ISO 15765-2

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ISO 15765-2, also short ISO-TP is an international standard for a transport via CAN-Bus. The Protocol allows the transport of messages that length exceed 8-byte payload of can Frames maximum. The OSI Model covers the layer 3 (network layer) and 4 (transport layer) and can carry up to 4095 bytes of payload per telegram. ISO TP segmented longer messages on multiple frames and complements the data packets to metadata that allow an interpretation of each frame by the recipient.

The typical application is the transfer of diagnostic messages from KWP2000 and UDS, but is not limited.

ISO-TP can be operated with its own addressing as so-called Extended Addressing or without address (so-called Normal Addressing). With extended addressing each frame is a byte address information in the first payload bytes of CAN frames. This is necessary, if in a CAN network an own CAN identifier to reply is assigned to each control unit. Addressing through different CAN identifier for request and response for each individual control unit must be ensured without address in the frame. When normal addressing a byte representing data transfer more user data in the respective CAN frame.

In addition to the existing extended addressing byte is always a Protocol control information one CAN frame with ISO TP byte (PCI) in the user data of CAN frames. This Protocol control information server drops byte ISO TP frame types and protocol-specific counters and status.

The ISO TP defines four frame types:

   * Single frame: the telegram to be transferred is payload, not segmented transfer up to 6 or 7 bytes (normal addressing).
   * First frame: first frame of the channel, if more than 6 or 7 bytes of data segmented must be communicated. The frame contains the entire length of the telegram.
   * Consecutive frame: transfers the other user data for segmentation.
   * Flow control frame: Response of the receiver, which lays down the arrangements for the transmission of further consecutive frames.

The chunked transfer introduces the channel frame with a first, which confirmed the recipient with a flow control frame. The recipient defined in this (first) flow control frame, how many consecutive frames directly consecutive receive (Block Size), and at what intervals (0-127 ms) these consecutive frames must be sent. A block of consecutive frames is transferred, is a further flow control frame by the recipient are transmitted to all user data. Modern ECU support also block size 0 (= off), unlimited consecutive consecutive frames can be sent by sender. Each consecutive frame contains a 4-bit sequence counter is incremented with each frame sent (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, but in practice the project-specific limit is considerably lower because usually less elected the receive buffer.

Literature

• Werner Zimmermann, Ralf Schmidgall: Bussysteme in der Fahrzeugtechnik – Protokolle und Standards. Vieweg+Teubner, 3. Auflage, 2008, ISBN 978-3-8348-0447-1