OBD-II UART board: Reading raw CAN bus data

Development Boards
1

Using the OBD-II UART board, I am able to read the standard vehicle data via ELM327 AT commands.

How / where should I connect to the board in order to connect its MCP2551 RX and TX lines to a CAN controller such as the MCP2515?

Thanks.

2

  1. Locate the PTH holes on the board labeled “CAN Hi” and “CAN Lo” - these provide direct access to the CAN bus differential signals from the MCP2551

  2. For connection to your MCP2515 CAN controller:

  • Connect the “CAN HI” test point on the SparkFun board to the CANH pin on your MCP2515 circuit
  • Connect the “CAN LO” test point to the CANL pin on your MCP2515 circuit
  • Connect a common ground between the SparkFun board and your MCP2515 circuit

Note that this will establish a parallel connection to the CAN bus - both the ELM327 and your MCP2515 will be able to receive CAN messages simultaneously, but you’ll need to be careful about both trying to transmit at the same time

3

Thank you for that info. Due to the close proximity of the CAN HI and CAN LO labeled holes on the board, I was not sure whether they were connected directly to the DB9 connector pins or if they were connected to the RX and TX of the MCP2551.

4

You can always poke around its schematic (from the ‘documents’ tab on product page) https://cdn.sparkfun.com/assets/a/3/f/9/c/OBD-II-UART-v13.pdf

5

From the schematic, it appears that the CAN HI and CAN LO holes in the board are connected directly to the DB9 connector pins which correlate to the OBD2 pins 6 and 14. This would put those holes before the MCP2551 in the circuit. If this is the case, if I were to connect at those points, I would need to first connect them to another CAN transceiver (e.g. another MCP2551) which would in turn connect to a MCP2515.
For clarity, I was asking where/how to connect to the MCP2551 output (RX and TX).

6

I have confirmed that the PTH holes on the board labeled CAN HI and CAN LO are input connections and not the output from the MCP2551 as you described. I simply used a custom OBD2 cable from which I directly connected only the VBAT, GND, CAN H and CAN L lines (OBD2 pins 16, 4/5, 6 and 14 respectively) to the corresponding holes in the board. I did not use the DB9 connector at all for this test. My ESP32 successfully reads the vehicle info from the board using the ELM327 AT commands.

So my initial question remains: Where/how can I connect to the board for the OUTPUT from the MCP2551 transceiver that I could then connect to a separate CAN controller such as an MCP2515?

Thanks.

7

There’s not a good place to do that unfortunately. Not without pulling up the CAD files and figuring out what traces to solder too.

The CAN bus shield might be a better option.