Access to our products and community spaces requires basic respect. We have zero tolerance for harassment, personal attacks, abusive behaviour or defamatory content toward our team or community. The principle is simple: don’t be abusive. If that principle is violated, access is revoked. We build for people who enjoy experimenting, not for people creating hostility.
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:
