Before You Buy
The SidecarTridge TOS 256KB decoder is designed to work ONLY with specific Atari Mega ST motherboards, together with a SidecarTridge TOS Emulator.
Already have a TOS Emulator on your Mega ST? The decoder connects to its carrier board with a cable, so what matters is the connector for the decoder on the carrier board: a 6-pin flat-cable connector or a 2x3 pin IDC header. Which decoder to order, below, shows both and which decoder to order for each.
Please follow these steps before purchasing the emulator:
Check Your Atari Mega ST Motherboard Model
To learn more about your Atari Mega ST motherboard model, open the case and look for the model number printed on the board, usually located in the middle bottom of the motherboard. Refer to the Compatibility section for a list of supported motherboards. If your motherboard model is not listed, please contact us for further assistance. If your motherboard model number is listed, proceed to the next step.
ROM Configuration
For Atari Mega ST systems, the online store currently offers the DUAL version for motherboards C103544-001, C103277, C100167, and C100501. If you have a special request, please contact us for more information.
ROMs Plugged or Soldered
If your ROMs are plugged, congratulations, you can proceed with the purchase. If they are soldered, you will need to desolder them and plug them into sockets.
If you are not comfortable with desoldering and soldering, please seek professional help to avoid damaging your Atari Mega ST motherboard. We strongly recommend using an experienced technician to perform this task.
A Note About the Blitter Patch in the Mega ST
Most of the Mega ST motherboards have a Blitter patch soldered on top of the Motorola 68000 CPU. The new SidecarTridge TOS 256KB Decoder is designed to work with the Blitter patch in place, so you do not need to remove it.
Purchase the Right SidecarTridge TOS 256KB Decoder Kit
Once you have verified the compatibility of your Atari Mega ST motherboard (C103544-001, C103277, C100167 or C100501), choose what to order.
Which decoder to order
New to the TOS Emulator? Order the 256KB Decoder + TOS Emulator Combo. It comes with everything needed: the decoder and its cable, and a complete TOS Emulator with the ROM Emulator board revision 4.
Already have a SidecarTridge TOS Emulator on your Mega ST? Order the 256KB Decoder on its own, in the version that matches your carrier board. Look at your carrier board, the board in the ROM sockets under the ROM Emulator board, and find the connector for the decoder. It tells you which decoder to order:
| Connector on your carrier board | Cable | Order |
|---|---|---|
| 6-pin flat-cable connector | 6-pin flexible flat cable (FFC) | 256KB Decoder, For REV 4.0.0 |
| 2x3 pin IDC header | 6-pin flat cable with IDC plugs | 256KB Decoder, For REV from 2.1.0 to 4.0.0 |
With a 2x3 pin IDC header, also check that the chip labelled MUX is fitted on the left of the connectors. If it is missing, or if your carrier board has neither connector, the decoder does not work with it as it is: see Upgrading an older Mega ST carrier board, or contact us.
The ROM Emulator board on top does not matter: the decoder only connects to the carrier board.
A final note about The Art of Modding
Modding a 35 to 40 year old computer is never completely risk free. Even if installing the device itself is straightforward, you have to accept that the machine will not remain in its original factory state. Working on old hardware often means removing the metal shielding, leaving the case open, adjusting wiring, or making other small changes that were never part of the original design.
These machines are fragile, and sometimes a simple slip can be fatal. If you decide to mod an Atari Mega ST, do it with patience, the right tools, and the understanding that this is part of the process. That is the art of modding.
The same principle applies in reverse: we only design, test and support the SidecarTridge TOS 256KB Decoder on Mega ST motherboards in their original stock configuration, with the standard Atari Blitter patch as the single supported exception. Any other prior modification (memory expansions, CPU accelerators, custom logic replacements, bodge wires, aftermarket ROM adapters) is out of scope and not covered by warranty. If your Mega ST has already been modified, please contact us before ordering so we can tell you whether the configuration is known to work.