ISO 15765-2

ISO 15765-2, ISO also short -TP is an international standard for a transport protocol on the CAN bus. The protocol allows the transport of messages whose length exceeds the maximum 8 bytes of user data a CAN frame. In the OSI model, it covers the layers 3 (network layer ) and 4 ( Transport Layer ) and can be up to 4095 bytes of user data per telegram transport. ISO TP longer segmented messages on a plurality of frames and adds the metadata to the data packets to enable an interpretation of the individual frames by the receiver.

The typical application is, however, the transmission of diagnostic messages from KWP2000 and UDS is not limited thereto.

ISO -TP can be operated with its own addressing a so-called Extended Addressing or without address ( so-called normal Addressing). With Extended Addressing each frame carries a byte address information in the first payload byte of the CAN frames. This is necessary, if not every controller has its own CAN Identifier is assigned to the response in a CAN network. Without Address in the frame addressing using different CAN - Identifier for the request and response for each control unit must be ensured. At normal addressing is in data transmission for one more byte of user data in each CAN Frame available.

In addition to the possibly existing Extended Addressing byte exists in a CAN frame at ISO -TP always a Protocol Control Information Byte (PCI) in the payload of CAN frames. In this Protocol Control Information Byte ISO -TP frame types and protocol-specific counters and status are stored.

The ISO -TP defines four frame types:

  • Single Frame: the transmitted message consists of up to 6 or 7 bytes (Normal Addressing) payload, non-segmented transmission.
  • First Frame: first frame of the transmitter when more than 6 or 7 bytes of data segments to be transmitted. The frame contains the entire length of the telegram.
  • Consecutive Frame: Transfers the further user data in segmentation.
  • Flow Control Frame: Answer the receiver, which sets out the procedures for the transfer of further Consecutive frames.

The segmented transmission directs the transmitter with a first frame that the receiver confirms with a flow control frame. In this (first) Flow Control Frame defines the receiver how many Consecutive frames can be directly consecutively received (block size ) and at what time intervals (0-127 ms) these Consecutive frames need to be sent. If a block is transferred from Consecutive frames, followed by a further flow control frame from the receiver until all payload is transmitted. Modern control units also support block size 0 ( = off), that is, there may any number of consecutive Consecutive frames are sent by the transmitter. Consecutive each frame includes a 4 -bit sequence counter that is incremented for each transmitted frame (1, 2, ..., 15, 0, 1, ...), which lost frames can be detected.

4095 bytes of user data are the defined upper limit for segmented messages of ISO -TP protocol, in practice, the project-specific limit is partially reduced, because the receiving buffer is smaller.

Standardize

ISO 15765-2:2011 Road vehicles - Diagnostic communication over Controller Area Network ( Docan ) - Part 2: Transport protocol and network layer services

List of International Organization for Standardization standards Frame (networking)
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