Chevrolet Equinox Service Manual: Data Link Communications Description and Operation

Note:This is an overview of different serial data buses used by GM devices to communicatewith each others. UseData Communication Schematicsto find out which serial data buses are configured for a specific vehicle.

Circuit Description

There are many components in a vehicle that rely on information from other sources,transmit information to other sources, or both. Serial data communication networksprovide a reliable, cost effective, way for various components of the vehicle to “talk”to one another and share information.

GM uses a number of different communication buses to insure the timely and efficientexchange of information between devices. When compared to each other, some of thesebuses are different in nature as far as speed, signal characteristics, and behavior.An example of this is the High Speed GMLAN and Low Speed GMLAN buses.

On the other hand, when other buses are compared to each other they have similar characteristicsand simply operate in parallel. In this case they are used to group together componentswhich have high interaction. Examples are the High Speed GMLAN, Powertrain Expansion,and Chassis Expansion buses. This allows them to communicate with each other on abus with reduced message congestion insuring faster and the more timely exchange ofinformationthan if all vehicle devices were on a single bus.

The majority of information that exists within a given network generally stays local;however some information will have to be shared on other networks. Control modulesdesignated as Gateway’s perform the function of transferring information between thevarious buses. A Gateway module is connected to at least 2 buses and will interactwith each network according to its message strategy and transmission models.

GMLAN provides the capability for a receiving device to monitor message transmissionsfrom other devices in order to determine if messages of interest are not being received.The primary purpose is to allow reasonable default values to be substituted for theinformation no longer being received. Additionally, a device may set a DiagnosticTrouble Code to indicate that the device it is expecting information from is no longercommunicating.

High Speed GMLAN Circuit Description

A High Speed GMLAN Bus is used where data needs to be exchanged at a high enough rateto minimize the delay between the occurrence of a change in sensor value and the receptionof this information by a control device using the information to adjust vehicle systemperformance.

The High Speed GMLAN serial data network consists of two twisted wires. One signalcircuit is identified as GMLAN-High and the other signal circuit is identified asGMLAN-Low. At each end of the data bus there is a 120-Ω termination resistor betweenthe GMLAN-High and GMLAN-Low circuits.

Data symbols (1’s and 0’s) are transmitted sequentially at a rate of 500-Kbit/s. Thedata to be transmitted over the bus is represented by the voltage difference betweenthe GMLAN-High signal voltage and the GMLAN-Low signal voltage.

When the two wire bus is at rest the GMLAN-High and GMLAN-Low signal circuits arenot being driven and this represents a logic “1”. In this state both signal circuitsare at the same voltage of 2.5-V. The differential voltage is approximately 0-V.

When a logic “0” is to be transmitted, the GMLAN-High signal circuit is driven higherto about 3.5-V and the GMLAN-Low circuit is driven lower to about 1.5-V. The differentialvoltage becomes approximately 2.0 (+/- 0.5)-V.

Chassis High Speed GMLAN Circuit Description

The GMLAN Chassis Expansion Bus is basically a copy of the High Speed GMLAN Bus exceptthat its use is reserved for chassis components. This implementation splits messagecongestion between two parallel buses helping to insure timely message transmissionand reception. Sometimes communication is required between the Chassis Expansion Busand the primary High Speed GMLAN Bus. This is accomplished by using the K17-ElectronicBrakeControl Module (EBCM) as the Gateway module. Since the High Speed GMLAN Chassis ExpansionBus and primary High Speed GMLAN Bus operate in the same manner, the diagnostics foreach are similar.

Object High Speed GMLAN Circuit Description

The GMLAN Object Bus is basically a copy of the High Speed GMLAN Bus except that itsuse is reserved for the enhanced safety system. This implementation is used to isolatethe heavy communication among the enhanced safety system devices from the other vehiclebuses, reducing congestion. The K124-Active Safety Control Module is connected tothe Object Bus as well as the Primary High Speed GMLAN Bus, the Chassis ExpansionBus, andthe Low Speed GMLAN Bus. The K124-Active Safety Control Module acts as a Gatewaymodule for all required communication between the Object Bus devices and devices onthese other vehicle buses. The GMLAN Object Bus operates in the same manner as theChassis Expansion and Primary High Speed buses and so the diagnostics are similar.The Object Bus is physically partitioned into a Front Object Bus and a Rear ObjectBus with each partitionhaving its own communication enable circuit to activate the partition, but functionaloperation of both is identical. The Front Object Bus standard devices are the K124-ActiveSafety Control Module, the K109-Frontview Camera Module, and the-B233B Radar SensorModule – Long Range. The Front Object Bus optional devices are the B233LF-Radar SensorModule – Short Range Left Front and the B233RF-Radar SensorModule – Short Range Right Front. The Rear Object Bus is optional and when presentwill have the K124-Active Safety Control Module and B233R-Radar Sensor Module – ShortRange Rear on the bus, and optionally the Radar Sensor Module – Short Range RightRear. All Object Bus components are powered by the K124-Active Safety Control Modulevia the communication enable circuits, except the K109-Frontview Camera Modulewhich is powered directly by battery.

