1. 15/09/2003
CAN-BUS
DATA SYSTEM

2. What is CAN?
CAN is short for Controller Area Network
First developed in 1984.
It is a network connecting the controllers on a machine e.g. tractor or combine, which allows information to be shared among the controllers.

3. Multiple use of signals,therefore there are less sensors.
Benefits of CAN:
Reduce the number of wires (quantity) and over all length of wiring harness.
To ensure all signal information is available to all controllers on the network (sharing).
Less wiring, less connectors, gives increased reliability and durability.
Multiple use of signals,therefore there are less sensors.
Better and more exact diagnostic possibilities.
1/ 25 to 30% of Vehicle costs are electric and electronic.
2/ Before CAN BUS a typical wiring loom could be about five and a half miles long, with the introduction of CAN BUS this could be reduced to about 3 miles.
3/ Improved EMC and therefore less trouble prone.
4/ Improved diagnostics with the use of Electronic service tools. `

4. ECU Discreet Wiring Processor CAN Stubs CAN Hi CAN Backbone CAN Lo
Sensors
ECU
Discreet Wiring
Processor
CAN Stubs
CAN Lo
CAN Hi
CAN Backbone
Terminators
1/ Maximum length of the CAN backbone is 40 metres
2/ Maximum length of CAN stubs is 1 metre
3/ Processors should be a minimum of 0.1 metres apart, and they should not be equally spaced along the CAN. The length of each leg should also be different. The reason for this is reflection, and signal propagation.
4/ The CAN HI is Yellow
The CAN Lo is Green
5/ The terminators are 120 Ohm resistors.
6/ Processors are positioned, near to the majority of there sensors, ie:- (engine contol processor situated in engine compartment).this results in less cabling being used, less plug connections, reducing the number of error sources.

5. Magnetic Field Around a Wire Carrying Current
1/ With electrical current flowing in a wire there is a magnetic field around it.
2/ In CAN Hi and Can Lo , current is flowing in the opposite direction, the magnetic fields cancel each other out, causing a vacumn around the wires. This prevents any unwanted noise (elecro magetic), getting off the BUS, but more importantly it stops any signals from an external source reaching the signal wires.

6. Wire Types Signal Wires 4 Wire Shielded Signal Wires 2 Wire Unshielded
1/ Twisted pair is used on most CAN systems, by twisting the wire you increase the magnetic field around the wires. ie:- 1.2 metres of wire is only 1 metre in length.
2/ Four wire shielded is used on some products, 12 volts is supplied on the red wire and ground on the black, they are dead headed at the processors, and at the terminator. This voltage, provides extra shielding around the signal wires.
Two wire shielded is 99.2% as efficient as four wire shielded.

7. Discreet Wiring (without C.A.N)
Flywheel sensor
Crankshaft sensor
Boost Pressure
Engine
Air Inlet Temp
Exhaust Temp
Coolant Temp
Engine Controller / Processor
Synchro Pots
Solenoids
Instrument Cluster Processor
Pressure switches
Transmission
Speed Sensors
Oil Temp
Clutch Pot
Transmission &
Hydraulic Controller
/ Processor
1/ Analogue systems consist of processors, hard wired to various sensors etc, around the vehicle, a sensor may send a signal to more than one processor, but through different wires.
2/ Problems with hard wiring:-
Weight
Cost
Reliability
Complexity
Connections
Size of the loom
Not compatible between manufacturers.
Crosshaft Pot
Load Sensing Pins
Drop Rate Pot
Hydraulics
Sensitivity Pot
Fender Switches
Solenoid

8. Discreet Wiring with C.A.N.
Flywheel sensor
Crankshaft sensor
Boost Pressure
Engine
Air Inlet Temp
Exhaust Temp
Coolant Temp
Engine Controller / Processor
Synchro Pots
Solenoids
Instrument Cluster Processor
Pressure switches
Transmission
Speed Sensors
Oil Temp
Clutch Pot
Transmission &
Hydraulic Controller
/ Processor
1/ With CAN bus the number of discreet wires has decreased now information is received by one processor / controller and it then shares the information with all the other processors / controllers on the bus.
Crosshaft Pot
Load Sensing Pins
Drop Rate Pot
Hydraulics
Sensitivity Pot
Fender Switches
Solenoid

