The expandable hard disk device for Atari ST/MegaST/STE/MegaSTE
The ACSI2STM Compact is the fully featured, open-source hard disk emulator for Atari ST computers, enabling the use of SD cards as storage devices. Powered by an STM32 microcontroller, it bridges the Atari’s ACSI port with modern SD cards, providing a reliable and versatile solution for expanding storage on vintage Atari systems.
Same core firmware and capabilities as the ACSI2STM Mini, with extra expansion options: chaining, external LED, and internal Mega installation.
Hardware and Design
After extensive testing of the original ACSI2STM, SidecarTridge introduces the Compact edition with one clear goal: maximum flexibility and expandability.
-
Expandable Design: The Compact edition adds extra connectors and options for advanced setups, including chaining and internal installation in Mega systems.
-
Direct DB-19 Connection: Built-in D-SUB DB-19 connector for a firm, reliable link to the Atari ACSI port, eliminating loose connections on aging systems.
-
Upgraded 4-Layer PCB Design: Better signal integrity and noise resistance, especially on older and electrically noisy machines.
-
Premium microSD Card Slots: Robust slots with built-in eject mechanism (same high-quality connectors used on SidecarTridge Multi-device).
-
Optimized Crystal Oscillator Circuit: Recalibrated capacitor values improve stability and eliminate rare boot issues. Details in the linked GitHub issue discussion.
-
External LED Connector: Optional activity LED output for clean Mega ST/Mega STE internal installs. Learn how to connect it in the documentation.
-
IDC20 Connector for SatanDisk / chaining: Support for SatanDisk-style adapters and daisy-chaining setups (feature not available on the Mini).
(“Castillian” is the SidecarTridge Compact board revision nickname. It remains 100% compatible with the original ACSI2STM firmware.)
Functional Features
- Broad Compatibility: Supports Atari models such as 520 STF, 1040 STE, Mega ST, and Mega STE.
- Operating Modes:
- GemDrive Mode: Offers seamless access to SD cards formatted with FAT16, FAT32, or ExFAT. This mode eliminates the need for additional drivers, making it easy to transfer files between your Atari and modern PCs.
- ACSI Mode: Emulates traditional ACSI hard drives and supports multiple partitions. This mode requires Atari-compatible hard disk drivers, offering enhanced compatibility with legacy software.
- Real-Time Clock (RTC): Includes UltraSatan-compatible RTC functionality, enabling accurate timekeeping on the Atari. A CR2032 3V lithium battery (not included) is required for this feature.
- Firmware Upgradeable: The ACSI2STM Compact firmware can be updated directly from the Atari, ensuring compatibility with future enhancements and bug fixes.
The Compact provides the same operating modes and firmware features as the Mini, while adding expansion and chaining options.
Place it inside your Megas: The Internal Riser Boards Option!
Traditionally, the ACSI2STM Compact device connects externally via the ACSI port. To enhance integration, we’ve developed two new riser boards that enable the internal installation of the ACSI2STM Compact device within Atari Mega ST and Mega STE computers. This allows users to seamlessly incorporate the ACSI2STM Compact into their systems, eliminating the need for external cabling and USB power adapters, while preserving the original aesthetics of the computer.
Benefits of Internal Installation:
- Internal Power Supply: No more external power wires; the device draws power internally.
- Preserved Aesthetics: Maintains the original look of your Atari by keeping the ACSI2STM Compact device concealed within the case.
- Reduced Wear: Minimizes wear on external ports and connectors by eliminating the need for constant plugging and unplugging.
- Space Efficiency: Frees up workspace by removing external devices and cables.
Kits Available:
ACSI2STM Compact Kit
When purchasing the ACSI2STM Compact kit, you’ll receive:
- A PCB with all components soldered and tested, including the DB-19 connector for the Atari ACSI port.
- A IDC-20 connector to expand the ACSI port to Satan Disk and other devices (not soldered).
- A 1x2-pin header for the external LED (not soldered).
- Pre-installed latest stable firmware.
The user will need to provide:
- A USB-C cable to power the ACSI2STM Compact.
- An SD, SDHC, or SDXC card to store files. They must be formatted in FAT16, FAT32, or ExFAT for GemDrive mode, or with Atari-compatible hard disk drivers for ACSI mode as described in the documentation.
- A functional Atari ST/MegaST/STE/MegaSTE/TT computer with an ACSI port.
- A 3V lithium battery (CR2032) for the RTC feature (optional).
- An external LED (optional).
Internal Riser Board for Mega STE + ACSI2STM Compact Kit:
- ACSI2STM Compact Kit
- Riser board specifically designed for Mega STE.
- Pre-soldered 30-pin female connector.
- Pre-soldered 20-pin female connector.
- Pre-soldered USB-A power-only female connector.
- USB-A to USB-C cable (length varies between 10 cm and 15 cm, depending on availability).
Internal Riser Board for Mega ST + ACSI2STM Compact Kit:
- ACSI2STM Compact Kit
- Riser board specifically designed for Mega ST.
- Pre-soldered 24-pin female connector.
- Pre-soldered 20-pin female connector.
- Pre-soldered USB-A power-only female connector.
- Two pre-soldered male pins for 5V power connection.
- 20 cm female-to-female Dupont red connector.
- USB-A to USB-C cable (length varies between 10 cm and 15 cm, depending on availability).
