Data Link Communications Description And Operation - Suzuki XL7 2007 Service Manual

10B-2 Data Communications:
Data Link Communications Description and
Operation
Circuit Description
The communication among control modules is
performed primarily through the GMLAN high speed
serial data circuit and the GMLAN low speed serial data
circuits. The modules that need real time communication
are attached to the high speed GMLAN network. The
body control module (BCM) is the serial data gateway
between the networks. The purpose of the gateway is to
translate serial data messages between the GMLAN
high speed buss and the GMLAN low speed buss. The
controller area network (CAN) is another serial data
communication network used on this vehicle which is
dedicated to the electronic brake control module (EBCM)
subsystem for stabilization. Below are more detailed
descriptions of the individual networks. 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.
GMLAN High Speed Circuit Description
NOTE
A loss of serial data DTC does not represent
a failure of the module that the code is set in.
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 two 120 ohms resistors, one is internal
to the engine control module (ECM) and the other is after
the BCM in either the AWD module or a separate
resistor connector assembly. The resistors are used to
reduce noise on the High Speed GMLAN buss 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. This code is mapped on the Tech 2 screen as a
code against the physical device.
GMLAN Low Speed Circuit Description
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 GMLAN low speed is up to 83.33 Kb/s. Under
normal vehicle operating conditions, the speed of the
buss is 33.33 Kb/s. This protocol produces a simple
pulse train sent out over the GMLAN low speed serial
data bus. When a module pulls the buss high, 5 volts,
this creates a dominant logic state or 0 on the buss.
When the buss is pulled low, 0 volts, it is translated as a
recessive logic state or 1. To wake the control modules
connected to the GMLAN low speed serial data buss, a
voltage wake up pulse is sent out over the buss, the
voltage level of the pules is +10 volts. Modules
connected to the GMLAN low speed buss can be part of
a virtual network as described in the previous paragraph.
The modules on the GMLAN low speed serial data buss
are connected to the buss in a parallel configuration.
Controller Area Network (CAN) Description
The EBCM communicates with the yaw rate sensor and
the steering wheel angle sensor exclusively, when
equipped. This serial data system does not
communicate with the scan tool however; there are loss
of communication DTCs for this system which set in the
EBCM. The yaw rate sensor and the steering wheel
angle sensors are in effect modules, exchanging
essential information with the EBCM. For more
information about these sensors refer to "ABS
Description and Operation: ".
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 GMLAN low speed communications 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
• Pin 14 High speed GMLAN serial data bus (−)
terminal
• Pin 16 Scan tool power, battery positive voltage
terminal