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The protocol allows for the transport of messages that exceed the eight byte maximum payload of CAN frames. ISO-TP segments longer messages into multiple frames, adding metadata that allows the interpretation of individual frames and reassembly into a complete message packet by the recipient.
It can carry up to bytes of payload per message packet. Extended addressing uses the first data byte of each frame as an additional element of the address, reducing the application payload by one byte. In total, six types of addressing are allowed by the ISO Protocol. ISO-TP prepends one or more metadata bytes to the payload data in the eight byte CAN frame, reducing the payload to seven or fewer bytes per frame.
The PCI is one, two or three bytes. The initial field is four bits indicating the frame type, and implicitly describing the PCI length.
A message of seven bytes or less is sent in a single frame, with the initial byte containing the type 0 and payload length bytes. With the 0 in the type field, this can also pass as a simpler protocol with a length-data format and is often misinterpreted as such. A message longer than 7 bytes requires segmenting the message packet over multiple frames.
A segmented transfer starts with a First Frame. The PCI is two bytes in this case, with the first 4 bit field the type type 1 and the following 12 bits the message length excluding the type and length bytes.
The recipient confirms the transfer with a flow control frame.
ISO – Wikipedia
The flow control frame has three PCI bytes specifying the interval between subsequent frames and how many consecutive frames may be sent Block Size. The next byte is the block size, the count of frames that may be sent before waiting for the next flow control frame. A value of zero allows the remaining frames to be sent without flow control or delay. The third byte is the Separation Time STthe minimum delay time between frames. ST values up to 0x7F specify the minimum number of milliseconds to delay between frames, while values in the range 0xF1 to 0xF9 specify delays increasing from to microseconds.
Note that the Separation Time is defined as the minimum time between the end of one frame to the beginning of the next. Robust implementations should be prepared to accept frames from a sender that misinterprets this as the frame repetition rate i. Even careful implementations may fail to account for the minor effect of bit-stuffing in the physical layer.
The sender transmits the rest of the message using Consecutive Frames. The sequence number starts at 1 and increments with each frame sent 1, 2, Each consecutive frame starts at 0, initially for the first set of data in the first frame will be considered as 0th data.
So the first set of CF Consecutive frames start from “1”. There afterwards when it reaches “15”, will be started from “0”.
The 12 bit length field in the FF allows up to bytes of user data in a segmented message, but in practice the typical application-specific limit is considerably lower because of receive buffer or hardware limitations. Transport protocol and network layer services. From Wikipedia, the free encyclopedia.
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