ISO 15765-2
ISO 15765-2 , also ISO-TP for short , is an international standard for a transport protocol via vehicle-specific bus systems such as CAN bus , but also FlexRay , LIN , MOST bus etc. The protocol enables the transport of diagnostic data imprinted on the ECU communication payload whose length exceeds the maximum 8 bytes of user data of a CAN frame , which in previous transport protocols led to restrictions and the like. a. when transferring larger amounts of data (ECU programming, flashing).
In the OSI model , it covers layers 3 (network layer) and 4 (transport layer) and can transport up to 4095 bytes of user data per telegram. ISO-TP segments longer messages into several frames and supplements the data packets with metadata, which enables the recipient to interpret the individual frames.
The typical application is the transmission of diagnostic messages from KWP2000 and UDS , but is not limited to this.
ISO-TP can be operated with its own addressing as so-called extended addressing or without an address (so-called normal addressing ). With Extended Addressing, each frame carries one byte of address information in the first user data byte of the CAN frame. This is necessary if not every control unit in a CAN network has its own CAN identifier assigned to answer it. Without an address in the frame, addressing must be ensured using various CAN identifiers for request and response for each individual control unit. With normal addressing, one more byte is available for user data in the respective CAN frame during data transmission.
In addition to the extended addressing byte that may be present, there is always a Protocol Control Information Byte (PCI) in the user data part of the CAN frame in a CAN frame with ISO-TP. ISO-TP frame types as well as protocol-specific counters and status are stored in this Protocol Control Information Byte.
The ISO-TP defines four frame types:
- Single frame: the telegram to be transmitted consists of up to 6 or 7 bytes (normal addressing) of user data, non-segmented transmission.
- First frame: first frame of the sender if more than 6 or 7 bytes of data have to be transmitted in segments. The frame contains the entire length of the telegram.
- Consecutive frame: Transmits the further user data when segmented.
- Flow Control Frame: Response from the recipient, which defines the method of transmission of further consecutive frames.
The sender initiates the segmented transmission with a first frame, which the receiver confirms with a flow control frame. In this (first) flow control frame, the receiver defines how many consecutive frames can be received in direct succession ( block size ) and at what time intervals (0–127 ms) these consecutive frames must be sent. Once a block of consecutive frames has been transmitted, another flow control frame is sent from the receiver until all user data has been transmitted. Modern control devices also support block size 0 (= off), i.e. H. Any number of consecutive consecutive frames can be sent by the sender. Each consecutive frame contains a 4-bit sequential counter that is incremented with each frame sent (1, 2,…, 15, 0, 1,…), so that lost frames can be recognized.
4095 bytes of user data are the defined upper limit for segmented messages of the ISO-TP protocol, whereby in practice the project-specific limit is sometimes lower because the receive buffers are selected to be smaller.
With the introduction of CAN FD and the associated expansion of the possible payload from 8 to 64 bytes per CAN frame, ISO 15765-2 was extended to the effect that theoretically up to 4 gigabytes of long segmented messages can now be transmitted, but this is in practice is never fully exploited due to the comparatively low data rates on the CAN.
Norms
ISO 15765-2: 2011 Road vehicles - Diagnostic communication over Controller Area Network (DoCAN) - Part 2: Transport protocol and network layer services
literature
- Werner Zimmermann and Ralf Schmidgall: Bus systems in vehicle technology - protocols, standards and software architecture. 5th edition, Springer Vieweg, 2014, ISBN 978-3-658-02418-5 .