Media Oriented Systems Transport (MOST) Circuit Description

The MOST Infotainment network is a dedicated high speed multimedia streaming databus independent from GMLAN. The MOST bus will be configured in a physical hardwiredloop with each device within the bus sends and receives data on an assigned MOST addressesin a set order. Each device on the MOST bus will be required to have twisted paircopper wires (2 transmit TX, 2 receive RX, and 1 electronic control line which isa 12-V wakeupsignal line). The A11-Radio is the MOST Master and will monitor the bus for vehicleconfiguration, Infotainment data messages and errors on the bus. The MOST initializationconsists of a short 100-ms low voltage pulse on the electronic control line (or MOSTcontrol line) connected to all devices contained on the MOST ring. This wakeup messageonce received by each device, will first respond with a generic device response. Oncethese initial responses on the MOST bus are reported successfully without error tothe A11-Radio, the second data request will record the MOST device addresses, theirfunctionality requirements and capabilities within. The A11-Radio will learn thisinformation and also record the address node sequence on the MOST bus at this point.This node address list will now be stored within the A11-Radio as the MOST bus configuration(called “Last Working MOST ID of Node 1 – 9” on scan tool data display).

When MOST receive, transmit, or control line faults are detected, transmit/receivemessages will not received as expected from the wakeup request. The A11-Radio andthe K74-Human Machine Interface Control Module will then perform diagnostics to isolatethese MOST faults. If the MOST control line is shorted low to 0-V for excess amountof time, the A11-Radio will set a U2098-DTC and K74-Human MachineInterface Control Module will set a U0029-02-DTC. At this point the MOST bus willbe unable to communicate until the shorted MOST control line is repaired.

Once the shorted MOST control line diagnostics pass, the A11-Radio will attempt toresend the initial short pulse attempts up to 3-times on the MOST control line. Ifthe expected responses are not received, the A11-Radio continues into a failure modesetting a U0028-DTC and will continue on to send one 300-ms long pulse, which willenable the furthest upstream transmitting device to become the surrogateMOST Master in this MOST fault/diagnostic mode. When the A11-Radio receives this newMOST Master identity, the surrogate MOST master device can be identified based onscan tool data parameter “Surrogate MOST Master Node Upstream Position”. The scantool should be used to determine the MOST bus configuration and direction by utilizingthe “Last Working MOST ID of Node 1 – 9” parameters from the A11-Radio data display.When a faultis present, it will indicate the newly enabled “Surrogate MOST Master Node UpstreamPosition” from the A11-Radio. This will assist in determining where the MOST bus/controlis at fault. The MOST device upstream from the surrogate MOST master device, transmit,receive, or control lines will be the suspect areas for diagnostics at this point.These faults can be associated with any of the MOST transmit, receive, or controlline twistedcopper wires or possibly an internal device fault.

The K74-Human Machine Interface Control Module will set a U0029-00-DTC when it diagnosesa MOST bus not communicating properly after one attempt. When the DTC-U0029-00 isset by the K74-Human Machine Interface Control Module without the corresponding DTC-U0028from the A11-Radio, it will be an indication of an intermittent wiring/device condition.

CAN Graphical Interface (CGI) Circuit Description

This bus is used by the Entertainment sub-system to transfer high-rate display graphicsbetween the A11-Radio and the P17-Info Display Module and/or Radio/HVAC Control. Theelectrical characteristics of the CAN Graphical Interface (CGI) Bus are very similarto the High Speed GMLAN Bus. The message strategy and construction of messages aredifferent however. Sometimes communication is required between the CAN Graphical InterfaceBus and the Low Speed GMLAN Bus. This is accomplished by using the A11-Radio as theGateway module. Since the CAN Graphical Interface Bus and primary High Speed GMLANBus have similar electrical characteristics, the diagnostics for each are similar.

In the case where the P17-Info Display Module and Radio/HVAC Control are separatedevices the P17-Info Display Module is responsible for passing information betweenthe A11-Radio and the Radio/HVAC Control. The A11-Radio interfaces only with the P17-InfoDisplay Module and the P17-Info Display Module then communicates with the Radio/HVACControl through a Local Interconnect Network (LIN) interface.