9. CAN System Function Processor Processor Processor Can Lo Can Hi
Sensors
Solenoid
Processor
Processor
Can Lo
Can Hi
1/ Sensor sends a signal via hard wiring down to the processor.
2/ Processor receives this information, it can use this to send signals out to other sensors, switches, solenoids,etc,which are hard wired, directly to it. It can also transmit this information down onto the CAN Network.
3/ All the other processors which are attached to the CAN network, can see this information, if they do not need this signal they can disregard it, if they require it they take the signal in and use it.
4/ The processor that has taken in the information, sends signals out to sensors, switches, potentiometers, which are directly attached to it.
Processor
Sensors

10. Printed circuit Boards, fibre optics, flat cable, standard cable
Overview of CAN System
Digital
Printed circuit Boards, fibre optics, flat cable, standard cable
CAN
Network
CAN
Network
Signal
Digital
Message
Switch
Processor
Digital
Message
Signal
Processor
Processor
End user
Signal
Digital
Message
Digital
Message
Sensor
Processor
Processor
Analogue
Outputs
Analogue
Inputs
1/ Inputs sent to processor down discreet wires (hard wiring), from switches, solenoids, and sensors, as a voltage.
2/ Voltage signal goes into processor, it is then processed internally, from an analogue to a digital signal, a data signal (message) is then created within the processor from this information.
3/ The data signal is then placed onto the CAN network, every processor that is connected to the CAN network can read and process the information from the data signal as required.

11. CAN HI CAN LO 1 1 t t t i i i t i i t t i i t t B B B i B B B B B 1 23 4 5 6 7 8 1
CAN HI t t i t i i t i t i t i i t t i B B B B B B B B 1 2 3 4 5 6 7 8
CAN LO 1
1/ Data is transmitted simultaneously down both line Can Hi and Lo). Each signal being a mirror, image of the other. ie:- CAN Hi is at 3.5 Volts
CAN Lo is at 1.5 Volts
Differential of 1 volt in each case.
2/ Processor has to see a differential of 0.8 volts before, it changes from 0 to 1. Electricity travels at 1/3 of the speed of light. It takes 5 nano seconds for electricity to travel down 1 metre of copper cable.
3/ One bus line is theoretically enough for BUS systems with a low clock speed. High speed BUS systems must always have two lines as the voltage level will otherwise be to low. Errors are also easily recognisable with two signals.
4/ A maximum of 160 bits are used in a single data telegram.
BUS speed is 250,000 BITs per second
250,000 / 160 = 1562 Telegrams per second.

12. Signal Transmission Breakdown
MESSAGE
CAN Hi
CAN Lo
SOF
EOF
29 BIT IDENTIFIER
8 BYTES DATA
CRC
1/ SOF, Start of frame tells the processors that this the start of a new frame.
2/ The 29 BIT identifier is broken down into different areas, its priority, where the signal has come from, where it is going to.
3/ The 8 Bytes of data, or information, 8BITs per BYTE 8 x 8 = 64 BITs
4/ CRC , Cyclic redundancy check checks and verifies data against EMC interference and corruption,ie:- If a voltage peak only occurs on one line (eg:emc problems) then the receivers can recognise this as an error and ignore the voltage peak.
5/ EOF, End Of Frame tells the processors that this is the end of this particular transmission.
6/ Data telegrams can only be transmitted when the CAN data Bus is free, There has to be a three bit space, left between messages.

13. CAN Controller Area Network ECU Electronic Control Module
Terminology:
CAN Controller Area Network
ECU Electronic Control Module
BUS The network i.e. the connection between the controllers (computing terminology)
CAN_HI Signal carrying wire
CAN_LO Signal carrying wire.
Terminator Electrical resistor placed at either end of a network.
Discreet wire A normal electrical wire to/from a sensor or solenoid to a controller
1/ International Specification requires that the main CAN is Yellow and Green.
CAN High = Yellow
CAN Low = Green
Other CAN systems on the same vehicle may be different colours.
2/ The terminator can be placed internally inside a processor.

14. Testing a C.A.N.- bus circuit
1/ International Specification requires that the main CAN is Yellow and Green.
CAN High = Yellow
CAN Low = Green
Other CAN systems on the same vehicle may be different colours.
2/ The terminator can be placed internally inside a processor.

