W‑BUS Protocol Implementation Notes

This document captures practical W-BUS protocol details for controlling Webasto heaters from ESP32 via the WBusSimple implementation.

Big picture

• W‑BUS is not TTL UART. It is a single‑wire, automotive “K‑line style” physical layer. You need an interface/level shifter/transceiver between the ESP32 UART pins and the heater’s W‑BUS pin.
• On the wire, the payload framing is UART at 2400 baud, 8E1 (8 data bits, even parity, 1 stop bit).
• Frames are short and checksum is simple (XOR), but timing/physical‑layer matters a lot (idle levels, dominant low, bus contention, etc.).

Physical layer / wiring assumptions

• The heater W‑BUS line is typically around vehicle battery voltage (12V system). The ESP32 UART pins are 3.3V.
• Use a proper transceiver (common approaches in public projects include K‑line transceivers; some builds use NXP MC33290/MC33660‑class parts or equivalents).
• ESP32 UART TX should not drive the bus directly.
• If your interface requires it, you may need:
  • An enable pin to switch the bus driver on/off during TX.
  • Open‑collector/open‑drain behavior on TX.

In this repo those knobs exist in configuration:

• WBUS_TX_PIN, WBUS_RX_PIN
• WBUS_EN_PIN (optional)
• WBUS_SEND_BREAK (optional “break” pulse before first command)

Frame format

W‑BUS messages are framed like:

• Header: 1 byte (source/destination encoded as nibbles)
• Length: 1 byte
• Payload: length - 1 bytes
• Checksum: 1 byte (included in the payload byte count)

Header

Header is a packed byte: high nibble is source address, low nibble is destination address.

In this repo we generate it as:

• header = ((src & 0x0F) << 4) | (dst & 0x0F)

Typical addressing used here:

• Controller address: WBUS_ADDR_CONTROLLER (commonly 0xF)
• Heater address: WBUS_ADDR_HEATER (commonly 0x4)

So the common bytes you’ll see are:

• Controller → Heater: 0xF4
• Heater → Controller: 0x4F

Important: some public implementations accept additional header bytes (other address pairs). In this repo we now treat “valid header” as whatever matches your configured addresses.

Length

The length byte counts (payload bytes + checksum byte) after the length field.

If you send a command byte plus N data bytes:

• length = 1 (cmd) + N (data) + 1 (checksum)

Checksum

Checksum is a simple XOR:

• csum = header XOR length XOR payload_bytes...

Where payload_bytes... means everything except the checksum itself.

So, when verifying a received frame, compute XOR across:

• header
• length
• payload bytes excluding the final checksum byte

and compare the result to the final byte.

Serial “break” pulse

Some heaters/interfaces expect an initial “break” before the first command.

In this repo WBUS_SEND_BREAK triggers a one‑time sequence roughly like:

• UART off
• idle/high for a while
• drive low for a short period
• drive high for a short period
• UART on again

Exact timing is hardware dependent. If you see unreliable first‑packet behavior, this is the first knob to experiment with.

Command/response conventions used here

Many interactions use a command byte and optionally a “sub‑index” (one data byte) to select a page of data.

Commands

Operating state

We use:

• request: command 0x50 with index 0x07
• response: 0xD0 0x07 <opState> ...

opState is a large heater state machine. In the receiver firmware we map it coarsely into Off vs Running.

Status polling: two styles

There are (at least) two commonly seen status mechanisms.

1) Multi‑status TLV snapshot (0x50 / 0x30)

This is the richer mechanism and the primary one used in this repo.

• request: 0x50 with data 0x30 followed by a list of status IDs
• response: begins 0xD0 0x30 ... and then a sequence like <id><value…> repeated

The catch: the response does not always include explicit per‑field lengths, and field sizes can vary by heater/firmware.

Implementation in this repo:

• Parser in WBusSimple::tryParseStatusTlv()
• Decodes common fields we care about (temperature, voltage, power) and also keeps raw “status_XX” fields.
• Uses a defensive heuristic for some ambiguous‑length IDs to avoid desynchronizing the parse.

2) “Simple status pages” (0x50 / index)

Some setups (and at least one public Arduino implementation) poll a small set of fixed pages:

In this repo we have readers for:

• readStateFlags() → page 0x03
• readActuators() → page 0x04
• readCounters() → page 0x06
• readOperatingState() → page 0x07

Keep-alive and auto-renewal

Per the esphome-webasto pattern, heaters may require periodic keep-alive messages to maintain an active heating/ventilation session.

The WBusSimple class now tracks:

• activeCmd: The currently running command (0x21=heat, 0x22=vent, 0=none)
• activeUntilMs: When the session expires
• lastKeepAliveMs: When the last keep-alive was sent

Helper methods:

• needsKeepAlive(nowMs): Returns true every 10 seconds while a command is active
• needsRenewal(nowMs): Returns true when <30 seconds remain (should re-issue the start command)
• setActiveCommand(cmd, minutes) / clearActiveCommand(): Manage state

Retry logic

Commands now retry up to 3 times (configurable via kCommandRetries) with ACK verification, matching the esphome-webasto pattern.

In this repo we added a fallback: if the multi‑status TLV snapshot doesn’t arrive/parse during the poll window, the receiver tries these pages and logs the raw bytes to Serial.

Logging / debugging workflow

When you’re bringing up real hardware:

• Start with verifying UART config: 2400 8E1.
• Confirm your interface wiring and idle level.
• Enable Serial logging and look for:
  • valid frames with passing XOR checksum
  • responses to 0x50 0x07 (operating state)
  • responses to 0x50 0x05 and 0x50 0x0F (fallback pages)

If you can capture raw byte streams from your heater, you can extend tryParseStatusTlv() safely by:

• adding IDs to the known‑ID set
• confirming fixed sizes for your heater firmware

Practical gotchas

• Many “it doesn’t work” problems are electrical (wrong transceiver, no common ground, incorrect pull‑ups, bus being driven too strongly, etc.).
• Some heaters require periodic keep‑alive (0x44 … patterns appear in public projects); in this repo there is an optional sendKeepAlive().
• Don’t assume every heater supports 0x50 0x30 multi‑status; keep the simple‑page fallback available.

Where this is implemented in this repo

• W‑BUS framing + checksum + RX state machine: lib/common/wbus_simple.*
• Receiver poll loop with TLV + simple‑page fallback and Serial logs: src/receiver/main.cpp