CAN-Multyplexing Worksheet

We've got Land Rover 2003 model which is on the photo. This twisted pair connected to the ABS connector is CAN system.
.

Have chosen 50us per division on the time axis scale and 2v per division on the voltage axis scale we captured this pattern:

  Here there are channel A which is L-CAN (yellow/black wire) and channel B which is H-CAN (yellow/brown wire).
Thanks to using double channel oscilloscope we can watch how high and low data signals oppose each other. Also, it's easier to justify that there is no aliasing affect in this case. Aliasing as event is situation when two signals with different frequency distort each other like one signal pattern lies above the other signal pattern. Some time it happens with signals which frequencies are as much as twice different. For instance the signal of 7 KHZ can be repeated in form and shape at the frequency of 14 KHZ. If equipment we use for sampling interpolates these two signals we can get wrong reading as some sort of combination. Good measuring equipment has special filters to avoid this misreading. Visual affect for digital signals aliasing can be understood easier on the example of digital picture pixels which are diffused slightly. That's why digital electronics tends to use higher frequency for higher definition.
For Ch. A main voltage in the line is 2.8 V and "talking” voltage is 3.6 V
For Ch. B main voltage in the line is 2.8 V and voltage to "talk" is 1.6 V

Below are photos of our attempts to get reading from the voltmeter in DC mode:

As these picture shows in comparison with oscilloscope reading above there is no sense to perform these measurements. They do not provide any useful information about modules and devices communication.

Scan Tool reading photo and video below express much more useful information reading. Video illustrates how by using “AutoBoss” scan tool we can simulate signal in to operate with certain device via CAN system. And once again, this process clarify that architecture of the system is divided onto the inputs, communication, and outputs. Eliminating any stages from this we can localise fault faster and more efficiently.
"As invaders" we got into the actuators department of “AutoBoss” scan tool, then chose a windscreen wiper and activated it:

Even though on the first photo we can see only on-board equipment available for scanning, the opportunity to test different CAN systems such as engine CAN system, ABS CAN system, VSA CAN system, suspension CAN system e. t. c. still exist. Eventually how far you can go into the system depends on the software of the scan tool. We didn't show others experiments video due to time frame during practical session although we activated also headlights, washer motor and windows....
The systems listened above are different in terms of the speed of the particular system. This is because the hierarchy of the vehicle modules. Engine, ABS and Stability control systems, Transmission system, SRS system have 128-1024 Kb per sec. Infotament system is slower and can be about 12b Kb per sec. Body system can be even slower, up to 32 Kb per sec. Manufacturers produce higher speed systems for most important from safety point of view systems and slow speed systems for the systems with less priority. For information exchange purposes a gateway is used between the systems. This gateway allows interfacing of different speed systems.

 

These two diagrams above Range Rover rated as fast CAN Bus system. And thes two diagram below company named as K Bus, I Bus with low speed:

 

There is Honda Accord Multiplex system wiring diagram below:

G- is Gateway which allows to interface modules with different speeds without aliasing and signals distortion. This is very important for data transferring.

Having been experimented on the CAN system we did not put it into the sleep mode because of connection of scan tool. However, when everything is deactivated, CAN system switches onto the sleep mode for the sake of battery energy saving. Otherwise it would draw battery current relatively fast and would bring problems with engine start. Normal figures for the current in sleep mode are about 30 mA depending on the manufacturer. Keeping in mind this it is worthwhile to check the vehicle on the subject of parasitic draw if fast battery discharge is suspected.  

Voltage changes while the system "awake" onto the base voltage. If it is high- CAN voltage rises, if it is low-CAN voltage decreases. Check the manufacturer spec for each particular case.

Some devices such as door module control must be in standby mode all the time to provide key entry or keyless access depending on the construction. Modules talk to each other before going to sleep confirming readiness for mode change. Also modules awake each other.

The diagnostics for the CAN system bases on the principle of separating inputs, communication, and outputs. Scan tool simply simulates inputs and we control if the communication takes place and the outputs activate controlled devices. All these experiments refer to this main idea.

The last video shows how the Controller Aria Network "goes to sleep". Creative guys deactivated bonnet switch and then recorded this signal from the ABS connector while CAN was given command to sleep. This is very informative video where you can see modules talking and after the command data transferring stopped.Impressive video.