Circuit Description
The serial data communication among the control modules is performed through
multiple serial data communication links. The class 2 serial data line is always
present. The high speed GMLAN or controller area network (CAN) may be present for
different vehicle options. The data link connector (DLC) allows a scan tool to communicate
with the modules on class 2 or high speed GMLAN serial data lines. In order
for the scan tool to communicate with the modules on high speed GMLAN link, a CANdi
module is needed. The CANdi module behaves as an interface between the scan tool and
the high speed GMLAN data link. The CAN is used for functional communication only,
among the modules connected to it.
Data Link Connector (DLC)
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:
• | Scan tool power battery positive voltage at terminal 16 |
• | Scan tool power ground at terminal 4 |
• | Common signal ground at terminal 5 |
• | Class 2 signal at terminal 2 |
• | High speed GMLAN serial data bus (+) at terminal 6 |
• | High speed GMLAN serial data bus (-) at terminal 14 |
Class 2 Serial Data Line
Class 2 serial data is transmitted on a single wire at an average of
10.4 kbps. This value is an average, class 2 uses a variable pulse width
modulation to carry data and depending on the message it may operate faster or slower.
The bus will float at a nominal 7 volts during normal operation. Each module
can pull this lower during the transmission. The bus is not at battery positive voltage
or ground potential during normal operation. When the ignition switch is in RUN, each
module communicating on the class 2 serial data line sends a state of health
(SOH) message every 2 seconds to ensure that the module is operating properly.
When a module stops communicating on the class 2 serial data line, for example
if the module loses power or ground, the SOH message it normally sends on the data
line every 2 seconds disappears. Other modules on the class 2 serial
data line, which expect to receive that SOH message, detect its absence; those modules
in turn set an internal DTC associated with the loss of SOH of the non-communicating
module. The DTC is unique to the module which is not communicating, for example, when
the inflatable restraint sensing and diagnostic module (SDM) SOH message disappears,
several modules set DTC U1088. Note that a loss of serial data DTC does not
normally represent a failure of the module that set it.
The class 2 serial data line on this vehicle is a star configuration.
The powertrain control module (PCM) has an additional class 2 serial data
circuit to body control module (BCM). If one of the class 2 serial data circuits
to the PCM opens, communication will not be interrupted. The following modules communicate
on the class 2 serial data line:
• | The audio amplifier (AMP), w/UQ7 and Y91 |
• | The body control module (BCM) |
• | The communication interface module (OnStar®), w/UE1 |
• | The digital radio receiver (DRR), w/U2K |
• | The driver door module (DDM) |
• | The electronic brake control module (EBCM) |
• | The generator battery control module (GBCM) |
• | The HVAC control module |
• | The hybrid control module (HCM), w/HP2 |
• | The inflatable restraint sensing and diagnostic module (SDM) |
• | The instrument panel cluster (IPC) |
• | The memory seat module (MSM), w/AN3 |
• | The passenger door module (PDM) |
• | The powertrain/engine control module (PCM/ECM) |
• | The rear seat audio (RSA) controller, w/UK6 |
• | The rear seat entertainment (RSE) assembly w/U42 |
• | The rear wheel steering control module (RWSCM), w/NYS |
• | The remote playback device - CD player (CDX), w/U1S |
• | The transfer case shift control module (TCSCM), w/NP1/NP8 |
• | The transmission control module (TCM), w/M74 |
The class 2 serial data line allows a scan tool to communicate with
these modules for testing purposes, checking for DTCs, and to activate/enable/disable
functions. These class 2 serial data circuits are bussed together via 2 splice
packs:
• | SP205--Located behind the instrument panel (I/P) near the headlamp
switch connector. |
• | SP207--Located in the I/P harness. |
Refer to
Computer/Integrating Systems Component Views
.
Controller Area Network (CAN) Serial Data Line w/6.6L Engine
The controller area network (CAN) serial data line does not communicate with
the scan tool via the data link connector (DLC). CAN serial data information is interpreted
by the modules that are on the class 2 also, and transmitted to the scan tool.
The following modules communicate on the CAN serial data line:
• | The engine control module (ECM) |
• | The transmission control module (TCM) |
• | The fuel injection control module (FICM) |
• | The glow plug control module (GPCM) |
The CAN serial data line, based on J1939 SAE standard, is high speed serial
data bus used to communicate information among the modules. Typical data-transmission
speeds must be high enough to ensure that the required real-time response is maintained.
The addressing scheme employed with CAN assigns a label to every message, with
each message receiving a unique identifier. The identifier classifies the content
of the message. Each module processes only those messages whose identifiers are stored
in the module's acceptance list. This is CAN's form of message filtering.
The identifier labels both the data content and the priority of the message
being sent. Each module can begin transmitting its most important data as soon as
the bus is unoccupied. When more than one module starts to transmit simultaneously,
the message with the highest priority is assigned first access. A module responds
to failure to gain access by automatically switching to receive mode, the module
then repeats the transmission attempt as soon as the bus is free again.
High Speed GMLAN Serial Data Line (PHT)
The data link connector (DLC) allows a scan tool to communicate with the high
speed GMLAN serial data circuit through the CANdi module. The serial data is transmitted
on two twisted wires that allow speed up to 500 Kbps. The twisted pair is terminated
with two 120-ohm resistors, one is internal to the engine control module (ECM) and
the other is internal to the energy storage control module (ESCM). 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
a recessive transmitted data and is interpreted as a logic 1. Driving the lines
to their extremes adds 1 volt to high speed GMLAN serial data bus (+) and subtracts
1 volt from 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's 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. This code is mapped as a code against the physical device.
Note that a loss of serial data DTC does not normally represent a failure of the module
that set it.
The powertrain control module (PCM) and hybrid control module (HCM) are functionally
communicating on both class 2 and high speed GMLAN data links. However the
diagnostic communication of the above modules with the scan tool is performed on class 2
data link only. All DTCs set by the high speed GMLAN modules, including HCM, are reported
by the PCM on class 2. Data displaying, output controls and programming of
the control modules connected to high speed GMLAN only, is performed through high
speed GMLAN link.
The GMLAN serial data communications circuit is in a linear topology. The following
modules are connected to the link, in order from DLC to the end of the linear configuration:
• | The energy storage control module (ESCM) |
• | The starter/generator control module (SGCM) |
• | The electro-hydraulic power steering (EHPS) |
• | The hybrid control module (HCM) |
• | The powertrain control module (PCM) |