CAN Bus Protocol

• Can-Bus is a serial communication protocol developed by Bosch for information exchange between electronic control units in automobiles.
• The term "Can" refers to Controller Area Network.
• In computer science, "Bus" is an element used to transport a large amount of information.
• The system shares information between control units subscribed to it.
• There is a significant reduction in the number of sensors and cables in the electrical installation.
• Functions in the car systems increase where the Can-Bus is employed without raising costs.
• These functions can be distributed among control units.
• Information circulating between the control units through the two cables are packets of 0s and 1s (bits).
• These packets have a limited length and a defined structure of fields that form the message.
• One of these fields acts as an identifier of the type of data that is transported.
• It also identifies the control unit that transmits it, and the priority to transmit it with respect to others.
• The message is not addressed to any specific control unit and each one recognizes whether the message is of interest to it via the identifier.
• All control units can be transmitters and receivers with variable quantities connected to the system within limits.
• A control unit can request certain information from another through one of the message fields if needed.
• Any control unit introduces a message in the bus with the condition that it is free.
• If another unit tries at the same time, the conflict resolves by the identifier of the message.
• The system ensures that the message is transmitted and received correctly.
• When a message presents an error, it is cancelled and re-transmitted correctly.
• A control unit with problems notifies the rest via the message and if the situation is irreversible, it goes out of service but the system continues to function.

Elements of the CAN bus

• Information circulates through two twisted cables connecting the system’s control units.
• Information is transmitted with the tension difference, where a high tension value represents 1 and a low represents 0.
• The combination of ones and zeros makes up the message to transmit.
• The tension values oscillate between 0V and 2.25V in the cable L (Low) and between 2.75V and 5V in the cable H (High).
• If the H line is interrupted or derived to ground, the system works with the Low signal with respect to ground, and vice versa for the L line.
• This allows the system to work with one of the cables cut or communicated to ground, or even with both.
• If both cables are cut, the system will be out of service.
• The braiding between both lines serves to cancel the magnetic fields and should not be modified under any circumstances.
• Resistors are connected to the ends of the cables H and L.
• Their values are acquired empirically and allow adequate functioning of the system to different lengths of cables.
• The resistors are housed inside some of the control units of the system for economic and safety reasons.
• Typically 120 ohms.
• The controller manages communication between the control unit’s microprocessor and the transmitter-receiver and prepares the information between the two components.
• The controller is located in the control unit, so there are as many controllers as there are units connected to the system.
• It works with very low voltage levels and determines the transmission speed of the messages - elevated or reduced depending on the system compromise.
• The Can-Bus line speed for the motor-brakes-automatic transmission is 500 K baud, and for the comfort system is 62.5 K baud.
• The controller also intervenes in the necessary synchronization between the different control units for proper emission and reception of messages.
• The transmitter-receiver receives and transmits the data, and prepares the information for the controllers.
• The preparation consists of situating the tension levels appropriately, amplifying the signal when the information flows into the line, reducing it when it is collected, and supplying it to the controller.
• The transmitter-receiver is situated in each control unit connected to the system, works with intensities close to 0.5 A and does not modify the message contents.
• It is situated between the cables that form the Can-Bus line and the controller.

CAN Bus Functioning

• The control units that connect to the Can-Bus system share information, whether or not they belong to the same system.
• In automotive, the motor, ABS and automatic transmission control units are connected to a line and the control units related to the comfort system are connected to another lower speed line.
• Can-Bus is oriented toward the message and not the recipient.
• The system transmits information in the form of structured messages and part of it is an identifier that indicates the class of data that it contains.
• All control units receive the message, filter it and those that need the data employ it.
• All subscribed control units can introduce and collect messages from the line and can begin to transmit a new message, when the bus is free.
• If one or various units intends to introduce a message at the same time, the one with the higher priority will do so - priority indicated by the identifier.
• In the data transmission process, a control unit receives information from the sensors it has associated.
• Its microprocessor passes the information to the controller where it is managed and prepared to be passed to the transmitter-receiver, where it is transformed into electric signals.
• In data transmission, the controller transfers the data with transmission start to all units and assumes that the message is correctly transmitted to all associated control units.
• To transmit the message, it has to encounter a free bus and if simultaneously, there is a collision with another control unit, the one with more priority will do so.
• The system’s units become receivers.
• Upon receiving the message, all control units verify the identifier to determine whether the message is to be employed by them.
• The ones that need the data process it, and if not, the message is ignored.
• The Can-Bus system detects errors in message transmissions so the receivers check the message analyzing a part of it using the CRC field.
• Mechanisms are applied in the emitting units monitoring the bus-level, the presence of fixed-format fields in the message and statistical analyses of the unit’s faults.
• Error probabilities are very low making it a reliable system.
• The Can-Bus system reduces wiring in the automobile because if a unit has information, it can be employed.
• For example, the motor temperature can be employed without needing each unit to receive information from a said sensor.
• One advantage is that there can be different units and increasing the unit’s functions does not assume additional cost.

Message Format

• The message is composed of strings of 0s and 1s (bits) represented by different tension levels in the Can-Bus cables.
• The message has a number of fields (number of bits) which leads to communication among units depending on the protocol defined by Bosch enabling identification of the unit, the start and end, the data, and different controls.
• The messages are introduced in the lines with cadences oscillating between 7 and 20 milliseconds depending on the speed and the unit introducing them.
• The message starts with a dominant bit for the units to synchronize.
• The 11 bit field is the identification that allows recognition of the message priority, where the lower value for the identifier makes the priority higher.
• Indication of whether the units contain data i.e. remote frame.
• Control bits indicate characteristics of data field, indicates bytes within the field, either “0” for standard or “1” for extended.
• In the data field, the message with the data appears to be introduced. It contains between 0 and 8 bytes.
• The field contains 16 bits
• The CRC has 2 bits, that are transmitted as recessive bits, the transmitting unit then recognises this and interprets if the messaged has generated an error.
• The field signals the end of the message using 7 recessive bits.
• The protocol can experience losing synchronisation.
• The CAN protocol resolves the situation by inserting a bit, if 5 previous are equivalent.
• If the bit needs to be used, it is discarded by a unit that uses it.

BUS Can: Diagnosis

• Security incorporated ensure communication is low, the cable can still present an issue.
• The analysis should present a unit that does not stop the unit working
• Implies that infomration is not being carried between some units.
• For example if a door is not connected, the closing function may not work.
• It is also possible in the busses cable to electronically short circuit and cause issues.
• Falls in the Can bus can mean that consulting the auto diagnosis and system may be challenging.
• To check an alternative is the CANalyser that allows visualsisation of data flow, the content, render errors and statistics.
• Most likely the tool needed is an osciloscape with two channnels to get great visuals of the messages.
• The bits that are added after 5 bytes need to be eliminated.