Lin Bus Operation

LIN BUS 

The LIN communication network is a single-wire, low-speed network different from the CAN networks. The LIN bus is a Class A data bus system that uses a centralized module, called a Master module, often referred to as the Master module, with a series of other modules, called Slave modules, directly wired to it on a dedicated, single-wire circuit. The communication over a LIN bus only occurs between the Master and Slave modules. Each module on the LIN bus can provide bias. Once the master module sends a request on the LIN bus, the slave modules do have the capability of both sending and receiving information over LIN. Even though it is limited, the LIN bus simplifies wiring in a vehicle by combining multiple input and output circuits on a single bus wire. The LIN bus utilizes stub (backbone) topology.

The Master module is central to the LIN bus system. All LIN modules connect to a Master module through the LIN bus circuit. The Master module is also wired to a CAN bus for information sharing and to allow for scan tool diagnosis. The Master module is responsible for LIN diagnostics and is capable of setting fault codes for any LIN module or circuit faults. Since a Slave module cannot communicate with any other module directly, the Master module relays LIN Slave module data over the CAN bus to the scan tool, similar to a Gateway Module. Controlled module inputs and commands must be given only to the Master module, which then sends the information to another Slave module or out onto the CAN bus.

The LIN bus is biased through the Master and Slave modules. When at rest, the voltage on the LIN network is close to battery voltage. When LIN bus communication occurs, the voltage is pulled low to nearly 0V, creating a digital signal. If a Slave module is disconnected or removed from the network, that module no longer pulls voltage low on the LIN bus, and the average network voltage level will be lower. LIN voltage reads 12V when in sleep mode. The Master module communicates with Slave modules by pulling the voltage from high to low, creating a digital signal. Controlled modules communicate with the Master by biasing the network voltage in the same manner. Even though the LIN Slave modules are wired to each other, they only recognize the voltage pulses sent by the Master module, making communication possible only between the Slave and the Master. However, LIN communication can occur between Slave modules by using the Master module to relay information. The power feed to the LIN Slave is determined by its function. Controlled modules with a battery feed voltage supply operate and communicate even if the ignition is in the OFF position. For example, to support the hazard lamp function, the instrument panel switch bank is battery fed. If the LIN module only needs to function with the ignition ON, such as the compass module, the power feed is ignition only. The lighting and wiper multifunction switch require both feeds. This allows the lighting functions to be independent of ignition status, while the operation of the wipers requires the ignition feed. LIN Slave modules that receive their power feed from an ignition circuit enter sleep mode when the ignition power is removed. LIN Slave modules that receive their power from a direct battery feed receive a sleep message when the vehicle data bus enters sleep mode. For circuits where the LIN modules are battery fed, either the Master or the Slave module can wake the other. If the Slave module wakes up the Master, the vehicle bus also wakes.

As with any single-wire communication bus, a LIN bus has low fault tolerance. If a short to ground or power occurs on the LIN bus, the entire LIN bus loses the ability to communicate with the Master module. If an open occurs within the network, modules on the other side of the break (that are no longer connected to the Master module) lose communication. If an open occurs within the network, modules still connected to the Master module continue to communicate. Loss of power or ground to a Slave module results in that particular Slave module losing communication as well, whereas loss of power or ground to a Master module results in no LIN bus communication. Generally, one Slave module is triggered for information at a time - meaning zero collision risk and no need for arbitration as is used in CAN (Controller Area Network). If multiple Slave modules respond, a collision occurs and the Master ECU defaults to unconditional frames.

When the LIN bus is shorted, no modules can communicate on the network and a loss of all LIN module functionality occurs. Loss of communication codes (U-codes) will set in the Master module. A short to ground or to power on the LIN bus does not affect Master module communication over the CAN bus. The voltage on the LIN bus should be checked to confirm if voltage is present. Modules may be isolated by disconnecting Slave modules one at a time and then checking for LIN communication to return when a particular Slave is disconnected. When all Slave and Master modules are disconnected, test the LIN circuit for a short to ground or to power. If an open occurs, modules downstream of the open lose functionality and communication codes will set. If a single LIN Slave module is not communicating but all other LIN modules are functioning normally, always verify power, ground, and LIN voltage at the suspect module before replacing any parts. As long as at least one module on the LIN network is connected, then the circuit is able to be biased.