| Function of data bus systems |
| Data bus is the term used to describe a data transfer and distribution system. |
| There are various data bus systems in the Audi A4. These systems operate on different levels. The control unit in dash panel insert -J285- forms the interface (gateway) between the various data bus systems. |
| The Controller Area Network is a data bus system employing two wires. These are referred to as data bus wires. Data are transmitted serially via the data bus wires to the connected control units. |
| These control units communicate with the aid of the data bus systems, i.e. they use the data bus systems to exchange data. → No.186 |
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| The convenience data bus system |
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| Operates at a transmission rate of 100 kbaud (100000 data units per second) |
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| Continues to operate even in the event of failure of one of the two wires, however only in emergency operation (e.g. on account of an open circuit). |
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| Examples of connected control units: |
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| Control unit in dash panel insert -J285- (interface with drive data bus) |
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| Operating and display unit, Climatronic control unit -J255- of air conditioner/heater |
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| Auxiliary heater control unit -J364- (vehicles with genuine Audi auxiliary heater only) |
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| Steering column electronics control unit -J527- |
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| The drive data bus system |
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| Operates at a transmission rate of 500 kbaud (500000 data units per second) |
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| Only operates if data can be transferred on both wires (safety circuit). |
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| Examples of connected control units: |
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| Control unit in dash panel insert -J285- (interface with convenience data bus system) |
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| Relevant engine control unit |
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| “CAN bus” is the term used to describe a data transfer and distribution system. |
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| The data are transferred simultaneously in the data bus systems by way of the “CAN-Hi” and “CAN-Lo” signal wires (two-wire bus systems). Depending on the version of the data bus system, these wires may form twisted pairs to minimise interference from other systems. They may also be screened against the other wires. → Current flow diagrams, Electrical fault finding and Fitting locations |
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| The wires between the control units which are used to transmit data are referred to as signal wires. |
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| These signal wires are used for serial transmission of data to the connected control units. The data cannot be checked using workshop equipment. |
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| The data are transmitted and received in the form of telegrams by the various control units at regular intervals. The interval between two telegrams transmitted by a control unit is governed by the priority of the data transferred and is between 0.01 and 0.2 seconds long. |
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| If a connected control unit does not receive any information within a specified period or if the data telegrams received cannot be evaluated, a fault is stored in the fault memory. |
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| The reception of information is displayed for example in the operating and display unit, Climatronic control unit -J255- of the air conditioner/heater and also in the auxiliary heater control unit -J364- (in the measured value block, display group“125”) → Chapter. |
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| If a control unit cannot provide the data required (or just one item of information) on account of a component fault, a fault message is transmitted together with the data telegram requesting readout of the fault memory in this control unit. |
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| The ambient temperature sensor -G17- is not connected and the control unit in dash panel insert -J285- can therefore not transmit the measured value of this sensor in its data telegram. |
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| The control unit in dash panel insert -J285- stores this fault and sets a fault message in its data telegram instead of the measured value. |
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| The operating and display unit, Climatronic control unit -J255- of the air conditioner/heater detects the fault message on evaluating the telegram and stores the information “Control unit in dash panel insert, read out fault memory” in the fault memory. |
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