Installation
The ACSI2STM Compact is already pre-assembled and ships with the latest firmware, making it ready for use out of the box. The only steps required are to prepare an SD card and connect to your Atari system:
1. SD Card Preparation
- For GemDrive Mode: Format the SD card with FAT16, FAT32, or ExFAT to allow the Atari to access files directly without additional drivers.
- For ACSI Mode: Create disk images or partitions compatible with Atari’s hard disk drivers for legacy software.
2. Operation
- Insert the prepared SD cards into the slots.
- Connect the ACSI2STM Compact to the Atari’s ACSI port.
- Power the ACSI2STM Compact using a USB-C cable (not included).
- Power on the system. The Atari should detect the SD card as a hard drive, ready for file storage and retrieval.
This product is intended for users familiar with vintage computer hardware. While we offer support for hardware setup and firmware installation, the firmware is provided as-is. It is actively and continuously developed, and we encourage users to contribute by reporting bugs and suggesting features on the GitHub repository.
Frequently Asked Questions
Should I choose the ACSI2STM Compact or the Mini?
Choose the Compact if you want maximum expandability: daisy-chaining, SatanDisk compatibility (IDC20), external activity LED, or internal Mega ST/Mega STE installation.
Choose the Mini if you want the smallest possible device with the same core firmware features in a clean, desk-friendly setup (with optional enclosure).
Is the SidecarTridge ACSI2STM Compact compatible with the original ACSI2STM firmware?
Yes, the SidecarTridge ACSI2STM Compact is 100% compatible with the original ACSI2STM firmware.
What is the difference between GEMDRIVE and ACSI modes?
- GEMDRIVE Mode: Allows easy file transfers between Atari and modern PCs using FAT16, FAT32, or ExFAT formatted SD cards. No additional drivers are needed.
- ACSI Mode: Emulates a traditional ACSI hard drive, supporting legacy Atari software with custom partitions. Requires Atari hard disk drivers like HDDRIVER or ICD Pro.
Choose GEMDRIVE for convenience and modern workflows, and ACSI for full compatibility with vintage software.
Why do I need a USB-C cable?
The ACSI2STM Compact requires a USB-C cable to power the device because it cannot draw enough power from the Atari’s ACSI port. The USB-C cable should be connected to a USB power source, such as a computer, USB charger, SidecarTridge Multi-device, or a USB power bank.
Can I use the ACSI2STM Compact with SidecarTridge Multi-device?
Yes, the ACSI2STM Compact is compatible with the SidecarTridge Multi-device, allowing you to connect multiple devices to your Atari simultaneously. The SidecarTridge Multi-device can power the ACSI2STM Compact using its USB-C port, simplifying the setup and reducing cable clutter.
My Atari STE suffers from the Bad-DMA issue. Can I use the ACSI2STM Compact?
The latest firmware of the ACSI2STM Compact includes a workaround for the Bad-DMA issue on Atari STE computers called PIO mode. You must enable this mode by following the instructions in the documentation. PIO mode may reduce the performance of the ACSI2STM Compact by a factor of 10, but it ensures compatibility with Atari STE computers affected by the Bad-DMA issue.
Can I upgrade the firmware of the ACSI2STM Compact from the Atari itself?
Yes, the latest firmware of the ACSI2STM Compact can be updated directly from the Atari using the firmware update utility. This feature ensures that you can benefit from future enhancements and bug fixes without needing additional hardware or tools.
What is the difference between the GEMDRIVE in the ACSI2STM Compact and the SidecarTridge Multi-device?
Both implementations are inspired by the Hatari emulator. The GEMDRIVE in the ACSI2STM Compact uses the specific messaging system of the ACSI device to communicate with the Atari, but in the SidecarTridge Multi-device, the GEMDRIVE uses the messaging system implemented over the cartridge port. Both try to bypass the limitations of the Atari ST operating system to access small disk drives.
I want a case for my device. Where can I find one?
My lack of 3D printing skills is legendary, so I let the community do the work for me. You can find a list of cases in the cases page. If you have a case that you want to share with the community, please let me know!
Can I connect the ACSI2STM Compact external LED connector to the Atari Mega STE activity LED?
Yes, you can connect the ACSI2STM Compact external LED to the Atari Mega STE activity LED. The default hard disk activity LED on the Mega STE is a 2-pin connector that can be used to connect the ACSI2STM Compact external LED. This allows you to use the existing activity LED on the Mega STE to indicate the status of the ACSI2STM Compact device, providing a convenient and integrated solution for monitoring disk activity. Please refer to the documentation for detailed instructions on how to connect the external LED to the Mega STE activity LED.
Can I use the ACSI2STM Compact external LED connector to the Atari Mega ST activity LED?
The Atari Mega ST does not have a dedicated hard disk activity LED like the Mega STE. However, you can still use the ACSI2STM Compact external LED connector to create a custom activity LED setup. This would involve connecting the external LED to a suitable location on the Mega ST case. Please refer to the documentation for detailed instructions on how to set up a custom activity LED for the Mega ST.
More information
- Documentation: You can read the guides in our documentation section.
- Project Repository: Detailed guides and tutorials are available on the project’s GitHub repository, covering hardware setup, firmware installation, and SD card preparation.
- License: The ACSI2STM is an open-source project released under the GNU License. You can access the source code and contribute to the project on the GitHub repository.
