CoCo Pico Keyboard Interface

A Raspberry Pi Pico board that connects to the keyboard matrix of a TRS-80 Color Computer 3. Its 16-pin keyboard header (J1) is wired pin-for-pin to the CoCo 3 keyboard connector, so the same board can sit on either side of that connector. Which firmware you flash decides which way the keystrokes go.

Designed in KiCad. Built and tested.

Two firmware loads, two directions

	1. QMK coco	2. usb-to-coco
Direction	Real CoCo keyboard → PC (USB)	Modern USB / Bluetooth keyboard → real CoCo
J1 connects to	The CoCo 3 keyboard's ribbon cable	The CoCo 3 motherboard's keyboard connector
Pico	Pico (RP2040)	Pico 2 W (RP2350 + Bluetooth)
What the Pico does	Scans the matrix and acts as a USB HID keyboard	Acts as a USB / Bluetooth host and emulates the matrix
Repository	wdathing/qmk_firmware → keyboards/coco	wdathing/usb-to-coco
Build system	QMK (make coco:default)	Pico SDK 2.3.0 / CMake

Both loads use the same pin assignment for the matrix (rows on GP0–GP6, columns on GP8–GP15) and share the same key layout. They are mirror images of each other: QMK reads the CoCo keyboard and sends modern keycodes, while usb-to-coco takes modern keycodes and reproduces CoCo key presses.

Both also handle the places where the two keyboards disagree: they remap shifted symbols so the character on the keycap you're pressing is the one that appears.

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1. QMK coco: use a CoCo 3 keyboard on a PC

Plug the original CoCo 3 keyboard into J1 and the Pico's USB port into a PC, a Raspberry Pi, or a CoCo emulator such as XRoar. The board shows up as a standard USB keyboard.

• CoCo shifted symbols. On the CoCo, Shift+2 is ", Shift+7 is ', Shift+8 is (, Shift+: is *, Shift+; is +, and so on. The firmware sends whatever is printed on the CoCo keycap, not the PC character in that position.

• Two layouts, switchable at runtime:

  • Ctrl + Alt + 0: CoCo layout (CoCo symbols, the default)
  • Ctrl + Alt + 1: PC layout (plain PC symbols)

  The selected layout is saved in EEPROM and survives unplugging.

• Windows / GUI key. The CoCo has no Windows key, so tapping Ctrl + Alt together and releasing them, with no other key in between, sends one. Holding them still works as normal Ctrl and Alt.

• Special keys. CLEAR sends Home and BREAK sends Esc. ALT, CTRL, F1, and F2 pass straight through.

• Joystick. An Atari-style digital joystick on the DE9 (J2) sends the arrow keys, and its fire button sends Space:

  DE9 pin	Function	Pico GPIO	Key sent
  1	Up	GP27	↑
  2	Down	GP20	↓
  3	Left	GP21	←
  4	Right	GP22	→
  6	Fire	GP26	Space
  8	Ground	—	—

• NKRO, bootmagic, extra keys, and mouse keys are enabled.

Build and flash, from a checkout of the QMK fork:

make coco:default          # builds coco_default.uf2
make coco:default:flash    # or hold BOOTSEL, plug in, and copy the .uf2 over

To enter the bootloader without the BOOTSEL button, hold the top-left matrix key (@) while plugging in.

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2. usb-to-coco: use a modern keyboard on a real CoCo 3

Remove the CoCo's own keyboard and plug J1 into the motherboard's keyboard connector. The Pico 2 W pretends to be the keyboard: it watches which column the CoCo is scanning and pulls the matching rows low for whatever keys are held. The CoCo can't tell the difference, so nothing on the CoCo side needs to change.

• USB keyboards plug into the Pico's own USB port, which runs in host mode. Use an OTG adapter, and power the board through J5.
• Bluetooth keyboards work too, both Classic and BLE (tested with an 8BitDo Retro Mechanical keyboard and an Anker compact keyboard).
  • Press the pairing button (SW1) to open a 60-second pairing window. The Pico scans for nearby keyboards and connects to the first one it finds.
  • The last keyboard is remembered and reconnects automatically on power-up.
• Status LEDs:
  • D1 lights while the CoCo is actively scanning the keyboard, so you can confirm the CoCo is alive and connected.
  • D2 lights while any key is held.
• Keycap-accurate symbols. On a modern keyboard, Shift+2 types @, Shift+; types :, and the '/" key types quotes. Each one is translated to the CoCo key (with or without CoCo SHIFT) that produces that character.
• Special keys. Esc and Pause send BREAK, Home sends CLEAR, Backspace sends ← (the CoCo's erase-left), and ` sends @.

