Bus system
The OBD-II communication takes place via buses of different types.
It is often to read "the various OBD-II protocols" or "possible OBD-II protocols". Personally, I do not like the expression protocol in this context, because it is too general for me. The OBD data or its structure is nearly always the same, only the bus system on which the OBD data is transported changes. That's why I'm talking about bus systems when it comes to the different transmission media.
However, I would like to make an exception: For the SAE J1939 the OBD-II data is exchanged in a very peculiar way, e.g. Some values need not be requested at all, but are always available on the bus. J1939 is based on the CAN bus with a 29-bit identifier. I will not go into J1939, because it is mainly used in the heavy commercial vehicle sector and is repressed in conjunction with OBD-II by the WWH-OBD.
There are various bus systems to transfer OBD data. Each of these bus systems then also has different characteristics.
| Bus system | Characteristics | Comment |
|---|---|---|
| J1850 | PWM | The J1850 PWM bus requires 2 lines (e.g Ford until about 2006) |
| VPW | The J1850 VPW bus requires 1 lines (e.g. old Chrysler Voyager) | |
| - | ||
| K-Line / L-Line | ISO 9141-2 | Initialization on K- and L-Line, communication on K-Line (e.g., Asian and European vehicles) |
| ISO 14230 5Baud | Slow initialization with 5 bits per second, only K-Line in use (e.g., Asian and European vehicles) | |
| ISO 14230 Fast | Fast initialization, K-Line only in use (e.g., Asian and European vehicles) | |
| - | ||
| CAN bus | CAN 11-bit Identifier 250 kbit/s | New vehicles from approx. 2007 |
| CAN 11-bit Identifier 500 kbit/s | New vehicles from approx. 2007 | |
| CAN 29-bit Identifier 250 kbit/s | New vehicles from approx. 2007 | |
| CAN 29-bit Identifier 500 kbit/s | New vehicles from approx. 2007 |
In the case of a connection setup via OBD-II, the entire bus systems including characteristics are tested, in the hope the OBD-II capable control units respond. (All control units with OBD-II installed in the vehicle must use the same characteristic of the bus system.)
This sampling (In tools often referred as "automatic".) can take a relatively long time. Therefore, many tools also offer the possibility to select a bus system (There is unfortunately also called protocol). Then only on this bus system is an attempt to establish a connection. This can be particularly helpful if OBD-II diagnostics are carried out very often with the same vehicle.
Example "Bus system (protocol)" selection menu
With most tools there is somewhere an entry, which bus system is used for OBD-II. This information can be obtained e.g. After the "automatic" to find out which bus system the vehicle is using for OBD-II.
