UDS Packet

UDS packet might also be called Network Protocol Data Unit (N_PDU). The packets are created during segmentation of a diagnostic message. Each diagnostic message consists of at least one UDS Packet (N_PDU). There are some packets which does not carry any diagnostic message data as they are used to manage the flow of other packets.

UDS packet consists of following fields:

Network Address Information

Network Address Information (N_AI) contains address information which identifies the recipient(s) and the sender between whom data exchange takes place. It also describes communication model (e.g. whether response is required) for the message.

Network Data Field

Network Data Field (N_Data) carries diagnostic message data. It might be an entire diagnostic message data (if a diagnostic message fits into one packet) or just a part of it (if segmentation had to be used to divide a diagnostic message into smaller parts).

Network Protocol Control Information

Network Protocol Control Information (N_PCI) identifies the type of UDS packet (Network Protocol Data Unit). N_PCI values and their interpretation are bus specific.

UDS Packet on CAN

In this chapter you will find information about UDS packets that are specific for CAN bus, therefore applicable only for UDS packets that are transmitted over CAN bus.

CAN Frame

CAN data frames are the only type of CAN frames that are used during normal UDS communication. CAN data frames are made up of many different fields, but the key in our case (these influenced by UDS protocol) are listed below:

  • CAN Identifier (CAN ID)

    CAN ID is a field that informs every receiving CAN node about a sender and a content of frames. CAN nodes shall filter out and ignore CAN frames that are not relevant for them. In a normal situation, a CAN node detects a transmission of incoming CAN frames and once the identifier value (of a CAN frame) is decoded, the CAN node shall stop further listening to the frame if the CAN node is not a recipient of the frame.

    There are two formats of CAN ID:

    • Standard (11-bit Identifier)

    • Extended (29-bit identifier)

  • Data Length Code (DLC)

    Data Length Code (DLC) informs about number of CAN frame payload bytes that CAN Data Field contains.

  • CAN Data Field

    CAN Data consists of CAN frame payload bytes. The number of bytes that CAN Data Field contains is determined by frame’s DLC values as presented in the table:

    DLC

    Number of CAN Data bytes

    Supported by CLASSICAL CAN

    Supported by CAN FD

    0x0

    0

    YES

    YES

    0x1

    1

    YES

    YES

    0x2

    2

    YES

    YES

    0x3

    3

    YES

    YES

    0x4

    4

    YES

    YES

    0x5

    5

    YES

    YES

    0x6

    6

    YES

    YES

    0x7

    7

    YES

    YES

    0x8

    8

    YES

    YES

    0x9

    12

    NO

    YES

    0xA

    16

    NO

    YES

    0xB

    20

    NO

    YES

    0xC

    24

    NO

    YES

    0xD

    32

    NO

    YES

    0xE

    48

    NO

    YES

    0xF

    64

    NO

    YES

Note

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CAN Packet Addressing Formats

Each CAN packet addressing format describes a different way of providing Network Address Information to all recipients of CAN packets.

The exchange of UDS Packets on CAN is supported by three addressing formats:

Warning

Addressing format must be predefined and configured before any CAN packet is received as every CAN packet addressing format determines a different way of decoding CAN packets information (Network Address Information, Network Data Field and Network Protocol Control Information) that is not compatible with other addressing formats.

Note

Regardless of addressing format used, to transmit a functionally addressed message over CAN, a sender is allowed to use Single Frame packets only.

Normal Addressing

If normal addressing format is used, then the value of CAN Identifier carries an entire Network Address Information. Basing on CAN Identifier value, it is possible to distinguish an addressing type, a sender and a target/targets entities of a packet.

Following parameters specifies Network Address Information when Normal Addressing is used:

  • CAN ID

Note

Correspondence between Network Address Information and the value of CAN Identifier is left open for a network designer unless normal fixed addressing subformat is used.

Note

Network Protocol Control Information is placed in the first byte of CAN frame data field if normal addressing format is used.

Normal Fixed Addressing

Normal fixed addressing format is a special case of normal addressing in which the mapping of the address information into the CAN identifier is further defined.

Note

For normal fixed addressing, only 29-bit (extended) CAN Identifiers are allowed.

Following parameters specifies Network Address Information when Normal Fixed Addressing is used:

  • CAN ID (with embedded Target Address and Source Address)

CAN Identifier values used for UDS communication using normal fixed addressing:

  • For physical addressed messages, CAN Identifier value is defined as presented below:

    CAN_ID = 0x18DATTSS

  • For functional addressed messages, CAN Identifier value is defined as presented below:

    CAN_ID = 0x18DBTTSS
where:

  • CAN_ID - value of CAN Identifier

  • TT - two (hexadecimal) digits of a 8-bit Target Address value

  • SS - two (hexadecimal) digits of a 8-bit Source Address value

Extended Addressing

If extended addressing format is used, then the value of the first CAN frame byte informs about a target of a UDS packet and remaining Network Address Information (a sending entity and an addressing type) are determined by CAN Identifier value.

Following parameters specifies Network Address Information when Extended Addressing is used:

  • CAN ID

  • Target Address (located in the first data byte of a CAN Frame)

Note

Network Protocol Control Information is placed in the second byte of CAN frame data field if extended addressing format is used.

