I am new to the field of automotive electronics and am currently trying to understand the working of the CAN bus. I understand that when we connect a CAN bus logger to the OBD2 port in a car we will be able to view the CAN traffic (raw CAN frames) and in order to get specific information from ECUs, we can send a request with ID 7DF and would get a response with IDs between 7E8 to 7EF. Is 7E8 actually the ID of the responding ECU or is it a general-purpose address just to relay the response back to the sender. Further, why 7DF has been chosen to send a request? Is there a specific reason? I apologize if any of the above statements are not right. But I could not find any material on the internet providing this clarification. Any help is much appreciated. Thank you.
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Welcome to Motor Vehicle Maintenance & Repair!– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Apr 21, 2021 at 0:54
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I am also just starting to dig into OBD2. I am electronics technician and auto technician. It seems that 7DF is the standard identifier for request messages. The response identifier can range from 7E8 to 7EF, depending on which ECU is responding. These 3-digit HEX addresses all start with 7 (binary 0111) so that the leading 0 can be dropped leaving 11 bits for the identifier.
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If you write out all these addresses in binary, you will see how the programmers chose a nice easy set of address headers. Commented Apr 27, 2021 at 5:45