ISO 17458
FlexRay
Deterministic, high-speed bus for x-by-wire systems.
Bitrate
10 Mbit/s per channel (dual channel)
Topology
Bus or star, optional dual-channel for redundancy
Introduced
2006
Overview
FlexRay uses a TDMA schedule of static and dynamic slots inside a fixed cycle, making message timing deterministic. Dual channels add redundancy for safety-critical applications such as steer-by-wire and active suspension.
Frame format
| Field | Bits | Description |
|---|---|---|
| Header | 40 | Frame ID, payload length, header CRC, cycle count |
| Payload | 0–2032 | 0 to 254 bytes |
| Trailer | 24 | 24-bit CRC over header + payload |
Byte structure — request & response
FlexRay — ISO 17458
Frame template
[Header:5B][Payload:0–254B][Trailer CRC:3B]Request
| Off | Size | Field | Value | Description |
|---|---|---|---|---|
| 0 | 5 b | Status bits | 00010 | Reserved, payload preamble, null, sync, startup indicators. |
| 0–1 | 11 b | Frame ID | 0x05 | Slot identifier (1–2047). |
| 1 | 7 b | Payload length | 0x08 | Words (16-bit) of payload. |
| 2–3 | 11 b | Header CRC | auto | CRC over Frame ID and length. |
| 3–4 | 6 b | Cycle count | 0x0C | Communication cycle (0–63). |
| 5… | 0–254 B | Payload | AA BB CC DD … | Application data (16 B in slot 5 example). |
| tail | 3 B | Frame CRC | auto | 24-bit CRC over header + payload. |
Response
| Off | Size | Field | Value | Description |
|---|---|---|---|---|
| 0–4 | 5 B | Header | FrameID=0x06 | Reply slot scheduled in the static or dynamic segment. |
| 5… | var | Payload | 01 OK ack… | Reply payload, delivery guaranteed within the cycle. |
| tail | 3 B | CRC | auto | Frame CRC. |
FlexRay schedules slots in advance — there is no on-demand request. 'Request' and 'response' are simply two scheduled slots producers/consumers agreed on.
Use cases
- · Steer-by-wire
- · Brake-by-wire
- · Active suspension
- · Chassis domain
Pros
- Deterministic timing
- Redundant channels
- High bandwidth
Cons
- Expensive controllers
- Complex schedule design
- Largely superseded by Auto-Ethernet
Request / Response examples
Static slot 5, cycle 12
Request
Slot5—FrameID0x05Payload:16bytessteeringdata
Response
(scheduled—guaranteeddeliverywithincycle)
Same slot fires every cycle giving sub-millisecond jitter for control loops.