The generic electronic module (GEM) knows which modules are in the Controller Area
Network (CAN) and checks whether all modules are communicating. If a module is not
communicating a trouble code (DTC) is set in the GEM. There is a trouble code (DTC) for
every module.
Faulty communications
If the Controller Area Network (CAN) is in operation then the generic electronic module
(GEM) continuously monitors the flow of information in the network. If the GEM determines
that, for some reason, the communications in the CAN bus are interrupted, it can set a
trouble code (DTC). The GEM has trouble codes for all parts of the CAN bus (one each for
the HS CAN bus and the MS/MM CAN bus).
With the exception of the GEM, which only knows one type of DTC, each module uses two
types of DTC in relation to communication faults. These are:
Faulty messages
Faulty configuration
As the GEM is the main module in the network, it does not have a DTC for faulty
configuration.
Faulty messages
The control modules continuously monitor the flow of information in the Controller Area
Network (CAN). If a module receives a message it cannot interpret it sends a fault message
via the CAN bus. The modules also have function which allows them to detect any faulty
messages they generate themselves. This prevents interference on the CAN bus. If there is
a lot of interference in the network then any modules which cannot communicate properly
can switch themselves off. This status is referred to as "databus OFF" - the module can no
longer send or receive information.
To prevent the vehicle from coming to a sudden standstill or suffering a total loss of
particular functions if a fault occurs in the CAN bus, certain modules have an emergency
running mode. This means that modules which are required for driving or for safety aspects
(e.g. the powertrain control module (PCM) or the transmission control module (TCM)) can
maintain a restricted functionality on the basis of predefined or estimated data. For
example, if communications to the transmission control module (TCM) are interrupted then
the transmission will use a preselected gear which will enable the customer to at least drive
to the nearest workshop. A module remains in "databus OFF" mode until the power supply
to the module is switched off. If the power supply is reconnected the module attempts to
re-establish communications.
Faulty configuration
The generic electronic module (GEM) transmits its configuration ID within the messages it
sends out to other modules. To enable the modules in the Controller Area Network (CAN) to
communicate with each other they must have the same configuration ID, as modules only
look at messages carrying their own configuration ID. If the signal configuration of a
module does not match the signal configuration of the GEM then a trouble code (DTC) is
stored in the module with the faulty configuration.
NOTE: A module will save a trouble code (DTC) if it does not receive any messages from
the generic electronic module (GEM). This also applies if there are any software errors in
the module. If the circuit in the Controller Area Network (CAN) is interrupted then certain
modules will also not receive any messages from the GEM.
Local Interconnect Network (LIN)
The Local Interconnect Network (LIN) bus is a standard which has been developed for cost-
effective communications between intelligent sensors and actuators in motor vehicles. LIN
is used wherever the bandwidth and versatility of CAN is not required.
The LIN specification comprises the LIN protocol, a standard format for describing a