A Controller Area Network CAN bus is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications without a host computer. It is a message-based protocol , designed originally for multiplex electrical wiring within automobiles to save on copper, but can also be used in many other contexts. For each device the data in a frame is transmitted sequentially but in such a way that if more than one device transmits at the same time the highest priority device is able to continue while the others back off. Frames are received by all devices, including by the transmitting device.
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In the reviewed and restructured ISO ? In addition to the high-speed CAN, the development of the low-speed CAN, which was originally covered by ISO , gained new means such as fault tolerant behaviour. The subject of this part of ISO is the definition and description of requirements necessary to obtain a fault tolerant behaviour as well as the specification of fault tolerance itself.
In particular, it describes the medium dependent interface and parts of the medium access control. The CAN is a serial communication protocol which supports distributed control and multiplexing. The following parts of the physical layer are covered by this part of ISO ? In addition, parts of the physical layer signalling PLS and parts of the medium access control MAC are also affected by the definitions provided by this part of ISO During simultaneous transmission of dominant and recessive bits, the resulting bus value will be dominant.
Only a few influences to the data link layer are given. The overall termination resistance of each line should be greater than or equal to ?. Figure 2 illustrates the normal termination of a designated bus node. Figure 2 — Termination of a single bus node In Figure 2, the termination resistors are denoted as optional.
That means that under certain conditions not all nodes need an individual termination, if the requirements of proper overall termination are fulfilled. It consists of a pair of twisted copper cables which are connected in a topology described in 5. The following basic model shown in Figure 3 and 4 is used for the calculations. Key a b Driver. Figure 3 — Substitute circuit for bus line? Figure 4 — Operating capacitance referring to network length l The operating capacitance is calculated using Equation 1.
Vth is assumed to 0,2 V. No groundshift is assumed. The total internal loop delay is assumed to 1,5? To distinguish between both states a differential voltage V? In general, this leads to a negative differential voltage Vdiff? The recessive state is transmitted during bus idle or during recessive bits.
The dominant state is represented by a positive differential voltage Vdiff? The dominant state overrides a recessive state and is transmitted during dominant bits. Key a b Recessive. V 40,0 40,0 58,0 58,0 12 V system 42 V system No destruction of transceiver occurs.
The transceiver should not affect communication on the net. The voltage levels may be applied without time restrictions. The common mode voltage, VCOM, for an undisturbed system in normal mode must be ensured within the ratings specified in Table 2.
VCC nominal — — — max. The differential voltage is determined by the input load of all ECUs during the recessive state. Therefore, Vdiff decreases slightly as the number of ECUs connected to the bus increases. Key 1 2 node 1 node 2 Figure 7 — Connecting model; bus structure with stub lines However, for any connecting concept, the following requirements shall be fulfilled, in order to provide the fault tolerant means:?
The overall network termination resistor shall be in a range of about ? For a detailed description of the termination concept please refer to 5. The actual number of nodes varies due to communication speed, capacitive network load, overall line length, network termination concept, etc.
However, it is possible to increase the overall network length by reducing the actual communication speed. Even some connecting lines star connector to node might be extended to several meters; no stub lines are recommended. Both the overall network length all star connection line lengths added and the maximum node to node distance affect the network communication. The star point connection method is with two star points. The network is terminated with an overall resistance of ?.
The node number is about The overall network length is about 40 m. The maximum node to node distance is 20 m. The dominant bus level overrides actively this recessive bus state. The transition between the dominant to recessive level is done by the termination, too. However, there is no designated termination network or circuit. Moreover, the termination is attached to most of the participating nodes.
Due to the failure management described in 7. By connecting the termination pins, the following requirements shall be considered:? The overall network termination resistor of one line all parallel resistors connected to RTL or RTH pins shall be about ? A single resistor connected to an individual transceiver device should not be below ? It is recommended that every node provide its own termination resistors. However, this is not a strict requirement.
A not-well-terminated node might be sensitive to false wake-up signals if a broken line error had occurred. They can be divided into two major groups:? In general, the detection of failure events causes the transceiver device to perform an internal state switch.
Hence, a reduced set of failure events is specified see Table 9. The remaining nodes continue communicating at least with reduced signal to noise ratio. All nodes continue communicating at least with reduced signal to noise ratio. No operation within the complete system. Nodes within the remaining subsystems might continue communicating. Due to the distributed termination concept, these failures do not affect the remaining communication and are not detectable by a transceiver device.
Hence, they are not treated and are not part of this part of ISO Both failures are treated together as power failures. These failures do not cause any corrective action within the transceiver and are tolerated implicitly. Hence, a transceiver device shall fulfil given constraints under all possible failure conditions.
Figure 9 — Timing example, differential operation without GND shift 7. Key a Failure generation. The test circuit allows applying different failure cases in combination with a local GND shift in positive and negative direction. The wiring harness between the nodes shall stay as short as possible and shall not exceed 1 m in total. Depending on the applied failure case, the transceiver operates in three main states:?
Destination node. Ground shift. Bus failure. Bus load. The sample rate of digital timer implementations shall be faster than 4? Analogue failure detection and recovery timer implementations shall react upon consecutive input conditions only.
A transceiver device should react in such a way to fulfil the requirements of the operating modes in 7.