TeensyEprom

Teensy 4.1 EPROM emulator — drop-in replacement for 27C256 / 27C512 chips.

⚠ Work in Progress — Use at Your Own Risk

This project is under active development. Always keep a known-good programmed EPROM as backup before using this on a vehicle. If you find a bug, please open an issue.

What It Does

Pull the EPROM chip out of your ECU, plug in a DIP-28 socket wired to a Teensy 4.1, and serve ROM data from internal flash. Upload ROM files over USB, switch between up to 16 maps with a button press — no chip burning, no socket swapping, no SD card required.

Powers itself from the EPROM socket Vcc — no external supply needed.

Tested ECUs

Any ECU using a DIP-28 27C256 or 27C512 with parallel address/data bus.

ECU	Engine	EPROM	Status
893906266D (MMS-05C)	7A 20v	27C256	Primary target
893906266B (MMS-04B)	7A 20v early	27C256	Supported
4A0906266 (MMS-100)	AAH 2.8 V6	27C256	Supported
8A0906266A (MMS-200)	AAH 2.8 V6	27C256	Supported
8A0906266B (MMS-300)	AAH 2.8 V6	27C512	Supported

Should also work with Bosch Motronic, Digifant, and other EPROM-based ECUs with matching pinout.

Hardware

Part	Qty	Purpose
Teensy 4.1	1	Microcontroller + internal flash storage
SN74HCT245N	2	Address bus level shift (5V → 3.3V)
1kΩ resistor	2	/OE, /CE level clamp
0.1µF ceramic	3	Bypass caps (3.3V rail)
DIP-28 socket	1	Mounts in ECU EPROM socket
Momentary button	1	Map switching (optional)

Total cost: ~$40

SD card is optional — only needed as fallback storage.

Quick Start

1. Wire it up — Follow LITE_BUILD.md for connections
2. Flash firmware — cd firmware && pio run --target upload
3. Upload ROMs — python app/teensy_eprom.py upload stock.bin
4. Install — Plug DIP-28 socket into ECU where the EPROM was
5. Go — Key on, 3 slow LED blinks = running. Button press = next map.

ROM Storage

Primary: Internal Flash (LittleFS) ROMs stored on Teensy's 8MB program flash. Upload via USB from the desktop app. No SD card, no moving parts — survives engine bay heat and vibration.

Fallback: SD Card If internal flash is empty, checks SD card /maps/ for .bin files. Useful for initial bulk loading or field recovery.

Power

DIP-28 pin 28 (Vcc, 5V from ECU) → Teensy Vin
DIP-28 pin 14 (GND)              → Teensy GND

USB Serial Interface

Connect at 115200 baud, or use the desktop app:

pip install pyserial

python app/teensy_eprom.py                     # interactive mode
python app/teensy_eprom.py upload stock.bin     # upload ROM
python app/teensy_eprom.py upload stage2.bin    # upload another
python app/teensy_eprom.py list                 # list maps
python app/teensy_eprom.py switch 1             # switch to map 1
python app/teensy_eprom.py download 0 out.bin   # download map 0

Serial commands (direct):

INFO           — firmware version, storage status, active map
LIST           — show all maps (* = active)
MAP <n>        — switch to map index n
DUMP           — hex dump first 256 bytes of active ROM
UPLOAD name sz — upload ROM (raw bytes + CRC16)
DOWNLOAD <n>   — download map n
DELETE <n>     — delete map n
FORMAT         — delete all maps
SCAN           — rescan maps

Project Structure

TeensyEprom/
├── README.md
├── LITE_BUILD.md          ← wiring guide + build instructions
├── firmware/
│   ├── platformio.ini     ← PlatformIO build config
│   └── src/
│       ├── config.h       ← pin assignments (single source of truth)
│       └── main.cpp       ← firmware (LittleFS + EPROM emulation)
├── app/
│   └── teensy_eprom.py    ← desktop ROM manager (Python + pyserial)
├── schematics/            ← interactive wiring diagrams
└── rom_files/             ← reference ROMs and documentation

Related Projects

Project	Description
HachiROM	ROM editor — create the .bin files for this emulator
M35Tool	ROM editor for Bosch Motronic M3.x (VR6)
DigiTool	ROM editor for Bosch Digifant
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