ACSI2STM Mini Hard Disk for Atari ST series

microSD storage for your Atari ST in the smallest possible footprint

The ACSI2STM Mini turns a microSD card into a fast, silent hard disk for any Atari with an ACSI port, on a 65 x 35 mm board with a built-in DB-19 connector. It is a derivative of the ACSI2STM open-source project by retro16 and runs the same firmware as the ACSI2STM Compact, with the same GemDrive and ACSI operating modes. Order the board on its own or with a desk-ready 3D-printed enclosure.

Ships from Spain. For the Atari ST, STE, Mega ST and Mega STE.

Key features

• 65 x 35 mm footprint: one of the smallest ACSI hard-disk emulators available. Sits flush against the Atari's ACSI port without dominating the desk or trailing cables across the setup.
• Direct DB-19 connection: a 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.
• 4-layer PCB, premium microSD slots: the same upgraded 4-layer PCB as the Compact for clean signal integrity on electrically noisy machines, plus two high-quality push-eject microSD slots from 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: an on-board real-time clock keeps the date right between cold boots when fed by a CR1220 or CR1225 lithium battery (not included).
• 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.
• Refined oscillator circuit: the crystal oscillator circuit was recalibrated with updated capacitor values, fixing rare boot and startup issues present in the original design. Full technical detail in the upstream issue thread.
• Optional desk-ready enclosure: order the board on its own or paired with a custom 3D-printed enclosure designed specifically for the Mini, for a clean look without firing up a 3D printer.

Pick a kit

Choose the variant that fits your setup:

• ACSI2STM Mini board only: board fully assembled and tested with the latest stable firmware preloaded, DB-19 connector factory-soldered. Bring your own enclosure or 3D-print one with the community STL files.
• ACSI2STM Mini board + enclosure: everything in the board-only kit, plus a custom 3D-printed enclosure designed specifically for the Mini for a desk-ready setup.

Both kits ship with the latest stable firmware preloaded. You will still need a USB-C cable, a USB power source and a microSD card. The CR1220 or CR1225 RTC battery is optional.

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 Mini 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. The ACSI port does not deliver enough current to power the board.
• Power on the Atari. The microSD card shows up as a hard drive ready for use.

Full documentation lives at docs.sidecartridge.com/acsi2stm-atari-st.

Compatibility

The Mini is an external device with a fixed DB-19 connector. It does not install inside a Mega ST or Mega STE. If you need an internal install with chaining or external LED support, pick the ACSI2STM Compact. The PIO workaround for the Atari STE Bad-DMA issue is part of the firmware; the trade-off is slower transfers, full detail in the documentation.

Mini or Compact?

Specs

• MCU: STM32 family (ACSI2STM reference design).
• Board size: 65 mm x 35 mm.
• 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, CR1220 or CR1225 lithium battery (not included).
• Power: USB-C, 5V. No power is drawn from the ACSI port.
• Firmware: open-source, OTA upgrade from the Atari itself.
• Enclosure: optional custom 3D-printed enclosure, sold separately or as part of the board + enclosure kit.
• Upstream project: retro16/acsi2stm, GNU License.

Good to know

• External device only. It does not install inside a Mega ST or Mega STE; for an internal install use the ACSI2STM Compact with its riser kit.
• You provide a USB-C cable, a USB power source and a microSD card. The RTC battery is optional.
• This is homebrew for hobbyists comfortable copying files onto a microSD card and reading documentation. It runs out of the box, but it is not a sealed appliance.
• The Mini runs upstream firmware. Bug reports and feature requests go on the retro16/acsi2stm GitHub repository. SidecarTridge handles hardware-side support for the boards it sells.