# CAN Protocol — CAN IO Board

Bus: **500 kbit/s**, 11-bit (standard) identifiers, classic CAN 2.0.
Base ID: **0x100** (configurable in `firmware/include/config.h`).

| ID           | Direction   | Purpose                       |
|--------------|-------------|-------------------------------|
| base + 0 = `0x100` | board → bus | Status frame            |
| base + 1 = `0x101` | bus → board | Command frame           |

Bit numbering in all bitmaps: bit0 = IN1/OUT1, bit1 = IN2/OUT2, … 1 = active.

## Status frame — ID 0x100, DLC 8

Sent every second (heartbeat), immediately on any input/output change,
in response to `GET_STATUS`, and once at boot.

| Byte | Meaning                                        |
|------|------------------------------------------------|
| 0    | Input bitmap (bit0=IN1 … bit3=IN4)             |
| 1    | Output bitmap (bit0=OUT1, bit1=OUT2)           |
| 2    | Reason: 0=periodic, 1=change, 2=request, 3=boot|
| 3    | Reserved (0)                                   |
| 4–7  | Uptime in seconds, uint32 little-endian        |

Example: `05 02 01 00 3C 00 00 00` → IN1+IN3 active, OUT2 on,
reason=change, uptime 60 s.

## Command frame — ID 0x101

Byte 0 selects the command. Malformed/unknown frames are ignored.

| Cmd  | Name        | DLC | Layout                                     |
|------|-------------|-----|--------------------------------------------|
| 0x00 | GET_STATUS  | ≥1  | `[00]` → board replies with a status frame |
| 0x01 | SET_OUTPUT  | ≥3  | `[01, index, value]` index 0-based, value 0=off / ≠0=on |
| 0x02 | SET_ALL     | ≥3  | `[02, mask, values]` outputs where mask bit=1 get the corresponding values bit |
| 0x03 | TOGGLE      | ≥2  | `[03, index]`                              |

Examples (send to `0x101`):

| Bytes        | Effect                    |
|--------------|---------------------------|
| `00`         | request status            |
| `01 00 01`   | OUT1 on                   |
| `01 01 00`   | OUT2 off                  |
| `02 03 03`   | OUT1 + OUT2 both on       |
| `02 03 00`   | OUT1 + OUT2 both off      |
| `03 00`      | toggle OUT1               |

Every accepted output change triggers an immediate status frame
(reason = change), so the bus always confirms what actually happened —
there is no separate ACK.

## Local rules (run on the board itself)

Evaluated on the activation edge of an input; releasing does nothing
(outputs latch):

| Input | Action    |
|-------|-----------|
| IN1   | OUT1 → ON |
| IN2   | OUT1 → OFF|
| IN3   | OUT2 → ON |
| IN4   | OUT2 → OFF|

Inputs already active at power-up are reported but do **not** fire rules.