PlantCtrl/rust/website/content/Hardware/CAN-Bus-Protocol/1745239260000-CAN-Bus-IDs-and-Wire-Format/index.md

title, date, draft, description, tags

title	date	draft	description	tags
CAN Bus IDs and Wire Format	2026-05-21	false	Quick reference for CAN bus identifiers, message formats, and on-the-wire data structures.	can protocol wire-format

CAN Bus IDs and Wire Format

A concise technical reference for the PlantCtrl CAN bus protocol.

Quick Reference Table

CAN ID (hex)	Message Type	Direction	Payload
0x03E8	Moisture Data - Plant 0, Slot A	Sensor → Controller	u8 plant + u8 slot + u16 hz
0x0400	Identify Command - Plant 0, Slot A	Controller → Sensor	(empty)
0x042F	Moisture Data - Plant 15, Slot A	Sensor → Controller	u8 plant + u8 slot + u16 hz
0x0467	Firmware Build - Plant 3, Slot B	Sensor → Controller	u32 build_minutes

ID Calculation Formula

ID = 0x03E8 + Message_Offset + Plant_Index (+ Slot_Offset if B)

Constants

Constant	Value (hex)	Description
SENSOR_BASE_ADDRESS	0x03E8	Base address for all messages
MOISTURE_DATA_OFFSET	0x00	Moisture data group
IDENTIFY_CMD_OFFSET	0x20	Identify command group
FIRMWARE_BUILD_OFFSET	0x40	Firmware build group
B_SLOT_OFFSET	0x10	Offset for Slot B within a group

Message Type Offsets

pub const MOISTURE_DATA_OFFSET: u16 = 0;      // sensor → controller
pub const IDENTIFY_CMD_OFFSET: u16 = 32;      // controller → sensor
pub const FIRMWARE_BUILD_OFFSET: u16 = 64;    // sensor → controller

On-the-Wire Formats

Moisture Data Frame (Sensor → Controller)

CAN ID: 0x03E8 + offset + plant_index (or + 16 + plant_index for Slot B)

Byte	Field	Type
0	plant	u8 (0–15)
1	sensor	SensorSlot (A=0, B=1)
2-3	hz	u16 big-endian

Example: Plant 7, Slot A, frequency 45 Hz

CAN ID: 0x0415
Payload: [07 00 00 2D]
         plant=7, sensor=A, hz=45 (0x002D)

Firmware Build Frame (Sensor → Controller)

CAN ID: 0x03E8 + 64 + plant_index (or + 80 + plant_index for Slot B)

Byte	Field	Type
0-3	build_minutes	u32 big-endian

Example: Build timestamp 1,745,239,200 minutes since epoch (May 2026)

CAN ID: 0x0440
Payload: [00 00 6A F8]
         build_minutes = 1,745,239,200

Identify Command Frame (Controller → Sensor)

CAN ID: 0x03E8 + 32 + plant_index (or + 48 + plant_index for Slot B)

Byte	Field	Type
(none)	(empty payload)	-

Example: Identify Plant 5, Slot A

CAN ID: 0x0410
Payload: (empty)

Addressing Scheme Details

Plant Index Range

• Valid: 0–15 (decimal) or 1–8 on hardware jumpers (mapped internally as 0–7)
• Slot A: plant_index = jumper value - 1
• Slot B: Same mapping, but ID offset differs by +16

Slot Selection

Hardware	Internal Value
Jumper on PA3 (Low)	Slot A (0)
Jumper on PA3 (High)	Slot B (1)

Error Detection IDs

The sensor module monitors for unexpected messages:

• Moisture Data collision: If a sensor receives moisture data addressed to itself, it triggers error code 1 (1 info blink, 2 warning blinks)
• CAN errors: Bus-off, EWGF, EPVF flags trigger warning LED blinking

Protocol Extensions

To add new message types:

1. Define offset in canapi/src/lib.rs:

   pub const NEW_MESSAGE_OFFSET: u16 = 96; // Next available slot
   

2. Implement message struct with bincode serialization

3. Add receive handler on both sides

4. Update documentation

Binary Protocol Reference

bincode v2 Serialization

• u8: Single byte, no sign extension
• u16: 2 bytes big-endian (network order)
• u32: 4 bytes big-endian (network order)
• No varints – fixed size for predictable CAN frame lengths

CAN Frame Structure

| Arbitration Field | Control Field | Data Field (8 bytes) |
|-------------------|---------------|----------------------|
| 11-bit ID         | RTR + IDE     | Payload (max 4-6 bytes)|

All PlantCtrl messages fit within the 8-byte data field with room for CAN overhead.