Why Does CAN Adopt Twisted Pair Layout?

CAN (Controller Area Network) is a serial communication protocol bus for real-time applications. It can use twisted pair to transmit signals, and it is one of the most widely used field buses in the world. CAN protocol is used for the communication between various components in the automobile, so as to replace the expensive and bulky distribution Wiring Harness.

In the application of CAN bus, it is generally recommended to use shielded twisted pair for networking and wiring. This article will explain in detail why CAN bus should adopt twisted pair layout.

Brief Introduction of Twisted Pair

Twisted-pair wire refers to a kind of general Wire Harness made by winding two insulated wires together according to certain specifications. The main function of twisted pair is to prevent external electromagnetic interference and reduce external interference of its own signal. (that is, it can not only prevent others from interfering with themselves, but also reduce their interference with others. )

Twisting a pair of mutually insulated metal wires together can not only resist part of electromagnetic interference from the outside world, but also reduce the mutual interference between pairs of twisted wires. The principle is: twist two insulated wires together, and the interference signal acts on these two twisted wires in the same way (this interference signal is called common-mode signal). The common-mode signal can be eliminated in the difference channel of the received signal, so as to extract useful signals (differential-mode signals).

The function of twisted pair is to make the noise generated by external interference on two wires (in the professional field, the useless signal is called noise) the same, so that the subsequent difference channel can extract the useful signal. Difference channel is a subtraction circuit, and the in-phase signals (common-mode signals) at two input terminals cancel each other out (m-n), while the inverted signal is equivalent to x-(-y), which is enhanced. Theoretically, in twisted-pair and difference channel, m=n, x=y, which means that the interference signal is completely eliminated and the useful signal is doubled, but there are some differences in actual operation.

In a cable sleeve, different pairs have different twist lengths. Generally speaking, the twist length is within 38.1 mm ~ 140 mm, and the twist length of adjacent pairs is within 12.7 mm. The length of a twisted pair period is called pitch. The smaller the pitch (the denser the twisted pair), the stronger the anti-interference ability.

Common Twisted Pair and Its Advantages

Common twisted-pair wires are Class 3, Class 5 and Super Class 5 wires, and the latest Class 6 wires. The former has a thin wire diameter while the latter has a thick wire diameter. The models are as follows:

1) Class I cable: mainly used for voice transmission (Class I standard is mainly used for telephone cables before the early 1980s), which is different from data transmission.

2) Class II line: the transmission frequency is 1MHZ, which is used for voice transmission and data transmission with the highest transmission rate of 4Mbps. It is common in the old token network using 4MBPS standard token passing protocol.

3) Class III cable: refers to the cable currently specified in ANSI and EIA/TIA568 standards. The transmission frequency of this cable is 16MHz, which is used for voice transmission and data transmission with the highest transmission rate of 10Mbps, mainly for 10base-T.

4) Four types of cables: the transmission frequency of these cables is 20MHz, and they are used for voice transmission and data transmission with the highest transmission rate of 16Mbps, mainly for token-based LAN and 10base-T/100base-T.

5) Category V cable: This kind of cable increases the winding density, and is coated with a high-quality insulating material with a transmission rate of 100MHz. It is used for voice transmission and data transmission with the highest transmission rate of 10Mbps, mainly for 100BASE-T and 10BASE-T networks. This is the most commonly used Ethernet cable.

6) Super Class V line: Super Class V line has small attenuation, less crosstalk, higher attenuation to crosstalk ratio (ACR) and signal-to-noise ratio (Structural ReturnLoss), smaller time delay, and greatly improved performance. Category 5 cable is mainly used for Gigabit Ethernet (1000Mbps).

7) Class VI cable: the transmission frequency of this kind of cable is 1 MHz ~ 250 MHz, and the comprehensive attenuation crosstalk ratio (PS-ACR) of Class VI cabling system should have a large margin at 200MHz, which provides 2 times the bandwidth of Class V. The transmission performance of Class VI cabling is much higher than that of Class V standards, and it is most suitable for applications with transmission rate higher than 1Gbps. An important difference between Class VI and Super Class V is that it improves the performance of crosstalk and return loss. For the new generation of full-duplex high-speed network applications, excellent return loss performance is extremely important. The basic link model has been cancelled in the six standards, and the wiring standard adopts star topology. The required wiring distance is: the length of permanent link should not exceed 90m, and the length of channel should not exceed 100m.

At present, twisted pair can be divided into unshielded twisted pair (UTP=UNSHILDED TWISTED PAIR) and shielded twisted pair (STP=SHIELDED TWISTED PAIR). The outer layer of shielded twisted-pair cable is wrapped by aluminum platinum to reduce radiation, but it can't completely eliminate radiation. The price of shielded twisted-pair cable is relatively high, so it is more difficult to install than unshielded twisted-pair cable.

Combination with CAN twisted pair

CAN (Controller Area Network) is a serial communication protocol bus for real-time applications, and it is one of the most widely used field buses in the world. CAN protocol is used for communication between various components in automobiles, instead of expensive and bulky distribution wiring loom. CAN bus itself has powerful anti-interference and error correction and retransmission mechanism.

When CAN is applied to new energy vehicles, it means that it has to work in a severe electromagnetic environment, so how to resist interference is the topic that engineers are most concerned about.

When CAN bus uses twisted pair, these interference problems CAN be solved well, and CAN interface adopts differential signal transmission mode. Differential signal transmission is a method of information transmission using two complementary electrical signals. Taking high-speed CAN as an example, different logic states are transmitted through two signal lines CANH and CANL, and the receiving circuit only recognizes the signal difference between the two signal lines.

Interference usually exists in the form of common mode. When the bus is disturbed, both buses will be affected at the same time, but their differential voltages will not be affected. Compared with single-ended signal transmission mode, differential signal transmission mode has better anti-interference ability.

Of course, adopting the differential transmission mode can't rest easy. CAN bus is often used for long-distance communication. With the increase of cable length, all kinds of interference are coupled to the bus through the cable, which greatly increases the probability of external interference to bus communication. If the cable is selected and used improperly, it is very likely to cause communication abnormality. For CAN bus applications, we, Kable-X, generally recommend the use of twisted pair.

Special "twisted pair" CNA

In addition to a two-wire CAN, there is also a single wire CAN(Single Wire CAN), which can reduce one transmission line, but requires good common ground characteristics between nodes (equivalent to the second signal line). The signal anti-interference ability of CAN is relatively weak. In the design, it is necessary to increase the signal amplitude to increase the signal-to-noise ratio, which will increase its own radiation ability. Therefore, its signal transmission rate must be reduced to meet the requirements of electromagnetic compatibility. To sum up, single-wire CAN is only suitable for low-speed vehicle body electronic unit, comfort and entertainment control. Because of the low signal speed of CAN bus, it can still work in single-wire mode when one signal line fails.