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CAN is a multi-master serial bus standard for connecting electronic control units ECUs also known as nodes automotive electronics is a major application domain. Two or more nodes are required on the CAN network to communicate. A node may interface to devices from simple digital logic e.
Such a computer may also be a gateway allowing a general purpose computer like a laptop to communicate over a USB or Ethernet port to the devices on a CAN network. All nodes are connected to each other through a physically conventional two wire bus. This bus uses differential wired-AND signals. A 0 data bit encodes a dominant state, while a 1 data bit encodes a recessive state, supporting a wired-AND convention, which gives nodes with lower ID numbers priority on the bus.
Receivers consider any differential voltage of less than 0. The dominant differential voltage is a nominal 2 V. With both high-speed and low-speed CAN, the speed of the transition is faster when a recessive to dominant transition occurs since the CAN wires are being actively driven. The speed of the dominant to recessive transition depends primarily on the length of the CAN network and the capacitance of the wire used.
High-speed CAN is usually used in automotive and industrial applications where the bus runs from one end of the environment to the other. Fault-tolerant CAN is often used where groups of nodes need to be connected together. The specifications require the bus be kept within a minimum and maximum common mode bus voltage, but do not define how to keep the bus within this range.
The CAN bus must be terminated. The termination resistors are needed to suppress reflections as well as return the bus to its recessive or idle state. Low-speed CAN uses resistors at each node. A terminating bias circuit provides power and ground in addition to the CAN signaling on a four-wire cable.
This provides automatic electrical bias and termination at each end of each bus segment. Each node is able to send and receive messages, but not simultaneously. A message or Frame consists primarily of the ID identifier , which represents the priority of the message, and up to eight data bytes.
The message is transmitted serially onto the bus using a non-return-to-zero NRZ format and may be received by all nodes. The devices that are connected by a CAN network are typically sensors , actuators , and other control devices.
CAN data transmission uses a lossless bitwise arbitration method of contention resolution. This arbitration method requires all nodes on the CAN network to be synchronized to sample every bit on the CAN network at the same time.
This is why some call CAN synchronous. Unfortunately the term synchronous is imprecise since the data is transmitted in an asynchronous format, namely without a clock signal. The CAN specifications use the terms "dominant" bits and "recessive" bits, where dominant is a logical 0 actively driven to a voltage by the transmitter and recessive is a logical 1 passively returned to a voltage by a resistor. The idle state is represented by the recessive level Logical 1. If one node transmits a dominant bit and another node transmits a recessive bit then there is a collision and the dominant bit "wins".
This means there is no delay to the higher-priority message, and the node transmitting the lower priority message automatically attempts to re-transmit six bit clocks after the end of the dominant message. This makes CAN very suitable as a real-time prioritized communications system. The exact voltages for a logical 0 or 1 depend on the physical layer used, but the basic principle of CAN requires that each node listen to the data on the CAN network including the transmitting node s itself themselves.
If a logical 1 is transmitted by all transmitting nodes at the same time, then a logical 1 is seen by all of the nodes, including both the transmitting node s and receiving node s. If a logical 0 is transmitted by all transmitting node s at the same time, then a logical 0 is seen by all nodes.
If a logical 0 is being transmitted by one or more nodes, and a logical 1 is being transmitted by one or more nodes, then a logical 0 is seen by all nodes including the node s transmitting the logical 1. When a node transmits a logical 1 but sees a logical 0, it realizes that there is a contention and it quits transmitting. By using this process, any node that transmits a logical 1 when another node transmits a logical 0 "drops out" or loses the arbitration.
A node that loses arbitration re-queues its message for later transmission and the CAN frame bit-stream continues without error until only one node is left transmitting. This means that the node that transmits the first 1 loses arbitration. Since the 11 or 29 for CAN 2. For example, consider an bit ID CAN network, with two nodes with IDs of 15 binary representation, and 16 binary representation, If these two nodes transmit at the same time, each will first transmit the start bit then transmit the first six zeros of their ID with no arbitration decision being made.
When this happens, the node with the ID of 16 knows it transmitted a 1, but sees a 0 and realizes that there is a collision and it lost arbitration. Node 16 stops transmitting which allows the node with ID of 15 to continue its transmission without any loss of data. The node with the lowest ID will always win the arbitration, and therefore has the highest priority.
Decreasing the bit rate allows longer network distances e. The improved CAN FD standard allows increasing the bit rate after arbitration and can increase the speed of the data section by a factor of up to ten or more of the arbitration bit rate.
