Body Electrical Diagnosis General Development of Multiplexing

Networking allows greater vehicle content flexibility because functions can be added through software changes. Existing systems require an additional module or additional I/O pins for each function added.

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LEXUS Technical Training General Development of Multiplexing
General Development of Multiplexing
In-vehicle networking involving time division is also known as multiplexing. It is a method for transferring data among distributed electronic modules via a serial data bus. Serial data is electronically coded information which is transmitted by one computer and received and displayed by another computer. The term serial data implies that the information is digitally coded and transmitted in a series of data words. Serial data gets its name from the fact that data parameters are transmitted, one after another, in series. The display on the receiving computer updates or refreshes once each data cycle, after all data has been received.
Transmission of Using an analog/digital circuit, the transmitting computer digitizes the Data and Rates data from sensors, actuators, and other calculated information.
Typically, this means that each sensor or actuator value is converted into a one byte (8 bits) binary word before it is transmitted to the receiving computer. The data transmission rate is referred to as the baud rate. Baud rate refers to the number of data bits that can be transmitted per second. For example, if a data stream has 12 parameters, and each parameter is converted into an 8 bit data word, the total size of the data transmission is 96 bits of data (12 words x 8 bits per word.) If this data can be transmitted once every second, the baud rate is 96 bits/second or 96 baud. Without serial networking, inter-module communication requires dedicated, point-to-point wiring resulting in bulky, expensive, complex, and difficult to install wiring harnesses. Applying a serial data bus reduces the number of wires by combining the signals on a single wire through time division multiplexing. Information is sent to individual control modules that control each function, such as anti-lock braking, turn signals, power windows, dashboard displays, and audio systems.
Advances and In-vehicle networking provides system-level benefits, many of which Standards are only beginning to be realized: * A decreased number of dedicated wires is required for each function, and thus reduces the size of the wiring harness. System cost, weight, reliability, serviceability, and installation are improved. * Common sensor data, such as vehicle speed, engine temperature, etc. are available on the network, so data can be shared, thus eliminating the need for redundant sensors.
Car manufacturers are discovering new features that are enabled by networking. For example, multiplexing allows driver\’s preference for such things as ride firmness, seat positions, steering assist effort, mirror positions, and radio station presets. Recently, there have been efforts to standardize protocols at the data link and physical layers. Systems designers are also seeing the benefits of standardized application layer protocols. Multiplexing Standards are also appearing in automotive applications. Examples include: SAE J1939, OSEK from the German automotive consortium, SDS from Honeywell, and DeviceNet from Allen-Bradley. These standards allow system designers to avoid low-level protocol details and focus on the application itself. However, the impact of this type of standardization is increased demand on the microcontrollers and protocol devices; and thus the need for efficient message handling and standardized protocol.

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