SidecarTridge® 256KB Decoder for Mega ST

Add 256 KB TOS support to your Mega ST, solderless

The SidecarTridge 256KB Decoder upgrades an Atari Mega ST to fully support 256 KB TOS images without modifying the original motherboard ROM configuration. It is a dedicated expansion board for Mega ST systems that works together with an existing SidecarTridge TOS Emulator. The board installs completely solderless through the Mega ST MEGABUS connector and transparently generates the additional address decoding and chip-enable signals required for 256 KB ROM operation.

This is the decoder board on its own, for owners who already run a SidecarTridge TOS Emulator on a Mega ST carrier board revision 4 or newer. Do not have the TOS Emulator yet? Get the full 256KB Decoder + TOS Emulator Combo instead.

Features

• Fully solderless installation through the Mega ST MEGABUS connector.
• Adds native 256 KB ROM decoding support to Atari Mega ST computers.
• Works with 192 KB and 256 KB TOS images.
• 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.
• No modifications required to the original motherboard ROM sockets.
• Compatible with systems fitted with the stock 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 the later 256 KB TOS layouts used by the Mega STe and other enhanced systems. The SidecarTridge 256KB Decoder solves this by generating the missing address lines and ROM select logic required by modern 256 KB TOS images. Once installed, the Mega ST can access larger ROM layouts exactly as if the machine had native support for them, while the original machine stays fully reversible and unmodified.

Installation

Installation is simple and requires no soldering:

1. Open the Mega ST and expose the MEGABUS connector.
2. Insert the 256KB Decoder into the MEGABUS slot.
3. Connect the included 6-pin flat cable or FFC to the SidecarTridge carrier board.
4. Reassemble the system and boot normally.

The decoder is designed to coexist with standard Mega ST hardware, including machines equipped with the stock Atari Blitter board. Full illustrated steps live in the hardware installation guide.

Important: if your TOS Emulator carrier board is revision 2.x or 3.x and does not include the IC labeled MUX on the left side of the connectors, read the Upgrading an older Mega ST carrier board guide before continuing.

Compatibility

Supported exclusively on Atari Mega ST systems using a SidecarTridge Mega ST carrier board revision 4 or newer. Compatible motherboard revisions:

Package contents

• SidecarTridge 256KB Decoder board.
• 6-pin flat interconnect cable or 6-pin flexible flat cable (FFC).

Requirements

• An existing SidecarTridge TOS Emulator setup on a Mega ST carrier board revision 4 or newer.
• An Atari Mega ST computer.

Good to know

• Mega ST only. This decoder does not apply to ST, STM, STF, STFM, STE or Mega STE machines.
• 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.
• TOS is copyrighted material. You should own a legal copy before using it with the board. EmuTOS is free.