3.8 KiB
ELM327 support for panda
The panda now has basic ELM327 support.
What is ELM327?
ELM327 is a command protocol for interfacing with cars using an OBD-II port to read [list](standard vehicle diagnostic codes). ELM327 originally referred to a line of programmable microcontrollers that implemented the ELM327 command protocol, and are still being developed.
ELM327 devices present a shell and commands are sent via a UART. The official ELM327 chips only support raw UART communication, but most devices built with the ELM327 devices (official or clones) expose the UART in a more modern way (Wifi, USB, etc).
Mechanics use ELM to diagnose vehicles, and reset fault codes in the car's computer after fixing the issue (turning off the dreaded check engine light). Car owners can use ELM devices to perform the same diagnostics in their garage using either a raw terminal to send commands to the ELM device directly, or using a GUI (like one of several popular smart phone apps) to translate the OBD error codes into readable messages. These GUIs also often allow monitoring of the performance of the car's speed, engine rpm, etc.
The panda natively supports sending all the important OBD diagnostic messages, but ELM327 support removes the need for the user to manually craft CAN or LIN packets using the native panda API, and grants compatibility with many existing tools.
Wikipedia can provide additional information.
OBD Protocols?
While the commands that the OBD standard describe are in fact standard, there are several different protocols that those messages can be sent and received with. All cars after 1991 support one of these protocols. Which one depends on the car's year and country, as legal requirements change over the years.
The panda supports the most popular/modern of these protocols, and all but two can be added as needed. Below is a chart of the OBD-II protocols supported by the panda.
| Protocol | Support Status |
|---|---|
| SAE J1850 PWM (41.6 kbit/s) | Never/Obsolete |
| SAE J1850 VPW (10.4 kbit/s) | Never/Obsolete |
| ISO 9141-2 (5 baud init, 10.4 kbit/s) | Unsupported |
| ISO 14230-4 KWP (5 baud init, 10.4 kbit/s) | Unsupported |
| ISO 14230-4 KWP (fast init, 10.4 kbit/s) | Supported |
| ISO 15765-4 CAN (11 bit ID, 500 kbit/s) | Supported |
| ISO 15765-4 CAN (29 bit ID, 500 kbit/s) | Supported |
| ISO 15765-4 CAN (11 bit ID, 250 kbit/s) | Supported |
| ISO 15765-4 CAN (29 bit ID, 250 kbit/s) | Supported |
| SAE J1939 (250kbps) | Unsupported |
The Implementation
The panda ELM327 implementation is not a full implementation of all the features of the official ELM327 microcontroller. Like most ELM327 clones, the panda reports its ELM version as the unreleased version 1.5, despite only implementing commands from protocol version 1.0.
Testing
These tests require two pandas. One to be tested, and the second to simulate the vehicle.
The panda used to simulate the vehicle must be plugged into a USB port of the testing computer.
The computer running the tests must be connected to the panda being tested's wifi network.
The following command will run the tests (nosetest should work fine instead of pytest if you still prefer using that). The CANSIMSERIAL environment variable will force the car simulator to use the correct panda as the simulator if multiple pandas are attached via usb to the host computer.
CANSIMSERIAL=car_sim_panda_serial pytest tests/automated/elm_wifi.py
A single test can be run by putting the test name after the file name and two colons, like so:
CANSIMSERIAL=car_sim_panda_serial pytest tests/automated/elm_wifi.py::test_important_thing
For more detail, provide the -s (show output) and the -vv (very verbose) flags.
CANSIMSERIAL=car_sim_panda_serial pytest -s -vv tests/automated/elm_wifi.py