Build and flash with the Pico SDK (or the Raspberry Pi Pico VS Code extension):

cmake -B build -G Ninja && ninja -C build    # -> build/usb-to-coco.uf2

A -DDEBUG_KEY_LOG=ON build is available for bring-up without a CoCo. It logs decoded keys over UART instead of driving the matrix, because GP0/GP1 are both matrix rows and UART0. See the usb-to-coco README for details.

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Key matrix

The scan order matches the CoCo's own PIA keyboard scan: columns are strobed through $FF02 (PB0–PB7), and rows are read through $FF00 (PA0–PA6). J1 matches CN2 on the CoCo 3 motherboard pin-for-pin, as documented in the Tandy CoCo 3 Service Manual, Figure 5-9. Pin 3 is unused on both.

• Rows (7): GP0 – GP6
• Columns (8): GP8 – GP15

       col0   col1   col2   col3   col4   col5   col6   col7
row0    @      A      B      C      D      E      F      G
row1    H      I      J      K      L      M      N      O
row2    P      Q      R      S      T      U      V      W
row3    X      Y      Z      Up     Down   Left   Right  Space
row4    0      1      2      3      4      5      6      7
row5    8      9      :      ;      ,      -      .      /
row6   Enter  Clear  Break  Alt    Ctrl   F1     F2     Shift

The CoCo keyboard is a diode-less matrix, so pressing certain combinations of three or more keys at once can produce ghost keypresses. This comes from the original keyboard, not from either firmware.

Hardware

Ref	Part	Pico GPIO	Used by
U1	Raspberry Pi Pico / Pico 2 W	—	both
J1	1×16 header: CoCo keyboard matrix	GP0–6, GP8–15	both
J3	1×16 header, in parallel with J1	GP0–6, GP8–15	—
J2	DE9 male, right angle: joystick	GP20–22, GP26–27	QMK
J6	2×5 header: remote DE9 (mirrors J2)	same as J2	QMK
D1 + R1	3 mm LED, 1 kΩ: CoCo scan activity	GP19	usb-to-coco
D2 + R2	3 mm LED, 1 kΩ: key held	GP18	usb-to-coco
SW1	Right-angle tactile switch: Bluetooth pairing	GP7	usb-to-coco
J4	1×4 header: optional SSD1306 I²C OLED	GP16 / GP17	not yet used
J5	1×2 header: power in (VSYS, GND)	—	usb-to-coco (USB-host mode)

DE9 pins 1 and 6 go to the Pico's ADC-capable pins (GP27 and GP26), so J2 can also carry a CoCo analog joystick in the future.

Repository contents

Path	What it is
Coco_pico_keyb.kicad_sch / .kicad_pcb / .kicad_pro	KiCad schematic, layout, and project
Coco_pico_keyb.step	3D model of the assembled board
Coco_pico_keyb.dsn / .ses / .rules	Freerouting autorouter exchange files
production/	JLCPCB fabrication outputs (Gerber zip, BOM, positions, IPC netlist)
Coco_pico_keyb-backups/	KiCad project backups

The production/ files were generated with the JLC Plugin for KiCad. Upload production/Coco_pico_keyb.zip to JLCPCB to order boards.

Related project

xroar-waveshare-rp2350-pizero is a CoCo emulator for the Waveshare RP2350-PiZero that takes USB keyboard input. Paired with the QMK load, it lets an original CoCo 3 keyboard drive the emulator.

License

Copyright (c) 2025-2026 William Athing.

The hardware design files in this repository are released under the CERN Open Hardware Licence v2 – Permissive (CERN-OHL-P-2.0). You may use, modify, manufacture, and sell boards based on this design, including commercially, as long as you keep the copyright and license notices. This license applies to every version of the design in this repository, including commits made before the license file was added.

The firmware is licensed separately:

• usb-to-coco: MIT License
• QMK coco: GPL-2.0-or-later, as required by QMK