Power Functions RC (IR)

A 38 kHz infrared carrier, one way. The 38 kHz remote every 2007-and-later LEGO motor answers to, and the one LEGO published a spec for. Four channels, two outputs, seven speed steps each way. A 38 kHz emitter cannot reach a VLL brick and a plain lamp cannot reach this one.

Link layer

• 38 kHz carrier (like consumer IR remotes); a "mark" = 6 carrier cycles (~158 µs ON min). This differs from baseband VLL — you need a 38 kHz-modulated IR emitter.
• Bit timing: low bit ≈ 316–526 µs, high bit ≈ 526–947 µs, start/stop ≈ 947–1579 µs.
• 16-bit message: 4-bit nibbles = [toggle+escape+channel] + [address+mode] + [data] + [LRC check nibble].

Modes

• Combo PWM — both outputs, 7 speed steps each fwd/rev + brake/float.
• Single output PWM / increment / toggle.
• 4 channels (select on remote + receiver).

Tooling

• Open-source emitter libraries (LEGOPowerFunctions), receiver decoders (github.com/matthiaszimmermann/ArduinoLegoIrReceiver), recorders.
• Contrast with RCX IR protocol (serial data link) and VLL — Visible Light Link (baseband light). All three are "optical", but mutually incompatible encodings.