# Message Overview This implementation sends **6 messages** every 20 ms (50 Hz cycle). All use 29-bit extended IDs and 8 data bytes, conforming to SAE J1939. ### Message table

ID (hex)	Name	PGN	Variable	Active bytes
`0x0CF00400`	EEC1 — Engine RPM	61444	`rpm`	3–4
`0x0CFE6CEE`	TCO1 — Vehicle speed	65132	`vel`	6–7
`0x18FEEE00`	ET1 — Engine temperature	65262	`temp`	0
`0x18FEFC21`	ZBR — Throttle	65276	`gas`	1
`0x18FEAE0B`	EBS — Air pressure	65198	`aire1`, `aire2`	2–3
`0x18EEFF21`	Heartbeat	—	—	all `0x00`

### 29-bit ID structure Every J1939 message ID encodes three fields:

Bits	Field	Description
28–26	Priority	0 (highest) to 7 (lowest)
25–8	PGN	Identifies the data type
7–0	SA	Source address — which ECU is sending

### Byte convention Unused bytes are always set to `0xFF`, which in J1939 means **parameter not available** (SNA — Specific Not Available).

``` Byte: 0 1 2 3 4 5 6 7 ┌─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┐ │ FF │ FF │ FF │ Lo │ Hi │ FF │ FF │ FF │ └─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┘ ↑ ↑ Actual data (little-endian) ``` ### Send cycle All 6 messages are sent sequentially every 20 ms:

```c++ void loop() { // send all messages // ... delay(20); // 50 Hz } ``` ### Source addresses in use

SA (hex)	Decimal	ECU
`0x00`	0	Engine ECU (RPM, temperature)
`0xEE`	238	Tachograph ECU (speed)
`0x21`	33	Simulator node (throttle, heartbeat)
`0x0B`	11	EBS brake ECU (air pressure)