A bus wake up signal will be generated by the A11-Radio or by the P17-Info DisplayModule when the system functionality is required. The communication function of theCAN Graphical Interface shall be enabled or disabled based on the voltage level ofthe Center Stack Wake. The network will stay awake as long as the circuit voltageis driven low, to less than 1.5-V. Communications are disabled with a high circuitvoltagearound 5.0-V.

The A11-Radio can execute a warm reset of the P17-Info Display Module if the P17-InfoDisplay Module fails to respond to the A11-Radio’s request. The Center Stack Resetis a low-asserted pull down output (less than 1.5-V) from the A11-Radio to the P17-InfoDisplay Module and has the same electrical characteristics as those for the CenterStack Wake signal defined above.

Mid Speed GMLAN Circuit Description

The Mid Speed GMLAN Bus is very similar to the High Speed GMLAN Bus except that ituses a slower transmission rate of 125-Kbit/s. This bus is intended for use wherethe system response time demands that a large amount of data be transmitted in a relativelyshort amount of time, such as updating a graphics display. As such it has usuallybeen used for infotainment applications. Sometimes communication is required betweenthe LowSpeed GMLAN Bus and the Mid Speed GMLAN Bus. This is accomplished by using the A11Radio as the Gateway module. Since the Mid Speed GMLAN Bus and primary High SpeedGMLAN Bus operate in a similar manner, the diagnostics for each are similar.

Low Speed GMLAN Circuit Description

Low Speed GMLAN Bus is used in applications where a high data rate is not requiredwhich allows for the use of less complex components. It is typically used for operatorcontrolled functions where the response time requirements are slower than those requiredfor dynamic vehicle control.

The Low Speed GMLAN Serial Data Network consists of a single wire, ground referencedbus with high side voltage drive. During on road vehicle operation data symbols (1’sand 0’s) are transmitted sequentially at the normal rate of 33.3 Kbit/s. For componentprogramming only, a special high speed data mode of 83.3-Kbit/s may be used.

Unlike the high speed dual wire networks, the single wire low speed network does notuse terminating resistors at either end of the network.

The data symbols to be transmitted over the bus are represented by different voltagesignals on the bus. When the Low Speed GMLAN Bus is at rest and is not being driven,there is a low signal voltage of approximately 0.2-V. This represents a logic “1”.When a logic “0” is to be transmitted, the signal voltage is driven higher to around4.0-V or higher.

Local Interconnect Network (LIN) Circuit Description

The Local Interconnect Network (LIN) Bus consists of a single wire with a transmissionrate of 10.417-Kbit/s. This bus is used to exchange information between a master controlmodule and other smart devices which provide supporting functionality. This type ofconfiguration does not require the capacity or speed of either a High Speed GMLANBus or Low Speed GMLAN Bus and is thus relatively simpler.

The data symbols (1’s and 0’s) to be transmitted are represented by different voltagelevels on the communication bus. When the LIN Bus is at rest and is not being driven,the signal is in a high voltage state of approximately Vbatt. This represents a logic“1”. When a logic “0” is to be transmitted, the signal voltage is driven low to aboutground (0.0-V).

Communication Enable Circuit Description

Devices on High Speed GMLAN Bus enable or disable communication based on the voltagelevel of the communication enable circuit. When the circuit voltage is high (around12-V), communications are enabled. When the circuit is low, communications are disabled.

Data Link Connector (DLC)

The X84-Data Link Connector (DLC) is a standardized 16-cavity connector. Connectordesign and location is dictated by an industry wide standard, and is required to providethe following:

Pin 1 Low speed GMLAN communications terminalPin 2 Class 2 communications terminalPin 3 Mid speed GMLAN serial bus (+) terminal or Object high speed GMLAN serial bus(+) terminalPin 4 Scan tool power ground terminalPin 5 Common signal ground terminalPin 6 High speed GMLAN serial data bus (+) terminalPin 7 Keyword communications terminalPin 11 Mid speed GMLAN serial bus (-) terminal or Object high speed GMLAN serial bus(-) terminalPin 12 Chassis high speed GMLAN serial bus (+) terminalPin 13 Chassis high speed GMLAN serial bus (-) terminalPin 14 High speed GMLAN serial data bus (-) terminalPin 16 Scan tool power, battery positive voltage terminalSerial Data Reference

The scan tool communicates over the various buses on the vehicle. When a scan toolis installed on a vehicle, the scan tool will try to communicate with every devicethat could be optioned into the vehicle. If an option is not installed on the vehicle,the scan tool will display No Comm (or Not Connected) for that optional device. Inorder to avert misdiagnoses of No Communication with a specific device, refer toData Link Referencesfor a list of devices, the buses they communicate with, and the RPO codes for a specificdevice.