The communication protocol is divided in two parts :
 | The first one is the communication initialization : we have to send the address of the control module we want to reach (engine, ABS, airbag, ...). |
| The second one is a data exchange between the PC and the control module we choose.. |
In the car, you will find several control modules linked to the ECU :
| Engine |
| ABS |
| Central locks |
| Auto transmission |
| A/C |
| ETC ...depending on the car's options |
During the com init, a PC will send a control module address trough its serial port with the help of an electrical adapter (RS232 <=> OBDII). The Tx signal (+/-10V) will become the K signal (0/12V). This 7 bits address is sent at 5 bits per second and will wake up the ECU.
When the ECU understands this message, it answers at 9600 bps (8 bits/no parity/1 stop bit) the three following bytes (hexa) :
| 55 01 8A .... 55 01 8A ... 55 01 8A ... (several times) |
The PC has to answer :
| 75 to confirm the correct reception (FF- last byte 8A) |
Then, block data exchanges can occur (see "Initialization")