Message IDs must be unique  on a single CAN bus, otherwise two nodes would continue transmission beyond the end of the arbitration field ID causing an error. In the early s, the choice of IDs for messages was done simply on the basis of identifying the type of data and the sending node; however, as the ID is also used as the message priority, this led to poor real-time performance.
All nodes on the CAN network must operate at the same nominal bit rate, but noise, phase shifts, oscillator tolerance and oscillator drift mean that the actual bit rate might not be the nominal bit rate.
Synchronization is important during arbitration since the nodes in arbitration must be able to see both their transmitted data and the other nodes' transmitted data at the same time. Synchronization is also important to ensure that variations in oscillator timing between nodes do not cause errors. Synchronization starts with a hard synchronization on the first recessive to dominant transition after a period of bus idle the start bit.
Resynchronization occurs on every recessive to dominant transition during the frame. The CAN controller expects the transition to occur at a multiple of the nominal bit time. If the transition does not occur at the exact time the controller expects it, the controller adjusts the nominal bit time accordingly.
The adjustment is accomplished by dividing each bit into a number of time slices called quanta, and assigning some number of quanta to each of the four segments within the bit: synchronization, propagation, phase segment 1 and phase segment 2.
The number of quanta the bit is divided into can vary by controller, and the number of quanta assigned to each segment can be varied depending on bit rate and network conditions.
A transition that occurs before or after it is expected causes the controller to calculate the time difference and lengthen phase segment 1 or shorten phase segment 2 by this time. This effectively adjusts the timing of the receiver to the transmitter to synchronize them. This resynchronization process is done continuously at every recessive to dominant transition to ensure the transmitter and receiver stay in sync. Continuously resynchronizing reduces errors induced by noise, and allows a receiving node that was synchronized to a node which lost arbitration to resynchronize to the node which won arbitration.
The CAN protocol, like many networking protocols, can be decomposed into the following abstraction layers :. Most of the CAN standard applies to the transfer layer. The transfer layer receives messages from the physical layer and transmits those messages to the object layer. The transfer layer is responsible for bit timing and synchronization, message framing, arbitration, acknowledgement, error detection and signaling, and fault confinement.
It performs:. CAN bus ISO originally specified the link layer protocol with only abstract requirements for the physical layer, e. The electrical aspects of the physical layer voltage, current, number of conductors were specified in ISO , which is now widely accepted. However, the mechanical aspects of the physical layer connector type and number, colors, labels, pin-outs have yet to be formally specified. As a result, an automotive ECU will typically have a particular—often custom—connector with various sorts of cables, of which two are the CAN bus lines.
Nonetheless, several de facto standards for mechanical implementation have emerged, the most common being the 9-pin D-sub type male connector with the following pin-out:. This de facto mechanical standard for CAN could be implemented with the node having both male and female 9-pin D-sub connectors electrically wired to each other in parallel within the node. Bus power is fed to a node's male connector and the bus draws power from the node's female connector. This follows the electrical engineering convention that power sources are terminated at female connectors.
Adoption of this standard avoids the need to fabricate custom splitters to connect two sets of bus wires to a single D connector at each node.
Such nonstandard custom wire harnesses splitters that join conductors outside the node reduce bus reliability, eliminate cable interchangeability, reduce compatibility of wiring harnesses, and increase cost. The absence of a complete physical layer specification mechanical in addition to electrical freed the CAN bus specification from the constraints and complexity of physical implementation. However it left CAN bus implementations open to interoperability issues due to mechanical incompatibility.
In order to improve interoperability, many vehicle makers have generated specifications describing a set of allowed CAN transceivers in combination with requirements on the parasitic capacitance on the line. In addition to parasitic capacitance, 12V and 24V systems do not have the same requirements in terms of line maximum voltage.
Indeed, during jump start events light vehicle lines can go up to 24V while truck systems can go as high as 36V. Noise immunity on ISO is achieved by maintaining the differential impedance of the bus at a low level with low-value resistors ohms at each end of the bus. However, when dormant, a low-impedance bus such as CAN draws more current and power than other voltage-based signaling busses.
On CAN bus systems, balanced line operation, where current in one signal line is exactly balanced by current in the opposite direction in the other signal provides an independent, stable 0 V reference for the receivers. Best practice determines that CAN bus balanced pair signals be carried in twisted pair wires in a shielded cable to minimize RF emission and reduce interference susceptibility in the already noisy RF environment of an automobile.
