This is an overview of different serial data buses used by control modules to communicate with each others.
Use Data Communication Schematics to find out which serial data buses are configured for a specific vehicle.
Data Link Communications Overview
There are many components in a vehicle that rely on information from other sources, transmit information to
other sources, or both. Serial data communication networks provide a reliable, cost effective, way for various
components of the vehicle to “talk” to one another and share information.
General Motors uses a number of different communication buses to insure the timely and efficient exchange
of information between control modules. When compared to each other, some of these buses are different in
nature as far as speed, signal characteristics, and behavior.
On the other hand, when other buses are compared to each other they have similar characteristics and
simply operate in parallel. In this case they are used to group together components which have high
interaction. Examples are the Controller Area Network (CAN), private CAN, and LIN buses. This allows them
to communicate with each other on a bus with reduced message congestion insuring faster and the more
timely exchange of information than if all vehicle control modules 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 modules designated as Gateway’s perform the
function of transferring information between the various buses. A Gateway module is connected to at least 2
buses and will interact with each network according to its message strategy and transmission models.
CAN provides the capability for a receiving control module to monitor message transmissions from other
control modules 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 the information no longer being received.
Additionally, a control module may set a Diagnostic Trouble Code (DTC) to indicate that the control module it
is expecting information from is no longer communicating.
K9 Body Control Module (BCM)
The body control system consists of the K9 Body Control Module, communications, and various input and
outputs. Some inputs, outputs and messages require other control modules to interact with the K9 Body
Control Module. The K9 Body Control Module also has discrete input and output terminals to control the
vehicle’s body functions. The K9 Body Control Module is wired to CAN bus and multiple Local Interconnect
Network (LIN) buses and acts as a gateway between them.
The various K9 Body Control Module input and output circuits are illustrated in the corresponding functional
areas on the K9 Body Control Module electrical schematics. Refer to the Body Control System Schematics for
more detailed information.
K56 Serial Data Gateway Module
The K56 Serial Data Gateway Module gates messages between the CAN networks described in the Controller
Area Network (CAN) Bus Description section below. The K56 Serial Data Gateway Module needs to know
what CAN control modules are present on a given vehicle in order to enable/disable loss of communication
DTCs and to know what CAN control modules to track for their communication status. The K56 Serial Data
Gateway Module has the ability to learn the diagnostic addresses list of CAN control modules to identify what
CAN control modules are equipped on the vehicle and what CAN buses they are on. If the K56 Serial Data
Gateway Module is replaced, this learn/verification process will have to be done again through K56 Serial
Data Gateway Module programming and setup procedure in SPS. This learn process will not cause any
previously learned contents to be forgotten/overwritten. If the learn process is not done on a new K56 Serial
Data Gateway Module, DTC U1977 will be set until the learn procedure is executed. If the learn is invalid due
to control module internal malfunction or a K56 Serial Data Gateway Module swap, DTC U3000 42 or DTC
U3002 56 will be set. If any of these DTCs sets, the K56 Serial Data Gateway Module will enable loss of
communication for all CAN control modules. This will result in loss of communication DTCs being set against
CAN control modules that are not equipped on the vehicle.
A fault can be localized by monitoring the normal mode messages on a CAN bus. The K56 Serial Data
Gateway Module will monitor one signal per CAN control module per CAN bus to determine control module
status. When a signal times out, a loss of communication event will be started.
Controller Area Network (CAN) Bus Description
The CAN buses are used where data needs to be exchanged at a high enough rate to minimize the delay
between the occurrence of a change in sensor value and the reception of this information by a control device
using the information to adjust vehicle system performance.
Each CAN serial data network consists of two twisted wires. One signal circuit is identified as CAN-High and
the other signal circuit is identified as CAN-Low. At each end of the data bus there is a 120 Ω termination
resistor between the CAN-High and CAN-Low circuits. Most CAN control modules have an internal resistance
of 4.950K Ω. There may be one or two CAN control modules that have a higher internal resistance like the
K60 Column Lock Module which has an internal resistance of 77.4K Ω. The internal resistance of CAN control
modules causes lower terminating resistor reading when splitting the CAN network to check for faults. The
more CAN control modules on the network the lower the terminating resistor will read.
