Direct DB-19 connection
Built-in D-SUB DB-19 connector mates the board straight into the Atari ACSI port. No loose ribbon cables, no adapters, no flaky contacts on an aging port.
ATARI ST
Drop-in hard disk emulator for Atari ST, STE, Mega ST and Mega STE. Boot from a microSD card through the ACSI port and leave the floppies behind.
What you get ↗
microSD storage for your Atari ST, STE, Mega ST or Mega STE
The ACSI2STM Compact turns a microSD card into a fast, silent hard disk for any Atari with an ACSI port. It is a derivative of the ACSI2STM open-source project by retro16, built on a compact 4-layer board with a direct DB-19 connector, IDC20 chaining header and external LED output. Same firmware, same modes, no soldering required.
From €35 plus taxes. Ships from Spain. Atari ST, STE, Mega ST and Mega STE.
Key features
4-layer PCB, premium microSD slots
Upgraded 4-layer PCB for better signal integrity on electrically noisy machines, paired with the same high-quality push-eject microSD slots used on the SidecarTridge Multi-device.
GemDrive mode, no drivers needed
Format the microSD card with FAT16, FAT32 or ExFAT, drop in files from any modern PC, and the Atari sees them straight away. No Atari-side drivers, no extra partitioning.
ACSI mode for legacy software
Emulates traditional ACSI hard drives with multiple partitions using Atari-compatible drivers, for full compatibility with classic software that expects a "real" hard disk.
UltraSatan-compatible RTC
On-board real-time clock keeps the date right between cold boots when fed by a CR2032 lithium battery (not included). Recognised by software that already speaks UltraSatan RTC.
Firmware upgrade from the Atari
Future fixes and features land directly through the Atari itself via the firmware update utility. No JTAG, no programmer, no host PC required.
IDC20 chaining + external LED
On-board IDC20 header for SatanDisk-style adapters and daisy-chaining, plus a 2-pin external LED output for clean activity feedback when installed inside a Mega ST or Mega STE. Features the Mini does not have.
Internal install in a Mega
Optional riser kits dedicated to Mega ST and Mega STE drop the Compact inside the case, drawing power from the internal supply. No external cables, no USB charger on the desk, original look preserved.
Pick a kit
ACSI2STM Compact
from €35 plus taxes
- ACSI2STM Compact board fully assembled and tested, latest stable firmware preloaded
- DB-19 connector for the Atari ACSI port, factory-soldered
- IDC-20 connector for SatanDisk / chaining, supplied unsoldered
- 1x2-pin header for the external LED, supplied unsoldered
Compact + Mega ST riser kit
from €45 plus taxes
- Everything in the standalone Compact kit
- Mega ST riser board with pre-soldered 24-pin and 20-pin female connectors
- Pre-soldered USB-A power-only female connector for internal power
- Two pre-soldered 5V power pins and a 20 cm female-to-female Dupont red cable
- USB-A to USB-C cable, 10 to 15 cm
Compact + Mega STE riser kit
from €45 plus taxes
- Everything in the standalone Compact kit
- Mega STE riser board with pre-soldered 30-pin and 20-pin female connectors
- Pre-soldered USB-A power-only female connector for internal power
- USB-A to USB-C cable, 10 to 15 cm
All kits ship from Spain with the latest stable firmware preloaded. Pick the riser kit that matches your motherboard; the board can be moved later if you change machines.
Installation overview
- Prepare a microSD card with FAT16, FAT32 or ExFAT for GemDrive mode, or with the appropriate Atari hard-disk driver image for ACSI mode.
- Insert the microSD card into one of the two slots on the board.
- Connect the Compact to the Atari ACSI port through the built-in DB-19 connector.
- Power the board with a USB-C cable plugged into a USB-C charger, a USB power bank, a host computer or a SidecarTridge Multi-device.
- Power on the Atari. The microSD card shows up as a hard drive ready for use.
- Optional: install the Compact internally with the matching Mega ST or Mega STE riser kit. Step-by-step photos for that flow live in the internal riser quickstart.
Full documentation lives at docs.sidecartridge.com/acsi2stm-atari-st.
Compatibility
| Atari model | Status |
|---|---|
| Atari 520 STF / STFM | Supported |
| Atari 1040 STF / STFM | Supported |
| Atari STE / 1040 STE | Supported (Bad-DMA workaround via PIO mode if needed) |
| Atari Mega ST | Supported (external or internal with Mega ST riser kit) |
| Atari Mega STE | Supported (external or internal with Mega STE riser kit) |
| Atari TT | Not supported |
| Atari Falcon | Not supported |
The Compact is a derivative of the upstream ACSI2STM project and tracks its compatibility scope. The PIO workaround for the Atari STE Bad-DMA issue is part of the firmware; trade-off is slower transfers, full detail in the documentation.
