I am using a raspberry Pi and arduino CAN schield which is using a MCP2515 and SPI to request a single OBD II PID.
I am able to request and receive a single PID from my OBD emulator (Freematics). I know that is possible to request multiple PIDs (up to 6 PIDs) in a single query.
Whenever I use other kind of messages, I receive only the first request. Can anybody maybe help?
Here is the message for single PID which is working (C++):
msg.id = 0x7DF; //ID_QUERY
msg.header.rtr = 0;
msg.header.length = 0x08;
msg.data[0] = 0x02;
msg.data[1] = 0x01;
msg.data[2] = PID; //Whatever PID I want!
Message for multiple request that is not working:
msg.id = 0x7DF; //ID_QUERY
msg.header.rtr = 0;
msg.header.length = 0x08;
msg.data[0] = 0x07; //! Also it is not working with 0x08
msg.data[1] = 0x01;
msg.data[2] = PID0;
msg.data[3] = PID1;
msg.data[4] = PID2;
msg.data[5] = PID3;
msg.data[6] = PID4;
msg.data[7] = PID5;
Sending canbus messages through mcp2515 is a litle bit tricky.
First of all Freematics OBD Emulator support no multiple PID requests. Secondly, the multiple PID request should be sent with ISO 15765 format. when you send a multiple request, he ECU would response only with one "First Frame" message and will wait for the "Flow Control" message from the sender. After receiving the flow control, ECU will continue to sending the responses based on your flow control setting.
For more information about the CAN-Bus messages and how "First Frame" and "Single Frame" works, read the below links: googleBooks, ISO_15765-2, hackaday (dot) com