15/09/2003 CAN-BUS DATA SYSTEM

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.

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. `

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.

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.

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.

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

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

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

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.

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 2 3 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.

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.

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.

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.

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

TESTING THE CAN CIRCUIT 60.00 Ω 240.00 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 / R1 + 1 / R2 1 / RT = 2 / 120 1 / RT = 0.002 RT = 60 Ω RT = R1+R2 120 + 120 = 240

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:- (2.5 + 3.5) = 3 Volts on the CAN high Processor

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

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

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.

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.

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