FR Automotive Network Utility

FlexRay Frame Decoder

Decode FlexRay frame header bytes and inspect frame-control flags, Frame ID, payload length, Header CRC, Cycle Count and payload data. Optional trailing Frame CRC bytes can also be separated when included in a raw capture.

✓ Frame ID ✓ Payload Length ✓ Header CRC ✓ Cycle Count ✓ Payload

FlexRay Frame Decode

● Ready

Input Structure The decoder expects already recovered FlexRay frame bytes, not physical coding symbols.

Payload-Length Interpretation For normal FlexRay frames, the Payload Length field represents the payload size in 2-byte words.

FlexRay Frame Hex Bytes Hex bytes may be separated with spaces, commas, colons or hyphens. Continuous hexadecimal input and 0x prefixes are also accepted.

Structural decoder: this tool decodes the standard five-byte FlexRay header bit fields and separates payload/trailing CRC bytes. It does not infer application signal names, ECU ownership, slot configuration or physical values from payload bytes without the relevant FlexRay database or vehicle specification.

FlexRay Frame Decode Result Decoded

Decoded FlexRay Header —

Frame ID —

Payload Length Field —

Expected Payload Bytes —

Actual Payload Bytes —

Header CRC —

Cycle Count —

Payload Preamble —

Null Frame —

Sync Frame —

Startup Frame —

Frame CRC —

Total Input Bytes —

Payload Bytes

—

Bit-Level Decode Breakdown

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What Is a FlexRay Frame Decoder?

A FlexRay Frame Decoder converts raw frame bytes into the structural fields of the FlexRay communication protocol. FlexRay was designed for deterministic, fault-tolerant automotive networking where predictable communication timing is important.

A FlexRay frame contains a five-byte header followed by a payload and a 24-bit frame CRC. The header describes how the frame should be interpreted and includes the Frame ID, payload length, Header CRC and Cycle Count.

FlexRay Frame Structure

FlexRay Frame:

Header
  5 bytes

Payload
  0 to 254 bytes in normal frame structure

Trailer
  24-bit Frame CRC

The exact physical transmission additionally contains symbols and coding details that are normally removed before a software decoder receives the frame bytes.

FlexRay Header Fields

Header Field	Size	Purpose
Reserved	1 bit	Reserved header bit
Payload Preamble Indicator	1 bit	Indicates payload preamble information
Null Frame Indicator	1 bit	Distinguishes null-frame behavior
Sync Frame Indicator	1 bit	Marks a synchronization frame
Startup Frame Indicator	1 bit	Marks a startup frame
Frame ID	11 bits	Identifies the FlexRay communication slot/frame
Payload Length	7 bits	Payload length in two-byte words
Header CRC	11 bits	Protects selected header information
Cycle Count	6 bits	Communication cycle number

FlexRay Frame ID

The FlexRay Frame ID is an 11-bit field. It identifies a scheduled frame or slot within the configured communication schedule.

Frame ID range:

1 to 2047 usable according to network configuration

Binary field width:
11 bits

The meaning of a particular Frame ID is network-specific. A raw frame can prove the numeric identifier, but it cannot reveal a signal or ECU name unless a matching system database is available.

FlexRay Payload Length

The seven-bit Payload Length field normally represents the payload length in two-byte words rather than individual bytes.

Payload Length field:
4

Payload size:
4 × 2
= 8 bytes

This is why the calculator displays both the raw header value and the expected byte length.

FlexRay Cycle Count

The Cycle Count field contains six bits, allowing values from 0 through 63. After the cycle sequence reaches its configured limit, cycle numbering repeats.

Cycle Count Width:
6 bits

Numeric Range:
0–63

Network schedules can use cycle filtering so that particular frames transmit only during selected cycles.

Payload Preamble Indicator

The Payload Preamble Indicator, or PPI, is one of the FlexRay header flags. Its significance can depend on the segment and network configuration. The decoder reports the actual bit without inventing application semantics.

Null Frame Indicator

The Null Frame Indicator is associated with whether the transmitted frame contains valid application payload for the scheduled slot. Its exact use should be interpreted together with the FlexRay communication-controller configuration.

Sync Frame Indicator

The Sync Frame Indicator marks frames involved in FlexRay clock synchronization. Synchronization nodes use specially configured frames to support the distributed timing model of the cluster.

Startup Frame Indicator

The Startup Frame Indicator marks frames used during cluster startup. Startup and synchronization behavior are tightly connected to the configured FlexRay network parameters.

FlexRay Header CRC

The header contains an 11-bit CRC designed to protect selected header fields. The CRC is distinct from the 24-bit Frame CRC transmitted after the payload.

Header CRC:
11 bits

Frame CRC:
24 bits

They are separate integrity fields.

This page extracts the Header CRC value from the frame header. CRC verification against a captured frame can depend on exact protocol bit ordering and frame construction, so the structural result is kept separate from application interpretation.

FlexRay Frame CRC

The frame trailer contains a 24-bit CRC protecting the frame data. Some packet capture APIs include these three bytes while others remove the trailer before presenting the captured data.

Select the input structure that matches the bytes supplied by your analyzer. The decoder will then separate the final three bytes instead of treating them as application payload.

FlexRay vs CAN

FlexRay and CAN are both automotive communication technologies, but their medium-access and timing models differ considerably. Classical CAN relies on priority-based arbitration, while FlexRay provides a time-triggered static segment and can also provide a dynamic segment.

Feature	FlexRay	Classical CAN
Scheduling	Time-triggered + dynamic	Priority arbitration
Typical Channels	Up to two channels	Single logical bus
Payload	Much larger frame payload	Up to 8 bytes
Cycle Concept	Yes	No equivalent communication cycle

FlexRay Frame Decoder FAQs

How long is the FlexRay frame header?

The standard FlexRay frame header occupies five bytes.

How many bits is the FlexRay Frame ID?

The Frame ID field is 11 bits wide.

How is FlexRay payload length encoded?

The normal Payload Length field is seven bits and represents payload length in two-byte words.

If payload length is 4, how many payload bytes are expected?

Four two-byte words correspond to eight payload bytes.

How many bits is the FlexRay Header CRC?

The Header CRC field contains 11 bits.

How many bits is the FlexRay Frame CRC?

The trailer Frame CRC contains 24 bits.

What is FlexRay Cycle Count?

It is a six-bit header field identifying the communication cycle associated with the frame.

Can this tool identify signals inside the payload?

Not without the corresponding vehicle, ECU or network database. Raw FlexRay payload bytes do not contain signal names, scaling or engineering units.

Should I include the FlexRay Frame CRC?

Only if your capture includes it. Select Header + Payload + 3-Byte Frame CRC when the trailing CRC bytes are present.

Does this tool decode physical FlexRay symbols?

No. It expects recovered frame bytes rather than physical bus waveform or symbol-level input.

Decode FlexRay Header and Payload Bytes

Inspect FlexRay Frame IDs, payload-length fields, communication-cycle values, header flags, CRC fields and application payload boundaries from raw frame bytes.