The data to be transmitted over a CAN bus is represented by the voltage difference between the CAN-High
signal voltage and the CAN-Low signal voltage. Data symbols (1’s and 0’s) are transmitted sequentially at
the following rate:
CAN 1 (circuits 4986 & 4987) = 500 Kbit/s
CAN 2 (circuits 4978 & 4979) = 2 Mbit/s
CAN 3 (circuits 4976 & 4977) = 500 Kbit/s
CAN 4 (circuits 4100 & 4101) = 500 Kbit/s
CAN 5 (circuits 4984 & 4985) = 500 Kbit/s
CAN 6 (circuits 4980 & 4981) = 5 Mbit/s
CAN 7 (circuits 4982 & 4983) = 5 Mbit/s
CAN 8 (circuits 4104 & 4105) = 2 Mbit/s
CAN 9 (circuits 4102 & 4103) = 2 Mbit/s
When the two wire bus is at rest the CAN-High and CAN-Low signal circuits are not being driven and this
represents a logic “1”. In this state both signal circuits are at the same voltage of 2.5 V. The differential
voltage is approximately 0 V.
When a logic “0” is to be transmitted, the CAN-High signal circuit is driven higher to about 3.5 V and the
CAN-Low circuit is driven lower to about 1.5 V. The differential voltage becomes approximately 2.0 (+/- 0.5)
V.
The CAN 1, CAN 2, CAN 3, CAN 4, CAN 5, CAN 8, and CAN 9 buses are used to communicate between the
K56 Serial Data Gateway Module and other CAN control modules.
The CAN 8, and CAN 9 buses are reserved for the following systems:
The CAN 8 bus is reserved for most control modules and sensors related to active safety system, if
applicable.
The CAN 9 bus is reserved for most control modules and sensors related to Hybrid/EV system, if
applicable.
The following CAN buses are between the X84 Data Link Connector and the K56 Serial Data Gateway
Module:
The CAN 6 bus is used for CAN diagnostics and programming.
The CAN 7 bus is used for programming by assembly plant only.
The Private Presentation CAN 1 bus (circuits 2577 & 2578) is used by Engineering to observe data
communications on CAN buses not directly accessible at the X84 Data Link Connector. It requires
special security access and will not be used in a service environment.
The Private Presentation CAN 2 bus (circuits 2579 & 2580) is used by Engineering to observe data
communications on CAN buses not directly accessible at the X84 Data Link Connector. It requires
special security access and will not be used in a service environment.
Private Powertrain CAN Bus Description
The Private Powertrain CAN bus (circuits 4054 & 4055) is reserved for Powertrain components. It has a
transmission rate of 500 Kbit/s. Sometimes communication is required between the Private Powertrain CAN
bus and another CAN bus. This is accomplished by using the K20 Engine Control Module (for gas vehicles) or
K16 Battery Energy Control Module (for electric vehicles) as the Gateway module. Since the Private
Powertrain CAN bus and other CAN buses operate in the same manner, the diagnostics for each are similar.
Local Interconnect Network (LIN) Bus Description
The LIN Bus consists of a single wire with a transmission rate of 10.417 Kbit/s. This bus is used to exchange
information between a master control module and other smart devices which provide supporting
functionality. This type of configuration does not require the capacity or speed of a CAN bus and is thus
relatively simpler.
The data symbols (1’s and 0’s) to be transmitted are represented by different voltage levels 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 about ground (0.0 V).
Ethernet Bus Description
Ethernet is a data communication technology that uses a single twisted copper pair of wires at speeds of 100
Mbit/s and 1000 Mbit/s. The Ethernet system uses point-to-point communication that is connected via an
Ethernet switch [Module <–> Switch <–> Module]. The Ethernet bus does not use terminating resistors.
The K56 Serial Data Gateway Module and the A11 Radio have an Ethernet switch that connects to other
Ethernet modules. The K56 Serial Data Gateway Module and the A11 Radio communicate with other devices
and systems in the vehicle via CAN and LIN buses. Diagnostic Trouble Codes will be read on CAN to diagnose
Ethernet, LIN and system faults.