TTEC 4825 Can System on board N1.

            
                                                       Can System

The CAN bus is a high reliability, high data rate bus for communicating

between electronic devices in situations where high data reliability is

 required. One use of the CAN bus is in the automotive applications where

the CAN bus is being used as a substitute for copper wiring looms in cars.

 In this article we take a look at what the CAN bus is. 

                                          Can System on Board

                             Signal from CAN System without fault and load

                                                          Right indicator

                                                      Left indicator

                                                      Rear wiper

                                                         Stop light

                                                       Fuel pump

                                                    Reverse light

                Every pattern is individual for every load, such: rear light, wipers,
                        windows, sensors, ABS, transmission, head lights etc.
               Both channels should mirror each other as shown in a picture above.

                          CAN L is being pulled down from about 2.5V
                           CAN H is being pulled up from about 2.5V
                  

     This board shows a typical CAN node on a network. All nodes have a

     microcontroller with I/O circuitry, have a CAN controller and a line

     driver which interfaces the CAN L and CAN H differential

     connections to the CAN controller. Some microcontrollers

     have the CAN controller embedded which reduces the cost of the

     node. WE can see that the connections to the CAN node are

     power, ground, CAN H, CAN L and then other connections to I/O

     as required CAN nodes in a car. In this context where

     a node is a complete functioning unit the node is often referred

     to as an Electronic Control Unit or ECU. Here you can see five ECUs:

     an engine temperature sensor, an instrument panel, a switch on a

     brake pedal and ECUs for the left and right hand rear light clusters.

     Although the wires are not shown here all power and grounds are

     connected, and all CAN H and CAN L terminals are connected

     by 100ohm terminated twisted pair wires. In practice you may

     find the foot brake connected to the instrument panel ECU etc.

     A key feature of CAN is reliability, and this is kept to a maximum

     by keeping traffic on the CAN bus to a minimum. In a conventional

     network you might think that the foot pedal would tell the central

     processor on the instrument panel that it has been pushed down,

     and the instrument panel would then tell the light cluster ECUs to

     turn the brake lights on. CAN works differently: When the pedal is

     depressed the brake pedal ECU issues a message effectively stating

     “brake pedal pressed”. This message is issued to the whole bus. The  

     ight cluster ECUs are programmed so that when they see the

     “brake pedal pressed” message on the CAN bus they power up the

     appropriate lamp. This has kept message flow to a minimum, and if

     the instrument panel ECU—or any other ECU—is not working then

     the core important functions of the network are still active. This is an

     example of one type of data exchange for important ‘mission critical’

     data: if your brake lights don’t come on then you could run into trouble,

     or it would run into you. However if this method was employed by all

     devices connected to the bus then the traffic would be quite large—with

     more traffic meaning reduced reliability. So a second technique for data

     exchange is used. Looking at temperature monitoring:  the central

     instrument cluster wants to know what the temperature of the block

     is so that temperature can be displayed on the instrument console

     and—if necessary—the warning light activated. The designers of the

     system will have decided that the temperature needs to be monitored

     at, say, 5 second intervals. So every 5 seconds the central console

     will issue a message saying “can anyone tell me what the block

     temperature is?” The ECU on the block is programmed to look for

     the message “can anyone tell me what the block temperature is?”,

     to then measure the temperature, and reply with a message stating

     “the block temperature is” followed by the temperature data.
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