A2m2053

A2M2053 Controller

A replacement controller board for the Apple UniDisk 3.5 (A2M2053).

Overview

The UniDisk 3.5 is Apple's 800K drive for the Apple II. When its controller board (IF-83, service part 661-0307) dies, there's nothing to fix it with. The board is built around a Sony CXD1013 gate array that has no public datasheet and no replacement, and the only spare board on eBay has sat unsold at $150 for months.

This project replaces the whole board. A Raspberry Pi RP2350 speaks SmartPort to the Apple IIe through the Liron card and drives the stock Sony 800K mechanism directly through its 20-pin connector. The 65C02, ROM, RAM, IWM and the CXD1013 are all gone. The mechanism, the case and the front panel stay.

It isn't a clone of the original. The firmware is new C on the Pico SDK. The original drive ROM is used as a test oracle for SmartPort behavior, status codes, errors and timing, never as a template.

Features

• SmartPort device on the Liron card: STATUS, READBLOCK, WRITEBLOCK, FORMAT, CONTROL (eject), INIT
• Reads, writes and formats 800K double-sided ProDOS disks
• Daisy chain out the rear DB-19, in front of or behind another drive
• Front-panel eject button and in-use LED
• Runs from the Apple's own +5V and +12V rails through the host cable
• Whole-track read buffer on its own core; every write verified on the next revolution
• USB console for bring-up: mechanism status, raw flux capture, block reads, whole-disk scans, bus counters

Scope

Apple IIe with 800K double-sided ProDOS disks. Macintosh disks (524-byte blocks, Mac interleave) and 400K single-sided disks show up in Apple's docs and the drive ROM but were never used on the IIe, so they're out of scope.

Specifications

Architecture

 Apple IIe / Liron card                                  Sony 800K mechanism
         | DB-19                                                | 20-pin
   [level shifters]                                    [level shifters]
         |                                                      |
   PIO0: SmartPort FM rx/tx                     PIO1: flux capture / GCR write
   PIO2: rear-port pass-through                                 |
         |                                                      |
   CORE 0: SmartPort device  <---- block request queue ---->  CORE 1: mechanism
   - bus phases, packets                                      - status, seek, motor, eject
   - commands, DIB, status                                    - speed zones
   - daisy-chain ID, LASTONE/                                 - whole-track buffer
                                                              - GCR decode/encode

Each SmartPort read, write or format becomes a request to core 1. Core 0 holds ACK until the answer is back, and the Liron card waits.

Project status

Perfboard prototype: done (October 2026). Milestones M0 to M9 passed on the bench with a Pico 2, five 8T245 breakout boards and a mechanism from a dead UniDisk.

Drop-in PCB: in design. The outline matches the IF-83 and fits the original mounting holes and case opening (checked with 3D prints). The board must stay hand-solderable and well under the $150 the spare board couldn't sell for. Requirements carried from the bench: FABRICATION.md.

Build notes and photos are also on bytehamr.com.

Repository structure

a2m2053_controller/
├── firmware/            # RP2350 firmware (C, Pico SDK)
│   ├── src/             # SmartPort, mechanism server, PIO programs, console
│   └── test/            # Host-side unit tests (fake drive, simulated Liron card)
├── tools/
│   ├── a2m/             # Python: console client, flux decode, GCR, PIO model
│   ├── apple/           # Test disk builders and SPCOMPARE (Applesoft)
│   └── tests/           # pytest
├── hardware/kicad/      # KiCad schematic and PCB
├── docs/
│   ├── build_log/       # Per-milestone notes and measurements
│   ├── captures/        # Scope shots, Apple screens, console logs
│   ├── photos/          # Bench photos
│   ├── superpowers/specs/  # Design spec
│   ├── research_notes.md   # Facts with sources; start here
│   ├── level_shifting.md   # Signal directions and shifter grouping
│   └── pin_budget.md       # GPIO map
└── FABRICATION.md       # PCB requirements and part choices

Getting started

Prerequisites

• Pico SDK 2.2.0, CMake, Ninja, arm-none-eabi-gcc
• A Raspberry Pi Debug Probe (or picoprobe) and Raspberry Pi's OpenOCD build for SWD flashing
• Python 3 for the tools (tools/requirements.txt)
• AppleCommander and ADTPro for the test disks

Building and flashing

cd firmware
make            # build
make test       # host-side unit tests
make flash      # program over SWD (OpenOCD)
make reset      # reset the target

Tool paths live in .env.local at the repo root, git-ignored. Copy .env.example, fill in PICO_SDK_PATH and, if they are not on your PATH, OPENOCD and GDB. Anything already in your environment or on the make command line wins over the file. The test-disk scripts in tools/apple/ read AC_JAR and PRODOS_SRC from the same file.

Console

The firmware exposes a USB CDC console. help lists the commands: mechanism status and control, flux capture, block reads and writes, whole-disk scans, format, and sp for the SmartPort bus state and counters. The Python tool drives the console for the scripted jobs:

cd tools
python -m a2m.cli readdisk --port /dev/cu.usbmodemXXXX --out disk.po --compare ref.po

Design references

This project builds on:

• Byte Hamr: the PicoPort SmartPort device for the RP2040 (the FM receive/transmit PIO programs and packet layout started there), its IWM work, and the annotated drive ROM disassembly by Eric Smith used as the oracle.
• bitsavers: Sony 800K drive ERS (699-0321), mechanism schematics, Apple drive command and status codes.
• tashnotes: GCR checksum and drive command tables.
• Apple II Technical Notes (UniDisk 3.5 #1-#5, Misc #6) and the IWM specifications, via the Asimov archive.

Datasheets, Apple and Sony documents, and photos of other people's boards were used during the work but aren't ours to host, so they're not in this repo. docs/SOURCES.md says where each one came from.

License

This project is released under the MIT License.

Author

Robert Rico