I am trying to make a software, which runs under Windows and communicates with an ELM327 device. I created the first version and I went in my SMART ForTwo (SMART 451) vehicle and I managed to connect with the Instrument Cluster (Transmit CAN ID is 782, Receive CAN ID is 783). However I have a huge problem with Flow Control. Here is the log:
TX: ATI RX: ELM327 v1.5a
TX: ATE0 RX: ATE0 OK
TX: ATSP6 RX: OK
TX: ATH1 RX: OK
TX: ATL1 RX: OK
TX: ATCFC1 RX: OK
TX: ATFCSM0 RX: OK
TX: ATAL RX: OK
TX: ATSH782 RX: OK
TX: ATCRA783 RX: ?
TX: ATST64 RX: OK
TX: 1092 RX: 783 02 1A 87
TX: 1A87 RX: 783 10 16 5A 87 05 6E 00 08
I used another tool and I saw that the ELM327 device sends the Flow Control Frame, immediately. It is like this:
891.438 782 02 1A 87
891.444 783 10 16 5A 87 05 6E 00 08
891.444 782 30 00 00 00 00 00 00 00
As you can see - Flow Control frame is sent at exactly the same time as the First Frame, that is sent from the other device. I am sure that the other device has a problem to receive the "Flow Control" frame. I studied the ELM327 documentation, but did not find any info about how to delay the Flow Control frame. How should I correctly send the request "1A 87" and receive the response?
It's very easy, I guess..
Use the ATCFC0 command, and then you can process all the response frames from the control unit, and you need to manually send the flow control frame.
It's an old post but might help others!
This is my Experience with First Frame (FF) and Flow Control (FC) on MCP2515 connected with SPI.
First of all you should always send the FC message after the FF message and not in the same time.
Secondly the diagnostic reader can use the ID in the ECU response frame to continue communication with a specific ECU. In particular, multi-frame communication requires a response to the specific ECU ID rather than to ID 7DF. In the easy language you should not send your FF message with ID 7DF and you should address the exact ECU that you want to receive the consecutive frames. For instance requesting car VIN (based on real information from Golf VII):
7DF 02 09 00 00 00 00 00 00 //sending request
7E8 10 14 49 02 01 57 56 57 //receving from main ECU
7E0 30 00 00 00 00 00 00 00 //adressing the main ECU and not 7DF anymore !!
7E8 21 5A 5A 5A 41 55 5A 45 //consecutive messages are sending by 7E0!!
7E8 22 50 35 33 30 36 38 35
Hopefully it helps!
来源：https://stackoverflow.com/questions/29213988/flow-control-message-while-receiving-can-message-with-elm327