15. Testing the Can Circuit
Processor
Processor
60 Ω
Can Lo
Can Hi
1/ With all the processors disconnected, except the terminators, the resistance across the CAN circuit, from Can Hi to CAN Lo should be approximately 60 Ohms, this can be tested anywhere on the CAN circuit.
Processor

16. TESTING THE CAN CIRCUIT
60.00 Ω Ω
1/ With resistors connected in parallel, there is more than one path for the current to flow. The resistors are side by side, and provide separate routes for current.
2/ The following formula for x resistors in parallel
1 / RT = 1/ R1 + 1/ R2……+1/ Rx
Where RT = the resistance total.
1 / RT = 1 / R / R2
1 / RT = 2 / 120
1 / RT =
RT = Ω
RT = R1+R2
= 240

17. Testing the Can Circuit
Testing CAN Hi +Lo
2.0 Volts
CAN Lo
3.0 Volts
CAN Hi
Processor
Processor
Can Hi
Can Lo
1/ To test the can circuit is receiving information from the processors, it is possible to test each of the Can lines.
2/ The CAN Hi circuit should be approximately 3 Volts
The CAN Lo circuit should be approximately 2 Volts
3/ Recessive Dominant
CAN Hi 2.5v 3.5v
CAN Lo 2.5v 1.5v
When processors become dominant, they put 3.5 volts out onto the Can Hi and 1.5 Volts out onto the Can Lo.
These are fluctuating up and down from 2.5 to 3.5 on the Can Hi and 2.5 to 1.5 on the Can Lo to make up the binary signal. The transmission speed of high speed CAN BUS is 250,000 BITs per second. Our Multimeter reading is an average of these values.
Ie:- ( ) = Volts on the CAN high
Processor

18. Testing the Can Circuit
Short to Battery Voltage
12 Volts
Processor
12V
12 Volts
Can Hi
Can Lo
1/ If there is a short from a 12v source, the processors cannot send information down onto the CAN circuit.
2/ Using a multimeter, check the voltage from the circuit to ground
3/ This needs to be carried out for both the CAN Hi and CAN Lo.
4/ If data line voltage is at battery voltage, data can no longer be transmitted, if such an error is found on a data line, localise the error by opening the circuit at intermediate connectors and repeating the measurements.
5/ A line short may occur within a controller itself. Disconnect controllers individually and check network functionality and voltage.
Processor

19. Testing the Can Circuit
Short to Earth
Continuity
0.00 Ω
Processor
12V
0.00 Ω
Can Lo
Can Hi
1/ If there is a short to ground, the processors cannot send information down onto the CAN circuit.
2/ Using a multimeter, check resistance from the circuit to ground
3/ This needs to be carried out for both the CAN Hi and CAN Lo.
4/ If data line voltage is shorted to ground, data can no longer be transmitted, if such an error is found on a data line, localise the error by opening the circuit at intermediate connectors and repeating the measurements.
5/ A line short may occur within a controller itself. Disconnect controllers individually and check network functionality and voltage.
Processor

20. Testing the Can Circuit
Open Circuit
Processor
Processor
Can Hi
Break in CAN
Can Lo
Processor
1/ If there is a break in the backbone of the CAN network, the processors cannot send or receive information down the circuit. In this case, some, or all of the processors may go off line.
2/ Disconnect all of the processors from the CAN network
3/ Using a multimeter, check resistance from either end of the circuit.
4/ This needs to be carried out for both the CAN Hi and CAN Lo.
5/ If the data line has an open circuit, data can no longer be transmitted, if such an error is found on a data line, localise the error by opening the circuit at intermediate connectors and repeating the measurements.

21. Testing the Can Circuit
Open Circuit
Processor
Processor
Break in CAN
Can Hi
Can Lo
Processor
1/ If there is a break in the stub of the CAN network, the processor attached directly to it cannot send, or receive information from the circuit. In this case, the individual processor may have gone off line
2/ Disconnect all of the processors from the CAN network
3/ Using a multimeter, check resistance from the end of the CAN circuit, to the end of the leg.
4/ This needs to be carried out for both the CAN Hi and CAN Lo.
5/ If one of the data lines has an open circuit, data can no longer be transmitted, if such an error is found on a data line, localise the error by opening the circuit at intermediate connectors and repeating then measurements.

22. Testing the Can Circuit
18k Ω
12k Ω
60 Ω
120 Ω
Continuity
36 kΩ
36k Ω
0.00 Ω
Processor
Processor
1/ Disconnect all processors from the machine.
2/ Using a multimeter to check the impedance of the network, and resistance of the processors when disconnected from the network.
3/ Plug a multimeter into the diagnostic connector, pins D+E
3/ Plug the processors in one at a time, and measure the resistance change.
Processor