Mixed Addressing

Mixed addressing format specifies that the first byte of a CAN frame is an extension of Network Address Information.

Note

Network Protocol Control Information is placed in the second byte of CAN frame data field if mixed addressing format is used.

Mixed Addressing - 11-bit CAN Identifier

If mixed addressing format is used with 11-bit CAN Identifiers, then the value of the first CAN frame byte extends the CAN Identifier and a combination of these data forms the entire Network Address Information of a CAN packet.

Following parameters specifies Network Address Information when Extended Addressing is used:

  • CAN ID

  • Addressing Extension (located in the first data byte of a CAN Frame)

Mixed Addressing - 29-bit CAN Identifier

If mixed addressing format is used with 29-bit CAN Identifiers, then the value of the first CAN frame byte extends the CAN Identifier (that contains Target Address and Sender Address values) and a combination of these data forms the entire Network Address Information of a CAN packet.

Following parameters specifies Network Address Information when Extended Addressing is used:

  • CAN ID (with embedded Target Address and Source Address)

  • Addressing Extension (located in the first data byte of a CAN Frame)

CAN Identifier values used for UDS communication using mixed 29-bit addressing:

  • For physical addressed messages, CAN Identifier value is defined as presented below:

    CAN_ID = 0x18CETTSS

  • For functional addressed messages, CAN Identifier value is defined as presented below:

    CAN_ID = 0x18CDTTSS
where:

  • CAN_ID - value of CAN Identifier

  • TT - two (hexadecimal) digits of a 8-bit Target Address value

  • SS - two (hexadecimal) digits of a 8-bit Source Address value

CAN Data Field

CAN frames that are exchanged during UDS communication must have Data Length Code (DLC) equal to 8 (for CLASSICAL CAN and CAN FD) or greater (for CAN FD). The only exception is usage of CAN Frame Data Optimization.

DLC

Description

<8

Valid only for CAN frames using data optimization

Values in this range are only valid for Single Frame, Flow Control and

Consecutive Frame that use CAN frame data optimization.

8

Configured CAN frame maximum payload length of 8 bytes

For the use with CLASSICAL CAN and CAN FD type frames.

>8

Configured CAN frame maximum payload length greater than 8 bytes

For the use with CAN FD type frames only.
where:

Note

Number of bytes that carry diagnostic message payload depends on a type and a format of a CAN packet as it is presented in the table with CAN packets formats.

CAN Frame Data Padding

If a number of bytes specified in a UDS Packet is shorter than a number of bytes in CAN frame’s data field, then the sender has to pad any unused bytes in the frame. This can only be a case for Single Frame, Flow Control and the last Consecutive Frame of a segmented message. If not specified differently, the default value 0xCC shall be used for the frame padding to minimize the bit stuffing insertions and bit alteration on the wire.

Note

CAN frame data padding is mandatory for CAN frames with DLC>8 and optional for frames with DLC=8.

CAN Frame Data Optimization

CAN frame data optimization is an alternative to CAN Frame Data Padding. If a number of bytes specified in a CAN Packet is shorter than a number of bytes in CAN frame’s data field, then the sender might decrease DLC value of the CAN frame to the minimal number that is required to sent a desired number of data bytes in a single CAN packet.

Note

CAN Frame Data Optimization might always be used for CAN Packets with less than 8 bytes of data to send.

Warning

CAN Frame Data Optimization might not always be able to replace CAN Frame Data Padding when CAN FD is used. This is a consequence of DLC values from 9 to 15 meaning as these values are mapped into CAN frame data bytes numbers in a non-linear way (e.g. DLC=9 represents 12 data bytes).

Example:

When a CAN Packet with 47 bytes of data is planned for a transmission, then DLC=14 can be used instead of DLC=15, to choose 48-byte instead of 64-byte long CAN frame. Unfortunately, the last byte of CAN Frame data has to be padded as there is no way to send over CAN a frame with exactly 47 bytes of data.

CAN Packet Types

According to ISO 15765-2, CAN bus supports 4 types of UDS packets.

List of all values of Network Protocol Control Information supported by CAN bus:

The format of all CAN packets is presented in the table below.

CAN N_PDU

Byte #1

Byte #2

Byte #3

Byte #4

Byte #5

Byte #6

…

Bits 7-4

Bits 3-0

Single Frame

DLC ≤ 8

0x0

SF_DL

Single Frame

DLC > 8

0x0

0x0

SF_DL

First Frame

FF_DL ≤ 4095

0x1

FF_DL

First Frame

FF_DL > 4095

0x1

0x0

0x00

FF_DL

Consecutive Frame

0x2

SN

Flow Control

0x3

FS

BS

ST_min

N/A

N/A

N/A

N/A
where:

Single Frame

Single Frame (SF) is used by CAN entities to transmit a diagnostic message with a payload short enough to fit it into a single CAN packet. In other words, Single Frame carries payload of an entire diagnostic message. Number of payload bytes carried by SF is specified by Single Frame Data Length value.

