vt1200/reference/keyboard-mouse/elkay/docs/protocol.md

LK201 / LK401 protocol

What the firmware relies on. Sources: the VCB02 technical manual (EK-104AA-TM-001, LK201 chapter) and Linux drivers/input/keyboard/lkkbd.c. Anything marked verify comes from secondary sources or memory and must be confirmed on this keyboard in Phase 3.

Physical layer

• Asynchronous serial, 4800 baud, 8 data bits, no parity, 1 stop bit, full duplex.

• RS-423-style single-ended levels. The keyboard has only a +12 V supply, so its output cannot swing negative; the real levels are measured in Phase 1.

• Polarity is RS-232 style: idle (mark) is the low voltage, which is the opposite of a microcontroller UART, so both directions go through an inverter.

• Connector: 4P4C modular. Per lkkbd.c, looking at the plug with the cable downwards and the latch away from you, pins 1–4 left to right:

  Pin	Signal
  1	Data to keyboard (host TX)
  2	Ground
  3	+12 V
  4	Data from keyboard (host RX)

  Modular cables can be straight or reversed, so verify at the end of the cable we use.

Power-up

On power-up, or after the host sends FD (power-up reset), the keyboard runs its self-test, flashes the LEDs, and sends four bytes:

Byte	Meaning
1	Keyboard ID, firmware (01)
2	Keyboard ID, hardware (verify: 00)
3	Error: 00 none, 3D key down at power-up, 3E self-test failed
4	Keycode of the key held down, or 00

The host then asks for the ID (AB), which returns two bytes. lkkbd.c tells an LK201 (01) from an LK401 (02) by the second of them (verify).

Because the converter and the keyboard power up together, the power-up bytes are usually sent before anyone is listening. The console's reset command sends FD to repeat them.

Keycodes and special responses

Every key sends a single byte from 56 to FB. The bytes B3–BB are responses, not keys:

Byte	Name	Meaning
B3	ALL UPS	The last down/up key was released: every key is now up
B4	METRONOME	Auto-repeat tick (not used: elkay turns auto-repeat off)
B5	OUTPUT ERROR	Keyboard's transmit buffer overflowed
B6	INPUT ERROR	Keyboard received a bad command
B7	KBD LOCKED	Acknowledges "inhibit transmission"
B8	TEST MODE ACK	Acknowledges test mode
B9	PREFIX KEYS DOWN	Precedes a list of held keys after a mode change
BA	MODE CHANGE ACK	Acknowledges a mode-set command

The full keycode table is in keymap.md.

Divisions and modes

Keys are grouped into 14 divisions, and each division is in one of three modes:

Mode	Bits	Behaviour
Down only	00	Keycode on press, nothing on release
Auto-repeat down	01	Keycode on press, then B4 repeats while held
Down/up	11	Keycode on press, and again on release

USB HID needs releases, so elkay sets every division to down/up and lets the host do the auto-repeat.

A mode-set command is one byte: 1 DDDD MM 0 (division in bits 6–3, mode in bits 2–1), so down/up for division d is 0x86 | d << 3.

Releases in down/up mode

A release is reported by sending the same keycode again, except when the released key was the only down/up key still held: then the keyboard sends B3 (ALL UPS) instead. So the firmware keeps a set of held keys, toggles a key's state each time its code arrives, and clears the whole set on B3. This is also what lkkbd.c does.

Commands (host → keyboard)

Parameter bytes have bit 7 set; the last byte of every command has bit 7 set, which is how the keyboard knows the command is complete.

Bytes	Command
FD	Power-up reset (self-test, then the four power-up bytes)
AB	Request keyboard ID (two bytes back)
D3	Reinstate defaults
E9	Enable LK401 mode: separate codes for the Alt Function keys, right Compose and right Shift
0x86 | d<<3	Division d to down/up mode (d = 1–14)
13 p	LEDs on, p = 0x80 | mask
11 p	LEDs off, p = 0x80 | mask
99	Disable keyclick
1B v	Enable keyclick, volume v = 0x80 | 0–7 (0 loudest)
B9 / BB v	Disable / enable Ctrl-key click
A1 / 23 v	Disable / enable bell, with volume
A7	Sound the bell

LED mask bits: 01 Wait, 02 Compose, 04 Lock, 08 Hold Screen.

Initialisation sequence used by elkay

Taken from lkkbd_reinit(), with keyclick off:

AB                 request ID
D3                 defaults
E9                 LK401 mode
8E 96 9E … F6      divisions 1–14 to down/up
99  B9             keyclick and Ctrl click off
11 8F              all LEDs off

The console's init command sends exactly this.