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