An open hardware and firmware platform for replacing dead chord logic ICs in vintage electronic instruments, built around the Waveshare RP2350-Zero.
Status: work in progress. Nothing here has been tested on real hardware yet. Treat all GPIO assignments, component values, and wiring details as unverified starting points.
A lot of instruments from the late 70s and early 80s put all their musical intelligence into one custom IC. When that chip fails, the instrument is usually dead for good — they were never reproduced, and the few that remain are getting harder to find every year.
The Suzuki Omnichord OM-27 is the immediate motivation for this project. Its chord logic lives in an AY-5-1317A that reads the 27-button chord matrix, selects the right top-octave frequencies from an M083A generator, and routes them to a pair of M747 octave dividers that feed the strum plate. None of those chips are obtainable anymore.
The idea here is that a single RP2350-Zero, sitting in the AY-5-1317A socket, can do everything those chips did — and since it's software rather than fixed silicon, it can do more.
The firmware is split into instrument-agnostic core modules and instrument-specific configuration. The hope is that someone with a different instrument can write a config file and be most of the way there without having to understand everything from scratch.
Currently:
- OM-27 — the main target, reasonably well mapped out, untested
- Hammond X-5 — a stub based on Filip Kindt's TOG work, very incomplete
| IC | Role | Approach |
|---|---|---|
| AY-5-1317A | Chord logic brain | RP2350-Zero sits in this socket |
| M083A | Top-octave generator | Frequencies generated in PIO from internal clock |
| 4069 | RC clock oscillator for M083A | No longer needed; its 78L05 power supply gets stolen for VBUS |
| 4001 | NOR gate routing 5th or 7th to bass M747 depending on chord type | Bypassed — firmware outputs the correct note directly |
| M747 ×2 | Octave dividers | Replaced by CD4520 adapter boards (passive pin translation) — unverified |
The AY-5-1315 (rhythm chip), 4011 (percussion), and all downstream audio circuitry are left alone.
The original hardware supported major, minor, and dominant 7th. The firmware adds:
- Minor 7th, major 7th (two buttons, same root)
- Diminished (major + minor buttons)
- Augmented (all three buttons)
- Sus2, sus4 (two different roots, interval-aware)
- Slash chords (two different roots, other intervals)
- Barry Harris voicings via solder jumper: maj6, min6, full dim7
The OM-27 runs on +12V and +5V rails only (no negative rail). The AY-5-1317A is P-channel MOS, almost certainly wired with VSS=+12V and VDD=GND — the standard approach for running P-channel ICs on a positive supply. Logic HIGH = +12V, logic LOW = GND. This is consistent with pin 5 (reset) sitting at +12V at rest and being triggered by grounding it.
The current plan uses NPN MMBT3904 transistors on the five tone outputs,
open-collector pulling up to +12V. Worth trying direct GPIO connection first on
the bench — 3.3V may register as a valid HIGH on a 12V CMOS input. See
docs/center-negative.md for full details.
The AY-5-1315 bass-select outputs swing to +12V and require voltage dividers (68kΩ + 33kΩ) before connecting to RP2350 GPIOs.
RP2350 Errata 9: PULL_DOWN is unreliable on floating GPIO pins. The key matrix uses PULL_UP with rows driven LOW instead — already handled in firmware.
The key matrix is 3 rows × 6 column pins. Three of those column pins are shared between two roots each (tritone pairs), giving 9 accessible roots from 6 pins. The frequency input pins (13–24) are dedicated M083A connections and are not used as matrix columns — they are left unconnected on the daughterboard.
| GPIO | Function | AY-5-1317A pin |
|---|---|---|
| 0 | ROOT output → CD4520 #1 clock A | 31 |
| 1 | 3RD output → CD4520 #1 clock B | 29 |
| 2 | 5TH output → CD4520 #2 clock A | 28 |
| 3 | 7TH output → CD4520 #2 clock B | 32 (silent on non-7th chords) |
| 4 | MO output (auto-bass) | 34 |
| 5 | 7th Select drive (LOW = 7th active) | 33 |
| 6 | Row drive: Major | 11 + 12 (tie both pads) |
| 7 | Row drive: Minor | 9 + 10 (tie both pads) |
| 8 | Row drive: Seventh | 7 + 8 (tie both pads) |
| 9 | Col sense: Eb / A | 39 |
| 10 | Col sense: Bb / E | 40 |
| 11 | Col sense: F / B | 3 |
| 12 | Col sense: C | 36 |
| 13 | Col sense: G | 37 |
| 14 | Col sense: D | 38 |
| 15 | MIDI TX (optional) | — (UART1, verify pin capability) |
| 16 | Power enable | — |
| 17 | Any Key Down drive | 30 |
| 18 | Memory switch input | 35 |
| 19 | Modifier button input (was reset) | 5 |
| 20 | Bass select B3 from AY-5-1315 | 25 (needs 68k+33k level shift) |
| 21 | Bass select B2 from AY-5-1315 | 26 (needs 68k+33k level shift) |
| 22 | Bass select B1 from AY-5-1315 | 27 (needs 68k+33k level shift) |
| 23 | JP1: flat/sharp select | — |
| 24 | JP2: Barry Harris mode | — |
| 25–29 | Free for future use | — |
AY-5-1317A pins 1 (VSS) and 2 (VDD) go to the supply rails (+12V and GND respectively). Pin 4 (OSC) is left unconnected — no external clock needed. Pins 13–24 (frequency inputs from M083A) are left unconnected — we generate frequencies internally. Pins 6 (m Sel) and 36–40 (column pins) connect to the key matrix as shown above. Pin 30 (AK) is driven by the RP2350.
The M747 is a 14-pin dual 7-stage divider. The CD4520 is a 16-pin dual 4-stage binary counter that should be functionally equivalent for the OM-27's purposes (only the first three divide stages are used). The plan is a passive adapter board in each M747 socket — just pin translation, no active components.
This is theoretically straightforward but hasn't been built or tested.
openchord/
├── firmware/
│ ├── core/ # instrument-agnostic modules
│ │ ├── freq_gen.py # PIO square wave generator
│ │ ├── tuning.py # frequency tables, ADC tuning
│ │ ├── chord_logic.py # matrix scanning, chord resolution
│ │ └── midi.py # optional MIDI out
│ └── instruments/
│ ├── om-27/
│ │ ├── config.py # GPIO map, matrix layout
│ │ └── main.py # entry point
│ └── hammond-x5/
│ └── config.py # stub, very incomplete
├── hardware/
│ ├── common/
│ │ └── cd4520-adapter/ # M747 replacement adapter (design not started)
│ └── instruments/
│ └── om-27/ # schematic refs, BOM (unverified)
└── docs/
├── contributing.md
└── center-negative.md # important: read before wiring outputs
The most useful thing right now would be someone with an OM-27, a scope, and some patience verifying the output stage behaviour — specifically whether the existing PCB pulldown resistors on the AY output pins will fight a 3.3V GPIO signal, or whether direct connection might actually work without transistors. That determines a lot of the hardware design.
If you have a different instrument with a dead chord IC and want to add support for it, see docs/contributing.md.
Filip Kindt's RP2040 TOG replacement for the Hammond X-5 at cctv.fm is where the PIO frequency generation idea came from. Erich Izdepski's OM-27 repair blog at erichizdepski.wordpress.com was an invaluable schematic reference.
Firmware: MIT
Hardware designs: CERN-OHL-S-2.0