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Why External Steering Request (XSR) J1939 message has checksum message within the 8-byte Data Field?

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jani12

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Camera ECU sends External Steering Request (XSR) J1939 message to the steering ECU. The PGN of this message is 0xF341. There is 4-bit checksum within the 8 byte data field. The SPN of this checksum is 21095.

This 4-bit checksum is used to verify the signal path from the demanding device to the active steering system. It is calculated using the first 7 data bytes, the 4 bit message counter and the bytes of the message identifier.

Since there is 16 Bit CRC Field in the J1939 frame (please see attached), why does J1939 specification include 4-bit checksum within the Data Field?

crc_field.jpg
 

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