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Understanding Controller Area Network

Automotive technology is dependent on a number of control modules and their ability to communicate with one another
Monday, May 2, 2016 - 07:00
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This means that each node has a crystal oscillator and can control the voltage levels on the bus. If one node fails, this bus network configuration will still function. CAN is a language and like Morse code, it uses on and off electrical pulses against time to convey data messages. These data messages are comprised of digital high and low voltage changes that are read as a binary code. This binary code is made up of 1s and 0s. The way in which the 1s and 0s are organized against time is what makes each network protocol different. Just as in different spoken languages, such as English and German, the same alphabet is used. What makes each of these languages unique is the way in which the letters and spaces are organized. In both of these spoken languages, the organization of the letters and spaces will have rules applied to them that will have to be followed.

Network protocol languages will also have certain rules that will govern them. These regulations are set by the Society of Automotive Engineers (SAE) and the International Standards Organization (ISO). The regulations mandated by these organizations insure that a system standard will be followed so that all systems under the standard will operate and work together. When Bosch wrote the CAN language, no physical layer or transfer media were given. This open architecture was done on purpose to allow the CAN language to be more versatile, thus making it more powerful. By not confining the use of the CAN language to a set physical layer, the engineering teams can be more creative.  This is why CAN is able to run at various speeds from 1-1,000 kilobytes per second (kbps) and be transmitted on different physical media such as 1 wire, 2 wire, twisted pair, shielded twisted pair, coax or fiber optic. The CAN network can also utilize different voltage levels to control the bus communications. To make sure that each of these CAN embodiments work correctly, SAE and ISO wrote standards for each of the various speeds and physical layers (Figure 2). 

Figure 2

CAN protocol has been divided into three groups based on the transmission/receive rates.  These rates are based on how many bytes can be transmitted in one second. CAN A is a low-speed CAN network that operates at 10 kbps. CAN A is used in a low-cost network scheme that will control body functions such as seat nodes, door nodes, mirror nodes, etc. These systems are not safety critical, so the transmission speed is not crucial. CAN B is a medium-speed CAN network that can operate from 33 kbps to 250 kbps. This would be used in body control functions or diagnostic functions that need to transmit larger amounts of data. CAN C is a high-speed CAN network that operates from 250 kbps to 1 megabyte per second (mbps). This would be used in safety-critical systems such as air bag, ABS, traction control, power train, etc. The network data transmission speed costs money. The faster the data transmission speed, the greater the cost. 

Traffic control

Since the cost of these networks will raise the cost to manufacture the vehicle, the appropriate CAN bus will be utilized. What this means is there may be three different CAN networks operating at the same time in the same vehicle, but at different speeds. It is also possible that other network protocols and media types as well as the CAN protocol may be in operation on the same vehicle. CAN A, CAN B and CAN C run at different speeds; however, each of these can be implemented on the same vehicle. If all the CAN speeds were to run on the same wire with this much variation in speed, there would be data collisions. Think of this as a single-lane highway with VW Beetles driving at 10 mph and Formula 1 cars overtaking the VW Beetles at 250 mph. This would ensure that collisions would occur. To avoid this problem, each network is isolated to the protocol and speed at which the network is transmitting and receiving. Now that each network protocol and speed is contained on its own isolated network, the collisions from running at various speeds will be avoided; but because the networks have been isolated, there will be no way for these networks to share information.

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