ISO -2 provides some immunity to common mode voltage between transmitter and receiver by having a 0 V rail running along the bus to maintain a high degree of voltage association between the nodes. Also, in the de facto mechanical configuration mentioned above, a supply rail is included to distribute power to each of the transceiver nodes.
The design provides a common supply for all the transceivers. The actual voltage to be applied by the bus and which nodes apply to it are application-specific and not formally specified. Common practice node design provides each node with transceivers which are optically isolated from their node host and derive a 5 V linearly regulated supply voltage for the transceivers from the universal supply rail provided by the bus.
This usually allows operating margin on the supply rail sufficient to allow interoperability across many node types. Typical values of supply voltage on such networks are 7 to 30 V. However, the lack of a formal standard means that system designers are responsible for supply rail compatibility. ISO -2 describes the electrical implementation formed from a multi-dropped single-ended balanced line configuration with resistor termination at each end of the bus.
As such the terminating resistors form an essential component of the signaling system, and are included, not just to limit wave reflection at high frequency. During a recessive state the signal lines and resistor s remain in a high impedances state with respect to both rails. A recessive state is present on the bus only when none of the transmitters on the bus is asserting a dominant state. During a dominant state the signal lines and resistor s move to a low impedance state with respect to the rails so that current flows through the resistor.
Irrespective of signal state the signal lines are always in low impedance state with respect to one another by virtue of the terminating resistors at the end of the bus. Multiple access on such systems normally relies on the media supporting three states active high, active low and inactive tri-state and is dealt with in the time domain. A CAN network can be configured to work with two different message or "frame" formats: the standard or base frame format described in CAN 2.
The only difference between the two formats is that the "CAN base frame" supports a length of 11 bits for the identifier, and the "CAN extended frame" supports a length of 29 bits for the identifier, made up of the bit identifier "base identifier" and an bit extension "identifier extension". The distinction between CAN base frame format and CAN extended frame format is made by using the IDE bit, which is transmitted as dominant in case of an bit frame, and transmitted as recessive in case of a bit frame.
CAN controllers that support extended frame format messages are also able to send and receive messages in CAN base frame format. All frames begin with a start-of-frame SOF bit that denotes the start of the frame transmission.
The CAN standard requires that the implementation must accept the base frame format and may accept the extended frame format, but must tolerate the extended frame format.
In the event of a data frame and a remote frame with the same identifier being transmitted at the same time, the data frame wins arbitration due to the dominant RTR bit following the identifier.
The overload frame contains the two bit fields Overload Flag and Overload Delimiter. There are two kinds of overload conditions that can lead to the transmission of an overload flag:. However, limit yourself to one plate rather than adopting an all-you- can -eat mindset. From Bungay in Suffolk comes the news that a water-wagtail has built its nest in a milk- can.
When he waded past Captain Can -dage he heard the old skipper trying to comfort the girl, his voice low and broken by sobs. He kept his worms there, between his cap lining and his hair; it saved the trouble of a bait- can. Bob, Bob, massa him want can -noo go see great big ship mighty quick. In addition to the idioms beginning with can. New Word List Word List. Save This Word! See synonyms for can on Thesaurus. Smoothly step over to these common grammar mistakes that trip many people up.
Good luck! Can but is equivalent to can only: We can but do our best. Cannot but is the equivalent of cannot help but: We cannot but protest against these injustices.
See also help. You can or may use it tomorrow. Sentences using can occur chiefly in spoken English. May in this sense occurs more frequently in formal contexts: May I address the court, Your Honor? In negative constructions, can't or cannot is more common than may not : You can't have it today.
WebBritannica Dictionary definition of CAN [modal verb] 1: to be able to (do something) I don't need any help. I can do it myself. I can't decide what to do. All we can do [=the only thing we can do] now is wait.: to know how to (do something) She can read, can't she? I can whistle.: to have the power or skill to (do something). WebMeaning of can in English can modal verb us strong / k?n / weak / k?n / uk strong / k?n / weak / k?n / can modal verb (ABILITY) A1 to be able to: Can you drive? She can speak . WebThe CAN is a message-based protocol, which means that message carries the message identifier, and based on the identifier, priority is decided. There is no need for node identification in the CAN network, so it becomes very easy to insert or delete it from the network. It is a serial half-duplex and asynchronous type of communication protocol.