问题
What I'm looking to do is request all desired PIDs via a .dbc file made in Vector db Editor++.
I understand enough about CAN communication to be able to do this with 1 or 2 PIDs because the DLC allows up to 8 bytes of data per CAN message. I am also familiar with this resource on querying and responses of PID https://en.wikipedia.org/wiki/OBD-II_PIDs#CAN_.2811-bit.29_bus_format
What I'm having trouble understanding is how diagnostic tools are able to query every PID the manufacturer of a particular vehicle decides to make available, so I feel that this is possible. Yet, if I use a request ID of $7DF, I can only use this message ID alone for my querying, this is the reason why I currently can only fit two PIDs (signals) in that CAN message.
回答1:
How diagnostic tools are able to query every PID the manufacturer of a particular vehicle decides to make available?
You cannot request whatever you want from ECU (at least in a normal way!). Only the OBD relevant PIDs you can request. All the OBD II PIDs and their definitions, scaling and etc. are available within ISO 15031 part 5. It means that all the PIDs are predefined. So any logger would firstly request mode 01 pid 00 to get all the available PIDs for that vehicle and then starts to scan over it.
if I use a request ID of $7DF, I can only use this message ID alone for my querying.
This is wrong cause 0x7DF has nothing to do with DLC and content of the message. It is only the header of message to tell the ECU from whom you have this request. 0x7DF is the OBD requests and even you can directly request different controllers their available data.
Every can message is 8 byte long. First byte is the mode of the request. Second byte tells the ECU the number of incoming bytes and then you have 6 bytes to send. Because of that they say you can request up to 6 PIDs simultaneously. your problem might be receiving multiple data from OBD which might be a bit tricky using Flow Control and First Frame messages. Here you can find some info about how to receive a message when it is longer that 8 bytes.
regards,
来源：https://stackoverflow.com/questions/42320827/is-it-possible-to-request-all-desired-odb-ii-pids-via-a-dbc-file