CAN-BUS & OBD2 Explained

CAN-BUS & OBD2 Explained

Postby Admin » Sun Oct 05, 2014 8:11 pm

There is allot of confusion between OBD2 and the CAN-BUS out on the internet. People confuse the 2 with one another and sometimes they even think their vehicles have either CAN or OBD2 which is not correct at all. And Unfortunately they also communicate it to other people incorrectly.

OBD2 Explained
OBD2 means On board Diagnostics version 2
It's a system designed by automakers to report vehicle issues to the operator or manufacturer.
When something goes wrong the vehicle will show the usual check engine light.

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From Wikipedia http://en.wikipedia.org/wiki/OBD-II_PIDs you can get a list of issues most vehicles are able to report to either the operator or manufacturer.
Some manufacturers extend the standard OBD2 system by adding extra sensors which you will not find in the Wikipedia link above.

The following is where IMO the confusion set in.
Some manufactures like Chrysler also allows Chrysler dealership technicians to access other aspects and features of the vehicle the scariest of them all allow to re-flash the ECU permanently. So this means that the OBD2 port can also be used to access non OBD2 related areas of the vehicle. Communicating with the OBD2 is done using the CAN Standard protocol. Most OBD2 Scanners such as the ELM327 based scanner only allows the user to read trouble codes and reset them which is about the only "Change" you can make.
But if you are able to reverse engineer everything you should also be able to send programmable messages into the CAN-BUS using the ELM327.

This is the best way i'm able to explain it, If you are reading this and you think differently I'm open for correction, but i think i'm pretty close to correct.

CAN-BUS Explained
What is the so called CAN-BUS ?
CAN means Controller Area Network. It allows devices such as electric windows, steering wheel controls, blinkers , lights, speedometer and many other devices aka CAN-Nodes to talk to one another in the vehicle.

Why do manufactures use a CAN-BUS ?
Because it neaten things and make it allot easier to manage. Instead of running hundreds of wires back and forth in the vehicle, the CAN-Node which can have various devices connected to it only needs 3 wires, Power and 2 wires for communication.

Example:
Say the rear lights and other functions (Left/Right Blinkers, Reverse Light, Night time rear lights , Brake Lights, Wiper blades,Wiper water pump, open door sensors, Number plate lights ect ect ) are all on the same CAN-Node.
So instead of running wires to the back of the vehicle for each sensor/light/pump the manufacturer only needs to lay down 1 cable for power and 2 very thin cables for communication to the actual node, From the node a couple of wires run to each sensor/light/pump.

How do CAN-Nodes Communicate?
Each node in the CAN Network has an ID, Each ID supports multiple instructions.
When the operator of the vehicle had to step on the brakes, the CAN-Node on the brakes will send a message in CAN format to all of the nodes in the vehicle.
If the CAN-Node where the Rear Brake Lights are connected to, Sees a massage with its own ID it will react accordingly.

CAN Message Example.
There are usually 9-BITS in a CAN Message
It contains a CAN-ID which is the ID of the message and the rest of the other 8-Bits just forms part of a message.

CAN Message Example.
Lets say the rear CAN-Node's ID as explained before is 1431.
When the operator of the vehicle steps on the brakes the Brake sensor will tell its Node to send the following message.
1431,22,00,00,00,00,00,00,00 Then when the operator of the vehicle turns on the radio it send a message like this 436,00,00,00,00,00,00,00,00

The node connected to the rear lights will see both messages 1431,22,00,00,00,00,00,00,00 ,but since its id is 1431 is will accept the message otherwise ignore it.
22 after 1421 might mean turn on brake lights and 20 might mean turn on rear wiper blades.

Lay-mans Explanation.
There a 3 Teams, Team1 ,Team2 & Team3
In Team1 we have John, Peter & James
In Team2 we have Steve, Shaun & John (This is not the same John its another John)
In Team3 we have Bob.

Bob Shouts to all the teams "Team1- Turn on Rear Lights."
Team1 knows its responsible for rear lights and a few other things, Peter in Team1 knows he is responsible for the rear lights and then reacts accordingly.
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