SD2IEC & IEC-2-SD Adapters

SD2IEC & IEC-2-SD Adapters

Caption	TFW8b SD2IEC v5B – turnkey unit with OLED, buttons and IEC-passthrough
Type	SD-card IEC serial mass-storage adapter (1541-compatible subset)
Designer	Community project originated as MMC2IEC by Lars Pontoppidan (2007); maintained by Ingo Korb (sd2iec firmware)
Manufacturer	Unknown
First released	2008 (sd2iec v0.1)
Latest revision	sd2iec firmware v1.1.1 (March 2024)
Operating voltage	+5 V from IEC bus • on-board 3.3 V LDO for SD-card
Layers / PCB	—
Compatibility	C64 / C64C / SX-64 / C128 / VIC-20 (CR) / Plus-4 / PET (via IEEE–IEC adapter)
Features	—
Model No.	uIEC/SD, SD2IEC (v4–v5B), SD2IEC Slim/OLED, IEC-2-SD ARM, SaRuMan-SD, numerous DIY

The SD2IEC family—and its ARM-based off-shoots usually sold as IEC-2-SD—are open-hardware adapters that let Commodore 8-bit machines read and write FAT/FAT32-formatted SD-cards through the standard IEC serial bus. Acting as a *smart* disk-drive clone they implement the most-used 1541/1571/1581 DOS commands (LOAD, SAVE, directory, block API, ↑, ←, etc.) plus a small command channel interpreter that maps files and sub-directories to disk images on the card.

Quick facts

• “Loader-ready in < 1 s” – no mechanical spin-up / head-seek delays.
• Capacity: 32 GB+ per card, thousands of ‘‘D64/D71/D81’’ images or plain PRGs.
• Speed: ≈ 45× stock 1541 with JiffyDOS fast-loader (≈ 12 kB s^-1).
• Firmware upgradable from the C64 itself (`LOAD "FW.BIN",8,1` & `RUN`).
• 100 % open-source: hardware KiCad / Eagle files & AVR/ARM firmware on GitHub.

📜 History

• 2006-07 – MMC2IEC — Lars Pontoppidan prototypes an ATmega32 board that translates IEC ↔ SPI-MMC.
• 2008 – sd2iec — Ingo Korb forks the code, adds FAT32, long filenames, D71/D81 and renames the firmware sd2iec.
• 2010-12 – Commercial kits — Jim Brain's uIEC/SD and NKC Electronics sell through-hole DIY modules.
• 2014-15 – TFW8b v4/v5 — TheFutureWas8bit releases compact SMD units with button board & OLED.
• 2018 – IEC-2-SD (STM32) — Petter Lundsten ports the protocol to the STM32 “Blue Pill”, bringing USB mass-storage gadget mode.

🛠️ Hardware architecture

Variant	MCU	Clock	RAM	Notes
Classic SD2IEC	ATmega1284P	20 MHz crystal	16 KB	SPI→SD, UART→debug, optional OLED/I²C
uIEC/SD	ATmega644P	16 MHz	4 KB	2× micro-SD sockets, daughter-card form factor
IEC-2-SD	STM32F103C8T6	72 MHz	20 KB SRAM	USB-C, full-speed USB gadget (card seen as drive on PC)
SaRuMan-SD	ATmega1284P	24 MHz (X-tal)	16 KB	Sidekick64 plug-on module, exposed JTAG

Most boards power entirely from the +5 V IEC bus (≈ 100 mA). A 3 V LDO or DC/DC step-down supplies the SD slot. Level-shifting is done by resistor networks or BSS138 FETs depending on revision.

⚙️ Firmware features

• DOS Commands: `LOAD, SAVE, OPEN, B-A (block execute), VALIDATE`, etc.
• Disk image hot-swap using buttons (NEXT / PREV) or via `OPEN 15,"CD:somedir"`.
• Long-file-name to C64 PETSCII mapping (configurable).
• GEOS support (`U0>M0`) and native fast-loader patches for JiffyDOS, Final Cartridge III, Epyx FastLoad, SJLoad, etc.
• Config file `sd2iec.cfg` – set default device #, Jiffy mode, drive # mapping, LED polarity.
• IEC passthrough so real 1541s can daisy-chain behind the adapter.

Known limitations

• No true 6502-cycle-exact 1541 CPU emulation ⇒ copy-protected titles that rely on drive-side code or nibble-level flux patterns fail.
• Relative files not supported.
• Burst-mode fast-loaders designed for the 1571/1581 may require a parallel-IEC mod or fail altogether.
• Command channel buffers are 254 bytes – very long `@` commands can overflow.

🚀 Fast-loader compatibility matrix

Loader	Stock C64	JiffyDOS	SD2IEC (AVR)	IEC-2-SD (STM32)
JiffyDOS (KERNAL)	✗	✔	✔ 45×	✔ 45×
Epyx FastLoad (cart)	✔ 5×	✔ 5×	✔ 30×	✔ 35×
Final Cartridge III	✔ 6×	✗	✔ 32×	✔ 34×
Dolphin DOS	✗	✗	—	—
Fast Load V5 PAL	✔	✔	⚠ (needs ``OPEN15,"UX"` patch)	✔

🔌 Using SD2IEC

<plainlist>

• Device number – shipped as *8*; change with `OPEN15,8,15,"U0>"+CHR$(10+new#):CLOSE15`.
• Browse directories – `LOAD"$",8` or use FB64 file-browser PRG on the card root.
• Mount D64 image – `OPEN1,8,15,"CD:DEMOS/DISK1.D64":CLOSE1`.
• Swap image – front buttons (NEXT/PREV) or `OPEN15,"XS:next":CLOSE15`.
• Firmware update – copy `firmware.bin` to card root → reset while holding “NEXT”.

</plainlist>

🆚 Pi1541 / 1541 Ultimate

Feature	SD2IEC / IEC-2-SD	Pi1541	1541 Ultimate-II+
MCU / SoC	8-bit AVR / STM32	Raspberry Pi (ARM)	FPGA (Cyclone III)
Cycle-exact 6502 emulation	✗	✔	✔
Copy-protected G64 / TAP	✗	✔	✔
Price (2024)	US$25-60	US$60-90	US$200+
Plug-and-play (no Pi setup)	✔	⚠ SD card image & config needed	✔
Real-time clock, Ethernet, RAM-expansion	✗	✗	✔

🛒 Popular ready-made units

Brand / Model	Notes	Approx. price (2024)
TheFutureWas8bit SD2IEC v5B	OLED, 3 buttons, IEC thru, 3-D printed shell	£34 GB
Retro-Port uIEC/SD	Daughter-card w. 6-pin header, fits internal C64 installs	US$44
Pi1541 Zero (not SD2IEC)	Cycle-exact but needs Pi Zero – see separate page	€49
IEC-2-SD “BluePill edition”	STM32, USB-C, optional OLED, DIY kit	€29 kit / €45 built

🔧 DIY resources

• PCB & schematic: <https://github.com/IngmarK/sd2iec-hardware>
• Firmware source: <https://github.com/sd2iec/sd2iec>
• IEC-2-SD repo: <https://github.com/plundstr/IEC-2-SD>
• Forum support: Lemon64 “sd2iec user thread”, retro-computing Stack Exchange tag ‘‘sd2iec’’.

📚 See also

• Pi1541 Full-Drive Emulator
• 1541 Ultimate-II+ Cartridge
• Commodore IEC Serial Bus

🌐 External links

• Official sd2iec wiki – documentation, binaries, parts list.
• The Future Was 8-bit – commercial SD2IEC hardware.
• Retroleum blog – tutorial “Compiling sd2iec on macOS”.