ELM327 communication on Linux

The goal is to get a vehicle speed with high frequency (5-10 Hz), using a super-cheap elm327 controller and a standardized OBD API.
The speed is usually encoded with two-digit hexadecimal number and can vary in range 0...255 km\h.
In my case, the vehicle speed needed to increase the filter's performance in localization.

Before begin:

• obd communication cheatsheet;
• elm reference;
• termios serial params;
• OBD-II PIDs;

Configuring COM-port

The main thing - is to configure serial port in a right way. The communication itself is straight forward.
Here are some basic serial port parameters for elm327 devices:

• Baudrate - 38400;
• 8 data bits;
• Stop bits - 1;
• No Parity;

Sellecting the Serial port Number on Linux:

• /dev/ttyUSBx - when using USB to Serial Converter, where x can be 0,1,2...etc;
• /dev/ttySx - for PC hardware based Serial ports, where x can be 0,1,2...etc;
  Remember, that in Linux, everything is a file - so you can apply standart read/write operations on /dev/tty*.

Also, for quick tests, I use screen:

sudo screen -L /dev/ttyUSB0 38400,cs8

After finishing the screen session, the serial port "saves" the last configuration, so it becomes really useful for debugging. To get the last serial port configs for particular device, just run stty:

sudo stty -F /dev/ttyUSB0 -a

Using the code

All you need is stored in obd_helpers.h. The elm_main.c is only to show the functionality.

Compile:

gcc -o elm_main elm_main.c

Run, passing the needed device name (you need to be root or superuser):

sudo ./elm_main /dev/ttyUSB0

Or run with debug flag to test the code without a car:

sudo ./elm_main /dev/ttyUSB0 debug

The output of the main script is csv-like formatted, so it convenient to dump it to file and analyze:

sudo ./elm_main /dev/ttyUSB0 > /tmp/elm_speed.csv 2>&1

To monitor the logfile while dumping:

tail -f /tmp/elm_speed.csv

Skoda Octavia A5 2011 actual OBD PIDs

Supported protocol: ISO 14230-4 (KWP 5BAUD).
Default ECU - engine (id#10 in the headers).
Here are some PIDs supported by tested car:

{
    '03': 'Fuel system status',
    '04': 'Calculated engine load',
    '05': 'Engine coolant temperature',
    '06': 'Short term fuel trim—Bank 1',
    '07': 'Long term fuel trim—Bank 1',
    '0B': 'Intake manifold absolute pressure',
    '0C': 'Engine RPM',
    '0D': 'Vehicle speed',
    '0E': 'Timing advance',
    '0F': 'Intake air temperature',
    '11': 'Throttle position',
    '12': 'Commanded secondary air status',
    '13': 'Oxygen sensors present (in 2 banks)',
    '15': 'Oxygen Sensor 2 A: Voltage B: Short term fuel trim',
    '1C': 'OBD standards this vehicle conforms to',
    '20': 'PIDs supported [21 - 40]',
}