SDD
3fd1e13b2e
Distils public docs into reference/keyboard-mouse/PROTOCOL.md for the level-1 Keyboard (27) / Mouse (26) device tests: - keyboard: 4800 8N1 RS-423, self-test via FD -> 01 00 00 00 (errors 3D/3E) - mouse: 4800 8O1 RS-232, self-test via 'T' -> 4-byte report (errors 3D/3E, >=0x20 fatal) - separate ports: DUART2 A=keyboard, B=mouse (NOT daisy-chained) MAME already has an LK201 device (src/mame/dec/lk201.cpp/.h) but no DEC mouse.
Hardware
A separate inline box: a 4P4C socket for the LK401's own cable, and USB out to the host.
Provisional. The interface below assumes the keyboard's output idles near 0 V and swings towards +12 V (RS-232 polarity, no negative rail). Phase 1 measures this; revise the circuit before building if the measurements disagree.
Block diagram
USB 5V ──┬──────────────────────────────── Arduino Micro (5V)
│
polyfuse 500 mA
│
12 V boost ──┬── 470 µF ── GND
│
4P4C pin 3 (+12 V) ◀──────┘
4P4C pin 2 (GND) ◀────────────────────────────── GND
4P4C pin 4 (kbd TX) ──▶ [RX inverter] ──▶ D0 / RX1
4P4C pin 1 (kbd RX) ◀── [TX inverter] ◀── D1 / TX1
The pin numbers are lkkbd.c's; replace them with the Phase 1 results.
Receive path (keyboard → Arduino)
An NPN inverter. It also protects the 5 V input from the keyboard's 12 V swing.
kbd TX ──[10k]──┬── B 2N3904
│
[10k] 5V
│ │
GND [4k7]
│
├──▶ D0 (RX1)
C
E ── GND
Keyboard idle (≈0 V) → transistor off → D0 pulled high → UART idle. Correct.
Transmit path (Arduino → keyboard)
Also an NPN inverter, with its collector pulled up to 12 V so the keyboard sees a full-swing signal.
D1 (TX1) ──[10k]── B 2N3904 12V
│
[4k7]
│
├──▶ kbd RX
C
E ── GND
UART idle (5 V) → transistor on → keyboard input ≈0 V (mark). Correct.
If an RS-232 transceiver (MAX232/MAX3232) or a 74HC14 turns up in the parts bin, it can replace both inverters. The transistor version needs nothing unusual.
Power budget
USB 2.0 supplies 500 mA at 5 V. With a boost module about 80 % efficient, each 1 mA the keyboard draws at 12 V costs about 3 mA at 5 V, and the Micro needs about 40 mA. The keyboard's current is measured in Phase 1; if its LED-flash peak goes over about 150 mA, we'll need a bigger bulk capacitor or a USB 3 / powered port.
Bill of materials
| Qty | Part | Notes |
|---|---|---|
| 1 | Arduino Micro (A000053) | Teensy 2.0 is a drop-in alternative |
| 1 | 5 V → 12 V boost module | on hand; set to 12.0 V before connecting |
| 1 | 4P4C (RJ9/RJ10) PCB socket or breakout | takes the LK401's cable |
| 2 | 2N3904 (or BC547) NPN transistor | |
| 3 | 10 kΩ resistor | |
| 2 | 4.7 kΩ resistor | |
| 1 | 470 µF 25 V electrolytic | 12 V rail bulk capacitor |
| 1 | 100 nF ceramic | 12 V rail, at the socket |
| 1 | 500 mA polyfuse | on USB 5 V into the boost |
| 1 | Enclosure | about 80 × 50 × 25 mm |
| — | 24 MHz 8-channel USB logic analyser | test equipment, optional; use with PulseView |
Arduino Micro pins
| Pin | Use |
|---|---|
| D0 (RX1) | From the keyboard, through the receive inverter |
| D1 (TX1) | To the keyboard, through the transmit inverter |
| D13 (LED) | Blinks on each byte received |
| USB | HID keyboard and debug console |