Specs
- MCU: STM32 family (ACSI2STM reference design)
- Interface to Atari: D-SUB DB-19 connector, ACSI bus
- Storage: two push-eject microSD slots, FAT16 / FAT32 / ExFAT (GemDrive) or Atari-formatted images (ACSI)
- Operating modes: GemDrive and ACSI
- RTC: UltraSatan-compatible, CR2032 lithium battery (not included)
- Expansion: IDC20 chaining header, 2-pin external LED output
- Power: USB-C, 5V (no power drawn from the ACSI port)
- Firmware: open-source, OTA upgrade from the Atari itself
- Upstream project: retro16/acsi2stm, GNU License
Frequently asked questions
Hardware & compatibility
Should I pick the Compact or the Mini?
The Compact is the right pick if you want IDC20 chaining and SatanDisk compatibility, an external activity LED output, or an internal install in a Mega ST or Mega STE through a dedicated riser kit. The Mini keeps the same core firmware and operating modes in a tiny desk-friendly form factor with an optional enclosure, but skips the chaining header, external LED output and Mega risers.
Is the Compact firmware-compatible with the original ACSI2STM?
Yes. The Compact is a derivative of the ACSI2STM project by retro16 and is 100% compatible with the upstream firmware. The internal board revision nickname is “Castillian” but it runs the same binary.
What about TT, Falcon or other Atari machines?
The Compact does not target TT or Falcon machines and is not supported on them. Stick to the Atari ST, STE, Mega ST and Mega STE families listed in the compatibility table.
My Atari STE has the Bad-DMA issue. Can I still use the Compact?
Yes. The firmware ships with a PIO-mode workaround for affected Atari STE machines. Enable it as documented; expect a slower transfer rate as the trade-off for clean compatibility.
Operating modes
What is the difference between GemDrive and ACSI modes?
GemDrive lets you drop files onto the microSD card from any modern PC formatted with FAT16, FAT32 or ExFAT, and the Atari accesses them without any Atari-side driver. ACSI emulates a classic ACSI hard disk with partitions and requires an Atari hard-disk driver, which gives you full compatibility with software that expects “real” hard-disk semantics.
For convenience and modern workflows, pick GemDrive. For maximum software compatibility on the Atari side, pick ACSI. The board supports both, you choose per microSD card.
How does GemDrive on the Compact compare to GemDrive on the SidecarTridge Multi-device?
Both implementations are inspired by the Hatari emulator. On the Compact, GemDrive runs over the ACSI bus messaging. On the Multi-device, GemDrive runs over the cartridge port. The end result is the same on the Atari side; the path each one takes through the operating system is different.
Installation & power
Why does the Compact need a USB-C cable?
The ACSI port does not deliver enough current to power the Compact, so the board draws 5V over USB-C. Any USB-C charger or power bank does the job; you can also feed it from a host computer’s USB-C port or from a SidecarTridge Multi-device’s USB-C connector.
Can I power the Compact from a SidecarTridge Multi-device?
Yes. The Multi-device exposes a USB-C power connector that is happy to feed the Compact, which lets you simplify the cabling on the desk.
Can I upgrade the firmware from the Atari itself?
Yes. The Compact supports firmware updates directly from the Atari through the bundled update utility. No JTAG, no extra programmer, no host PC required.
Can I connect the external LED output to the Mega STE activity LED?
Yes. The Mega STE has a 2-pin hard-disk activity LED on the chassis that you can wire to the Compact’s external LED output. Detailed wiring lives in the documentation.
What about the Mega ST? It does not have a hard-disk activity LED.
The Mega ST does not ship with a dedicated hard-disk LED. The Compact’s external LED output is still usable, you just have to wire an LED of your choice to a suitable spot on the case. The documentation walks through the wiring.
I want an enclosure for the Compact. Where can I find one?
A community enclosure for the Compact is published on Printables and can be 3D-printed by anyone. It is not sold by SidecarTridge. More cases collected by the community are listed in the cases page.
Community & policy
Is this product for me?
Yes, if you like running real Atari hardware and you are happy to copy files onto a microSD card and read documentation. The firmware is an actively developed open-source project; you get a board that runs out of the box, plus the option to follow firmware releases as they come. If you want a sealed appliance with a single-vendor warranty stack, the Compact is not that.
Where do I report bugs or contribute?
The Compact runs upstream firmware. Bug reports and feature requests go on the retro16/acsi2stm GitHub repository, which is where development happens. SidecarTridge handles hardware-side support for the boards it sells.
Why can’t I access the store even though you ship to my country?
Access to our products and community spaces requires basic respect. We have zero tolerance for harassment, personal attacks, abusive behaviour or defamatory content toward our team or community. The principle is simple: don’t be abusive. If that principle is violated, access is revoked. We build for people who enjoy experimenting, not for people creating hostility.
Certifications & compliance
The Compact has been assessed for European (CE) and UK (UKCA) conformity. Official Declarations of Conformity and EMC Assessment reports for revision ACSI2STM-COMPACT REV2.1:
EU / CE
UK / UKCA
