Hardware Installation

This chapter will guide you through the installation of the current SidecarTridge TOS 256KB Decoder in your Atari Mega ST computer. It applies to Mega ST computers with a SidecarTridge TOS Emulator whose carrier board has a connector for the decoder. Once the decoder is installed, you can continue with the installation of the SidecarTridge TOS Emulator as described in its Mega ST installation guide.

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. Before You Buy shows both and which decoder to order for each.

Table of contents
  1. Accessing the MEGABUS Connector
    1. Overview
    2. Tools Required
    3. Safety Precautions
  2. Locate the MEGABUS Connector
    1. Alignment and Orientation of the Decoder
    2. Plug the Decoder into the MEGABUS Connector
    3. A Note to Mega ST Owners
  3. Verify the Installation
  4. Continue with the SidecarTridge TOS Emulator Installation
  5. Connect the Decoder to the Carrier Board
    1. 6-pin flat-cable connector
    2. 2x3 pin IDC header
    3. Both connectors
  6. Metal Shielding
  7. Power On and Test
    1. TOS images and firmware

Accessing the MEGABUS Connector

Overview

To install the current SidecarTridge TOS 256KB Decoder, you need to open the Atari Mega ST and access the MEGABUS connector on the motherboard. This version is a solderless design, so the decoder board plugs into the bus connector instead of being attached to the Motorola 68000 CPU.

The decoder works together with the carrier board of the SidecarTridge TOS Emulator, through the supplied cable.

Tools Required

  • Phillips-head screwdriver
  • Anti-static wrist strap (recommended)
  • Small container for screws
  • Insulating tape

Safety Precautions

  1. Power Off and Unplug: Ensure the Atari Mega ST is powered off and unplugged from any power source before opening the case.
  2. Ground Yourself: Wear an anti-static wrist strap or touch a grounded metal object periodically to avoid static damage.
  3. Work on a Clean Surface: Use a clean, static-free workspace and keep screws and loose parts organized.

Locate the MEGABUS Connector

Alignment and Orientation of the Decoder

The current SidecarTridge TOS 256KB Decoder plugs into the Atari Mega ST MEGABUS connector. The decoder board must be inserted straight and evenly so it sits squarely in the connector without mechanical stress.

When positioning the board:

  1. Keep the components facing up: The chips and silkscreen on the decoder board should face upward once installed.
  2. Route the flat cable toward the carrier board: The flat cable should leave the decoder board toward the area where the TOS Emulator carrier board is installed.
  3. Do not force the connector: If the board does not seat easily, remove it and check the orientation again before applying pressure.

The following images show the current Mega ST decoder board on its own, making it easier to identify the connector side and the component-facing top side before installation.

SidecarTridge TOS 256KB Decoder board top view for Mega ST

SidecarTridge TOS 256KB Decoder board perspective view for Mega ST

Before pressing the board into place, double-check that the connector is aligned correctly and that the cable will not be twisted or pinched once installed.

Plug the Decoder into the MEGABUS Connector

  1. Open the Mega ST case: Remove the top cover and metal shielding as needed to gain clear access to the motherboard.
  2. Find the MEGABUS connector: It is the long expansion connector on the Mega ST motherboard.
  3. Align the decoder board: turn the Mega ST so the power connector is on your right. Leave the first 5 columns of the MEGABUS free: the decoder covers pins 11 and 12 to pins 41 and 42. Its components and the pin numbers printed on it face up, and the small arrows printed on it point to the CPU.
Top view of the Mega ST MEGABUS connector, with the power connector on the right and the CPU below it. The connector has 32 columns of two pins: even pins on the top row, 2 to 64, and odd pins below, 1 to 63. The decoder covers 16 columns, from pins 11 and 12 to pins 41 and 42, boxed in red at its first column. The first 5 columns on the left and the last 11 on the right stay free.
The decoder starts at pins 11 and 12, the sixth column from the left, and covers 16 columns.
  1. Press evenly: Apply gentle, even pressure until the board is fully seated.
  2. Check clearance: Make sure the decoder board is stable and that nearby cables or shielding will not push against it.

The current decoder is a solderless Mega ST design. No soldering is required on the Motorola 68000 CPU.

A Note to Mega ST Owners

Most Mega ST motherboards have a Blitter patch soldered on top of the Motorola 68000 CPU. The current SidecarTridge TOS 256KB Decoder is designed to work with that Blitter patch left in place, so there is no need to remove or relocate it.

Verify the Installation

After plugging the SidecarTridge TOS 256KB Decoder into the MEGABUS connector, inspect the installation carefully before powering on the computer.

