Solderless 256KB ROM Decoder for Atari Mega ST
Upgrade your Atari Mega ST to fully support 256KB TOS images without modifying the original motherboard ROM configuration. The SidecarTridge 256KB Decoder is a dedicated expansion board designed exclusively for Mega ST systems and works together with the existing SidecarTridge TOS Emulator.
This revision installs completely solderless through the Mega ST MEGABUS connector and transparently generates the additional address decoding and chip-enable signals required for 256KB ROM operation.
Perfect for users who already own the SidecarTridge TOS Emulator and want to unlock support for TOS 2.06, EmuTOS 256KB builds, and Mega STe-compatible ROM layouts on a standard Mega ST.
Features
- Fully solderless installation through the Mega ST MEGABUS connector
- Adds native 256KB ROM decoding support to Atari Mega ST computers
- Compatible with 192KB and 256KB TOS images
- Enables support for:
- TOS 2.06
- SuperTOS
- EmuTOS 256KB
- Mega STe-compatible ROM mappings
- Works with existing SidecarTridge TOS Emulator setups
- No modifications required to original motherboard ROM sockets
- Compatible with systems fitted with the Atari Blitter board
- Transparent ROM address mapping for maximum software compatibility
What the Decoder Does
The Atari Mega ST was originally designed around smaller ROM configurations and does not natively support later 256KB TOS layouts used by the Mega STe and other enhanced systems.
The SidecarTridge 256KB Decoder solves this limitation by generating the missing address lines and ROM select logic required by modern 256KB TOS images. Once installed, the Mega ST can access larger ROM layouts exactly as if the machine had native support for them.
This allows advanced TOS switching and compatibility features while keeping the original machine fully reversible and unmodified.
Installation
Installation is simple and requires no soldering:
- Open the Mega ST and expose the MEGABUS connector
- Insert the 256KB Decoder into the MEGABUS slot
- Connect the included 6-pin flat cable or FFC to the SidecarTridge carrier board
- Reassemble the system and boot normally
The decoder is designed to coexist with standard Mega ST hardware configurations, including machines equipped with the Atari Blitter expansion.
Compatibility
Supported exclusively on Atari Mega ST systems using the SidecarTridge Mega ST carrier boards.
Compatible motherboard revisions include:
- C100167-001 Rev. 4.0
- C100167-001 Rev. B
- C100167-001 Rev. 5.0
- C100501-001
- C103277 Rev. 2.2
- C103544-001
WARNING: If you are using a TOS Emulator carrier board revision 2.x or 3.x and your board does not include the IC labeled “MUX” located on the left side of the connectors, please read the Upgrading an older Mega ST carrier board guide before continuing.
Package Contents
- SidecarTridge 256KB Decoder Board
- 6-pin flat interconnect cable or 6-pin Flexible Flat Cable
Requirements
- Existing SidecarTridge TOS Emulator setup
- Atari Mega ST computer