Single Frame Data Length

Single Frame Data Length (SF_DL) is 4-bit (for CAN packets with DLC<=8) or 8-bit (for CAN packets with DLC>8) value carried by every Single Frame as presented in the table with CAN packet formats. SF_DL specifies number of diagnostic message payload bytes transmitted in a Single Frame.

Note

Maximal value of SF_DL depends on Single Frame addressing format and DLC of a CAN message that carries this packet.

First Frame

First Frame (FF) is used by CAN entities to indicate start of a diagnostic message transmission. First Frames are only used during a transmission of a segmented diagnostic messages that could not fit into a Single Frame. Number of payload bytes carried by FF is specified by First Frame Data Length value.

First Frame Data Length

First Frame Data Length (FF_DL) is 12-bit (if FF_DL ≤ 4095) or 4-byte (if FF_DL > 4095) value carried by every First Frame. FF_DL specifies number of diagnostic message payload bytes of a diagnostic message which transmission was initiated by a First Frame.

Note

Maximal value of FF_DL is 4294967295 (0xFFFFFFFF). It means that CAN bus is capable of transmitting diagnostic messages that contains up to nearly 4,3 GB of payload bytes.

Consecutive Frame

Consecutive Frame (CF) is used by CAN entities to continue transmission of a diagnostic message. First Frame shall always precede (one or more) Consecutive Frames. Consecutive Frames carry payload bytes of a diagnostic message that was not transmitted in a First Frame that preceded them. To avoid ambiguity and to make sure that no Consecutive Frame is lost, the order of Consecutive Frames is determined by Sequence Number value.

Sequence Number

Sequence Number (SN) is 4-bit value used to specify the order of Consecutive Frames.

The rules of proper Sequence Number value assignment are following:

Flow Control

Flow Control (FC) is used by receiving CAN entities to instruct sending entities to stop, start, pause or resume transmission of Consecutive Frames.

Flow Control packet contains following parameters:
Flow Status

Flow Status (FS) is 4-bit value that is used to inform a sending network entity whether it can proceed with a Consecutive Frames transmission.

Values of Flow Status:

  • 0x0 - ContinueToSend (CTS)

    ContinueToSend value of Flow Status informs a sender of a diagnostic message that receiving entity (that responded with CTS) is ready to receive a maximum of Block Size number of Consecutive Frames.

    Reception of a Flow Control frame with ContinueToSend value shall cause the sender to resume ConsecutiveFrames sending.

  • 0x1 - wait (WAIT)

    Wait value of Flow Status informs a sender of a diagnostic message that receiving entity (that responded with WAIT) is not ready to receive another Consecutive Frames.

    Reception of a Flow Control frame with WAIT value shall cause the sender to pause ConsecutiveFrames sending and wait for another Flow Control frame.

    Values of Block Size and STmin in the Flow Control frame (that contains WAIT value of Flow Status) are not relevant and shall be ignored.

  • 0x2 - Overflow (OVFLW)

    Overflow value of Flow Status informs a sender of a diagnostic message that receiving entity (that responded with OVFLW) is not able to receive a full diagnostic message as it is too big and reception of the message would result in Buffer Overflow on receiving side. In other words, the value of FF_DL exceeds the buffer size of the receiving entity.

    Reception of a Flow Control frame with Overflow value shall cause the sender to abort the transmission of a diagnostic message.

    Overflow value shall only be sent in a Flow Control frame that directly follows a First Frame.

    Values of Block Size and STmin in the Flow Control frame (that contains OVFLW value of Flow Status) are not relevant and shall be ignored.

  • 0x3-0xF - Reserved

    This range of values is reserved for future extension by ISO 15765.

Block Size

Block Size (BS) is a one byte value specified by receiving entity that informs about number of Consecutive Frames to be sent in a one block of packets.

Block Size values:

  • 0x00

    The value 0 of the Block Size parameter informs a sender that no more Flow Control frames shall be sent during the transmission of the segmented message.

    Reception of Block Size = 0 shall cause the sender to send all remaining Consecutive Frames without any stop for further Flow Control frames from the receiving entity.

  • 0x01-0xFF

    This range of Block Size values informs a sender the maximum number of Consecutive Frames that can be transmitted without an intermediate Flow Control frames from the receiving entity.

Separation Time Minimum

Separation Time minimum (STmin) is a one byte value specified by receiving entity that informs about minimum time gap between the transmission of two following Consecutive Frames.

STmin values:

  • 0x00-0x7F - Separation Time minimum range 0-127 ms

    The value of STmin in this range represents the value in milliseconds (ms).

    0x00 = 0 ms

    0xFF = 127 ms

  • 0x80-0xF0 - Reserved

    This range of values is reserved for future extension by ISO 15765.

  • 0xF1-0xF9 - Separation Time minimum range 100-900 μs

    The value of STmin in this range represents the value in microseconds (μs) according to the formula:

    (STmin - 0xF0) * 100 μs

    Meaning of example values:

    0xF1 -> 100 μs

    0xF5 -> 500 μs

    0xF9 -> 900 μs

  • 0xFA-0xFF - Reserved

    This range of values is reserved for future extension by ISO 15765.