| Function of data bus systems |
| Data bus is the term used to describe a data transfer and distribution system. |
| Two different bus systems are used in the Audi A2. These systems work on different levels. The control unit in the dash panel insert -J285- forms the interface (gateway) between the two data bus systems. |
| “Controller Area Network” refers to a data bus system which operates with two wires. They are called data bus wires. These data bus wires are used for serial transmission of data, i.e. data is sent to each of the connected control units in turn. |
| These control units communicate, i.e. exchange data, with the aid of the data bus system → No.186 |
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| The convenience data bus |
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| Operates at a transmission rate of 100 kbaud (100,000 data units per second) |
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| Continues to operate even if there is a fault in one of the two wires, albeit in emergency operation mode (e.g. on account of an open circuit in the wiring) |
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| The following control units are connected, for example |
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| The control unit in the dash panel insert -J285- (interface to drive data bus) |
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| The operating and display unit for air conditioner/Climatronic -E87- of the air conditioner |
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| The central control unit for the convenience system -J393- |
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| Operates at a transmission rate of 500 kbaud (500,000 data units per second) |
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| Only operates if data can be transmitted on both wires (safety circuit) |
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| The following control units are connected, for example |
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| Control unit in the dash panel insert (interface) |
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| “CAN bus” is the term used to describe a data transfer and distribution system. |
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| The data is transmitted in the data bus systems simultaneously via the “CAN-Hi” and “CAN-Lo” signal wires (two-wire bus systems). Depending on the design of the data bus system, these wires take the form of twisted pairs to keep interference from other systems as low as possible, they may also be screened from the other wires. → "Current Flow Diagrams, Electrical Fault-Finding and Fitting Locations" |
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| The wires between the control units on which data is transmitted are referred to as signal wires. |
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| Data is transmitted in series via these signal wires to the connected control units and cannot be checked using workshop equipment. |
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| Data is transmitted at regular intervals by the various control units and received in the form of messages. The interval between two messages transmitted by a control unit is governed by the priority rating of the data transmitted and amounts to between 0.01 and 0.2 seconds. |
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| If a connected control unit does not receive information within a specified period or if the data messages received cannot be evaluated, a fault is stored in the fault memory. |
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| In the air conditioner for example, the reception of the information is displayed in measured value block, display group “016” → Chapter. |
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| If a control unit cannot provide the data requested (or only a certain item of information) due to a component fault, transmission of the data message is accompanied by a fault message requesting readout of fault memory in this control unit. |
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| The sender for the ambient temperature sensor -G17- is not connected; therefore, the control unit in the dash panel insert -J285-cannot transmit the value measured by this sender in its data message. |
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| The control unit in the dash panel insert stores this fault and inserts a fault message in its data message instead of the measured value. |
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| On evaluating the message, the operating and display unit for air conditioner/Climatronic -E87- detects the fault message and stores the following information in the fault memory “control unit in dash panel insert; read out fault memory”. |
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