Data Link Communications Description and Operation
Note: This is an overview
of different serial data buses used by GM control modules to
communicate with each others. Use
Data Communication Schematics to 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 networks provide 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 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. 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 characteristics and simply operate in parallel.
In this case they are used to group together components which have
high interaction. Examples are the High Speed GMLAN, Powertrain
Expansion, and Chassis Expansion 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.
GMLAN 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 to
indicate that the control module it is expecting information from
is no longer communicating.
High Speed GMLAN Circuit Description
A High Speed GMLAN Bus is 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.
The High Speed GMLAN serial data network consists of two twisted
wires. One signal circuit is identified as GMLAN-High and the other
signal circuit is identified as GMLAN-Low. At each end of the data
bus there is a 120 Ω termination resistor between the
GMLAN-High and GMLAN-Low circuits.
Data symbols (1's and 0's) are transmitted sequentially at a
rate of 500 Kbit/s. The data to be transmitted over the bus is
represented by the voltage difference between the 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 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 GMLAN-High signal
circuit is driven higher to about 3.5 V and the GMLAN-Low circuit
is driven lower to about 1.5 V. The differential voltage 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 except that its use is reserved for chassis
components. This implementation splits message congestion between
two parallel buses helping to insure timely message transmission
and reception. Sometimes communication is required between the
Chassis Expansion Bus and the primary High Speed GMLAN Bus. This is
accomplished by using the Electronic Brake Control Module (EBCM) as
the Gateway module. Since the High Speed GMLAN Chassis Expansion
Bus and primary High Speed GMLAN Bus operate in the same manner,
the diagnostics for each are similar.
CAN Graphical Interface (CGI) Circuit Description
This bus 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. The electrical characteristics of
the CAN Graphical Interface (CGI) Bus are very similar to the High
Speed GMLAN Bus. The message strategy and construction of messages
are different however. Sometimes communication is required between
the CAN Graphical Interface Bus and the Low Speed GMLAN Bus. This
is accomplished by using the Radio SilverBox as the Gateway module.
Since the CAN Graphical Interface Bus and primary High Speed GMLAN
Bus have similar electrical characteristics, the diagnostics for
each are similar.
In the case where the Info Display Module and Radio/HVAC Control
are separate control modules the Info Display Module is responsible
for passing information between the Radio and the Radio/HVAC
Control. The Radio interfaces only with the Info Display Module and
the Info Display Module then communicates with the Radio/HVAC
Control through a Local Interconnect Network (LIN) interface.
A bus wake up signal will be generated by the Radio or by the
Info Display Module when the system functionality is required. The
communication function of the CAN Graphical Interface shall be
enabled or disabled based on the voltage level of the Center Stack
Wake. The network will stay awake as long as the circuit voltage is
driven low, to less than 1.5 V. Communications are disabled with a
high circuit voltage around 5.0 V.
The Radio can execute a warm reset of the Info Display Module if
the Info Display Module fails to respond to the Radio's request.
The 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.
Mid Speed GMLAN Circuit Description
The Mid Speed GMLAN Bus is very similar to the High Speed GMLAN
Bus except that it uses a slower transmission rate of 125 Kbit/s.
This bus is intended for use where the system response time demands
that a large amount of data be transmitted in a relatively short
amount of time, such as updating a graphics display. As such it has
usually been used for infotainment applications. Sometimes
communication is required between the Low Speed GMLAN Bus and the
Mid Speed GMLAN Bus. This is accomplished by using the Radio
(Silverbox) as the Gateway module. Since the Mid Speed GMLAN Bus
and primary High Speed GMLAN 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 required which allows for the use of less complex
components. It is typically used for operator controlled functions
where the response time requirements are slower than those required
for dynamic vehicle control.
The Low Speed GMLAN Serial Data Network consists of a single
wire, ground referenced bus with high side voltage drive. During on
road vehicle operation data symbols (1's and 0's) are transmitted
sequentially at the normal rate of 33.3 Kbit/s. For component
programming 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 not use terminating resistors at either end of
the network.
The data symbols to be transmitted over the bus are represented
by different voltage signals 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 around 4.0 V or higher.
Local Interconnect Network (LIN) Circuit Description
The Local Interconnect Network (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 either a
High Speed GMLAN Bus 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 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).
Communication Enable Circuit Description
Control modules on GMLAN high speed type networks, excluding the
Mid Speed GMLAN Bus and CAN Graphical Interface Bus, enable or
disable communication based on the voltage level of this circuit.
When the circuit voltage is high (around 12 V), communications are
enabled. When the circuit is low, communications are disabled.
The CAN Graphical Interface Bus is similar but uses different
voltage levels. See description above for the CAN Graphical
Interface Bus.
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:
• |
Pin 1 Low speed GMLAN
communications terminal |
• |
Pin 2 Class 2 communications
terminal |
• |
Pin 3 Mid speed GMLAN serial
bus (+) terminal |
• |
Pin 4 Scan tool power ground
terminal |
• |
Pin 5 Common signal ground
terminal |
• |
Pin 6 High speed GMLAN serial
data bus (+) terminal |
• |
Pin 7 Keyword communications
terminal |
• |
Pin 11 Mid speed GMLAN serial
bus (-) terminal |
• |
Pin 12 Chassis high speed
GMLAN serial bus (+) terminal |
• |
Pin 13 Chassis high speed
GMLAN serial bus (-) terminal |
• |
Pin 14 High speed GMLAN serial
data bus (-) terminal |
• |
Pin 16 Scan tool power,
battery positive voltage terminal |
Serial Data Reference
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 control 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 optional
control module. In order to avert misdiagnoses of No Communication
with a specific control module, refer to
Data Link References for a list of control modules, the buses
they communicate with, and the RPO codes for a specific control
module.
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