Understand CAN Data Bytes: A Complete Guide

Hi everyone,


I am currently in the process of reading the CAN bus of a tractor and want to retrieve the engine speed.

In the J1939-Companion-Spreadsheet, the engine speed, also known as 'engine speed', has the PGN 61444 (decimal), which corresponds to F004 (hexadecimal). Also, the engine speed should be found in bytes 4-5.

Now, I've found the corresponding CAN message.

1592) 1926.5 Rx 0CF00400 8 0C FF 7D D0 03 27 F0 86

The position bytes 4-5 are D0 03. Therefore, I convert this hexadecimal number into a decimal number, but I get 53251. According to the unit, I have to multiply this number by 0.125 again, resulting in 6656.

As everyone knows, this value is certainly not correct.
What did I do wrong?

Thank you for your help, Best regards, Michael.

maybe an endianness issue?
It's not big-endian, but little-endian.
In your case: 0x03D0 corresponds to 976dec.
With that factor, the value is quite small.

Sure, you're welcome!

On some other parameters, it fits quite well.

Perhaps it's because I've just started the engine, and the engine speed is currently increasing... (And because of the high transmission rate).