Data Link Communications Description and Operation
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Data Link Communications Description and Operation
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The communication among control modules is performed through the high speed GMLAN serial data circuit, the chassis high speed GMLAN serial data circuit, the mid speed GMLAN serial data circuit, and the low speed GMLAN serial data circuit. The modules that need real time communication are attached to the high speed GMLAN network and the chassis high speed GMLAN network. The body control module (BCM) is the gateway between the networks. The purpose of the gateway is to translate serial data messages between the high speed GMLAN bus and the low speed GMLAN bus. The gateway will interact with each network according to that network's transmission protocol. Refer to
Body Control System Description and Operation
for more information about the gateway.
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High Speed GMLAN Circuit Description
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The data link connector (DLC) allows a scan tool to communicate with the high speed GMLAN serial data circuit. The serial data is transmitted on two twisted wires that allow speed up to 500 Kb/s. The twisted pair is terminated with 120 ohms resistors. The resistors are used to reduce noise on the high speed GMLAN bus during normal vehicle operation. The high speed GMLAN is a differential bus. The high speed GMLAN serial data bus (+) and high speed GMLAN serial data (-) are driven to opposite extremes from a rest or idle level. The idle level, which is approximately 2.5 volts, is considered recessive transmitted data and is interpreted as a logic 1. Driving the lines to their extremes, adds one volt to the high speed GMLAN serial data bus (+) and subtracts one volt from the high speed GMLAN serial data bus (-) wire. This dominant state is interpreted as a logic 0. GMLAN network management supports selective start up and is based on virtual networks. A virtual network is a collection of signals started in response to a vehicle event. The starting of a virtual network signifies that a particular aspect of the vehicles functionality has been requested. A virtual network is supported by virtual devices, which represents a collection of signals owned by a single physical device. So, any physical device can have one or more virtual devices. The signal supervision is the process of determining whether an expected signal is being received or not. Failsofting is the ability to substitute a signal with a default value or a default algorithm, in the absence of a valid signal. Some messages are also interpreted as a heartbeat of a virtual device. If such a signal is lost, the application will set a no communication code against the respective virtual device. Note: a loss of serial data DTC does not represent a failure of the module that the code is set in.
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Chassis High Speed GMLAN Circuit Description
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The GMLAN high speed chassis expansion bus functions the same as the GMLAN high speed bus, and the two buses operate in parallel. The expansion chassis bus was added to reduce message congestion on the primary high speed bus. Since the GMLAN high speed chassis bus and primary GMLAN high speed bus operate in the same manner, the diagnostics for each are the same.
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Mid Speed GMLAN Circuit Description
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The data link connector (DLC) allows a scan tool to communicate with the mid speed GMLAN serial data circuit. Modules connected to the mid speed GMLAN serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules when the ignition switch is in any position other than OFF. The mid speed GMLAN serial data bus uses a terminating resistor that is in parallel with the mid speed GMLAN (+) and (-) circuits.
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Low Speed GMLAN Circuit Description
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The data link connector (DLC) allows a scan tool to communicate with the low speed GMLAN serial data circuit. The serial data is transmitted over a single wire to the appropriate control modules. The transmission speed for low speed GMLAN is up to 83.33 Kb/s. Under normal vehicle operating conditions, the speed of the bus is 33.33 Kb/s. This protocol produces a simple pulse train sent out over the low speed GMLAN serial data bus. When a module pulls the bus high, 5 volts, this creates a dominant logic state or 0 on the bus. When the bus is pulled low, 0 volts, it is translated as a recessive logic state or 1. To wake the control modules connected to the low speed GMLAN serial data bus, a high voltage wake up pulse is sent out over the bus, the voltage level of the pulse is +10 volts. Modules connected to the low speed GMLAN bus can be part of a virtual network as described in the previous paragraph. The modules on the low speed GMLAN serial data bus are connected to the bus in a parallel configuration.
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Local Interconnect Network (LIN) Circuit Description
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Serial data is transmitted over a Local Interconnect Network (LIN) single wire network circuit bus between a primary control module and other control modules within a particular subsystem. If serial data communication is lost between any of the control modules on the LIN bus network, a no communication code against the non-communicating control module will be set.
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CAN Graphical Interface (CGI) Circuit Description
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The CAN graphical interface (CGI) is a high speed UART-based serial data interface utilizing a CAN physical layer. It is used by the Entertainment sub-system to transfer high-rate display graphics between the Radio and the Info Display Module and/or Radio/HVAC Control. In the case where the Info Display Module and Radio/HVAC Control are separate control modules, the Radio interfaces only with the Info Display Module and the Info Display Module communicates with the Radio/HVAC Control through a LIN interface. The Info Display Module therefore is responsible for passing information between the Radio and the Radio/HVAC Control. A bus wake up signal will be generated by the Radio or by the Info Display Module when the systems functionality is required. Once the CGI network is awake, all message transfers on the CGI are initiated by the Radio. Slave devices such as the Info Display Module and Radio/HVAC Control may not transmit unless they are responding to a request from the Radio.
Dual wire CGI networks have a termination resistance between the CGI+ (CAN- High) and CGI- (CAN- Low) conductors at each end of the bus. Each termination resistance is 120 ohms.
The communication function of the CGI shall be active or inactive based on the voltage level of a specific input (typically called "CSW", Center Stack Wake ). The network will stay awake as long as this discrete signal is driven low to less than 1.5 V. When the Radio determines that the communication bus needs to be operational or active, it shall generate the wake-up signal by pulling this signal circuit low.
The Radio can execute a warm reset of the Info Display Module if the Info Display Module fails to respond to Radio"s request via the CGI interface. Center Stack Reset is a low-asserted pull down output (less than 1.5 V) from the Radio to the Info Display Module and has the same electrical characteristics as those for the Center Stack Wake signal defined above.
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Data Link Connector (DLC)
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The data link connector (DLC) is a standardized 16-cavity connector. Connector design and location is dictated by an industry wide standard, and is required to provide the following:
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Pin 1 Low speed GMLAN communications terminal
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Pin 3 Mid speed GMLAN serial bus (+) terminal
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Pin 4 Scan tool power ground terminal
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Pin 5 Common signal ground terminal
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Pin 6 High speed GMLAN serial data bus (+) terminal
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Pin 11 Mid speed GMLAN serial bus (-) terminal
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Pin 12 Chassis high speed GMLAN serial bus terminal
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Pin 13 Chassis high speed GMLAN serial bus terminal
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Pin 14 High speed GMLAN serial data bus (-) terminal
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Pin 16 Scan tool power, battery positive voltage terminal
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The scan tool communicates over the various buses on the vehicle. When a scan tool is installed on a vehicle, the scan tool will try to communicate with every module that could be optioned into the vehicle. If an option is not installed on the vehicle, the scan tool will display No Comm for that options control module. In order to avert misdiagnoses of No Communication with a specific module, refer to
Data Link References
for a list of modules, the buses they communicate with, and the RPO codes for a specific module.