Check the following points:

  1. The decoder board is fully seated and level in the MEGABUS connector.
  2. The flat cable is not twisted, pinched, or under tension.
  3. The carrier board for the SidecarTridge TOS Emulator is installed correctly in the ROM sockets.
  4. No part of the decoder, cable, or carrier board can touch the metal shielding.

Do not power on the Atari Mega ST until you have confirmed that the decoder board and cable are installed securely and have enough clearance from the shielding and surrounding components.

Continue with the SidecarTridge TOS Emulator Installation

Once the SidecarTridge TOS 256KB Decoder is plugged into the MEGABUS connector, install the SidecarTridge TOS Emulator in the FC-L and FC-H ROM sockets, as described in its Mega ST installation guide: the carrier board goes into FC-L (LO-0), and the HI board on its flat cable into FC-H (HI-0). This is the same with the universal ST/STF/STFM carrier board of current kits and with the Mega ST carrier board of earlier kits.

If your ROM Emulator board is board revision 3, follow the board revision 3 guide instead; the board picker tells you which board you have.

Connect the Decoder to the Carrier Board

Once the decoder board is installed in the MEGABUS connector and the TOS Emulator carrier board is installed in the ROM sockets, connect both boards using the supplied flat cable.

6-pin flat-cable connector

On the universal ST/STF/STFM carrier board, the decoder’s 6-pin flexible flat cable (FFC) goes from the connector on the decoder (1) to the small connector at the bottom left of the carrier board (2), next to the DUAL/HEXA jumper. The wider connector at the bottom right of the carrier board (3) is for the HI board’s cable: do not swap them.

The 256KB Decoder kit with its connectors numbered in red. 1: the connector on the 256KB Decoder. 2: the small connector at the bottom left of the carrier board, where the decoder's flat cable goes. 3: the wider connector at the bottom right of the carrier board, for the HI board's flat cable.
The decoder's cable joins connector 1 on the decoder to connector 2 on the carrier board. Connector 3 is for the HI board.

2x3 pin IDC header

Plug the 6-pin flat cable into the 2x3 pin IDC header of the carrier board and into the decoder. Check first that the chip labelled MUX is fitted on the left of the connectors; if it is not, upgrade the carrier board before you connect the decoder.

Both connectors

Insert the cable fully into the connectors on both boards and route it so it is not twisted or trapped under the shielding. The cable should follow a relaxed path between the decoder and the carrier board, as shown below with the decoder installed in the MEGABUS connector.

The 256KB Decoder and the TOS Emulator installed on a Mega ST motherboard C103544-001. The black decoder sits in the MEGABUS connector at the top, above the CPU. Its flat cable runs down the left side to the small connector of the universal carrier board, which sits in the FC-L socket at the bottom with the ROM Emulator board on it. The HI board sits in the FC-H socket, to the right of the carrier board, on its own flat cable.
The 256KB Decoder in the MEGABUS connector (top), with its flat cable to the universal carrier board in FC-L (bottom left). The HI board is in FC-H, on its own cable.

Metal Shielding

The current Mega ST decoder is lower profile than the discontinued CPU-mounted versions, but you still need to check the available space carefully before reinstalling the metal shielding.

If you choose to reinstall the shielding:

  1. Make sure the decoder board is not being pushed sideways.
  2. Ensure the flat cable is not trapped between metal parts.
  3. Add insulation tape wherever a board or cable could touch the shield.

If the shielding does not fit cleanly without pressing on the hardware, do not force it.

Power On and Test

After completing the installation of the SidecarTridge TOS 256KB Decoder and the TOS Emulator, power on the Atari Mega ST computer and verify that it boots normally.

If the installation was successful, you should be able to use compatible 192KB and 256KB TOS images with the matching SidecarTridge TOS Emulator setup. If the computer does not boot correctly, power it off and recheck the MEGABUS connection, the carrier board alignment, and the flat cable routing.

TOS images and firmware

  • A 256 KB image to try: the board comes with EmuTOS 1.4 192 KB. Copy a 256 KB image, such as EmuTOS 256 KB, to the board’s ROMEMUL drive (Getting Started), or install the full firmware file with EmuTOS 256 KB (Firmware installation).
  • Rescue mode: TOS 1.06, 1.62 and 2.05 do not run on a Mega ST. If you start one by mistake, hold the Atari’s RESET button while you switch it on to start the rescue switcher (Rescue mode). Set RESCUE_TIMEOUT once, while you set the board up.
  • Firmware: the current firmware for each board, and how to update it.

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