VLL — Visible Light Link

VLL (Visible Light Link) is LEGO's protocol for sending commands as plain on/off light pulses. A light sensor on the brick measures brightness; the sender flashes light with precise timing to encode bits. Shared by Micro Scout, Code Pilot and Scout, which all decode the same codes.

The communication protocol was covered by a LEGO patent application — period LEGO license text notes it as "protected by a pending patent application."

Frame

One command = 10 bits = 3-bit checksum + 7-bit data, wrapped in init/start/stop:

1. Init / preamble — light ON ≈ 400 ms (some impls use 1000 ms, see Implementations below) before anything.
2. Start — OFF 20 ms.
3. Checksum — 3 bits, MSB first (see Checksum below).
4. Data — 7 bits, MSB first.
5. Stop — ON 20 ms, OFF 60 ms, ON 120 ms (some impls: just ON 20 / OFF 60).

The frame transmits [checksum:3 bits][data:7 bits], both MSB first.

Bit encoding (unit = 20 ms)

Total per bit = 60 ms either way (only the ON/OFF ratio differs).

Checksum

A 3-bit checksum is prepended (MSB first) to each 7-bit data value:

function checksum(n) {        // n = 7-bit data value
  return 7 - ((n + (n >> 2) + (n >> 4)) & 7);
}

Verified against all 27 commands in rcxvll.nqh (pbrick) and used verbatim by Flash Force, lego-blockly and elecbrick (Doug Eaton).

Opcode table

The 7-bit data values the Micro Scout recognises (shared codebase with Code Pilot / Scout). Decoded from rcxvll.nqh (pbrick) and cross-checked with elecbrick (Doug Eaton) / Flash Force, and validated against the official Scout SDK "VLL Command Set" table (Nov 1999, pp. 66–68) and the printable MicroScout VLL barcode card.

Direct commands (execute immediately)

Script commands (16–32, stored into a program; up to 15 steps)

Build a program = Delete (34) → send script steps 16–32 → Run (33).

Notes / gaps

• Stop motors: the official Scout SDK and rcxvll.nqh both define code 10 = Motor Stop (Direct). So 10 is the correct, sanctioned stop code — but it did nothing on a test Micro Scout (firmware/timing-dependent?). The old elecbrick "15" is not in any official list.
• 70 (Next) / 71 (Reset) are real, documented MicroScout codes (Scout SDK + barcode card), but likewise produced no effect on-device. Kept here for completeness; removed from Flash Force's active buttons as unreliable in practice. No direct "stop" code worked reliably in testing.
• Codes 2, 3, 9, 11–14 are unused/unknown for the MicroScout. (On Code Pilot, 9 = Sound Mute, 11 = Motor & Sound, 12–15 = fixed programs, 96–127 = touch/tones/numbers/speed/tacho — see the SDK table.)
• Device-confirmed working: motor 0/1, beeps 4–8, Run 33, Delete 34, and script builder 16–32.

Implementations

Four independent implementations agree on the core protocol:

The protocol is identical everywhere; the tunable parts are preamble length and stop-bit tail (reliability knobs).

Practical

• Timing tolerance is moderate — Flash Force works at scale 1.05–1.35× (default 1.2×).
• Needs strong ON/OFF contrast; hold the emitter close (1–3 cm), a dim room helps.
• Works with any light the sensor sees — visible or IR (see IR LED (invisible VLL)).

Links

• elecbrick VLL reference (Doug Eaton)
• Ben Jackson — "VLL Transmit for legOS"
• RCX → MicroScout VLL writeup (pbrick.info)

See also

• Phone torch (Flash Force)
• adriansieber