Principles of Operation

Communications Network

Principles of Operation

NOTE: The Smart Junction Box (SJB) is also known as the Generic Electronic Module (GEM).

Both the High Speed Controller Area Network (HS-CAN) and Medium Speed Controller Area Network (MS-CAN) use an unshielded twisted-pair cable of data (+) and data (-) circuits.

The HS-CAN operates at a maximum data transfer speed of 500 Kbps and is designed for real time information transfer and control. The MS-CAN operates at a maximum data transfer speed of 125 Kbps for bus messages and is designed for general information transfer.

ISO 9141 Network Operation

The ISO 9141 communications network is a single wire network, used for diagnostic purposes only.

The ISO 9141 communications network is used for the following modules:

- 4X4 control module

High Speed Controller Area Network (HS-CAN) Modules

The HS-CAN communicates using bussed messages. The HS-CAN uses an unshielded twisted pair cable, data bus (+) and data bus (-) circuits. In addition to scan tool communication, this network allows sharing of information between all modules on the network.

The HS-CAN is a communication network used for the following modules:

NOTE: The Electronic Variable Response Turbocharger (EVRT) actuator does not communicate with the scan tool.

- ABS module
- Accessory Protocol Interface Module (APIM)
- Auxiliary Heater Control Module (AHCM)
- EVRT actuator
- Instrument Cluster (IC)
- PCM
- Restraints Control Module (RCM)
- Trailer Brake Control (TBC) module
- Transmission Control Module (TCM)

Medium Speed Controller Area Network (MS-CAN) Modules

The MS-CAN communicates using bussed messages. The MS-CAN has an unshielded twisted pair cable, data bus (+) and data bus (-) circuits. In addition to scan tool communication, this network allows sharing of information between all modules on the network.

The MS-CAN is a communication network used for the following modules:

- Audio Control Module (ACM)
- Accessory Protocol Interface Module (APIM)
- Circuit Deactivation Ignition Module (CDIM)
- Rear Entertainment Module (RETM)
- Driver Seat Module (DSM)
- Parking Aid Module (PAM) (if equipped)
- Global Positioning System Module (GPSM)
- HVAC module
- IC
- Satellite Digital Audio Receiver System (SDARS) module
- SJB

Controller Area Network (CAN) Fault Tolerance

NOTE: The oscilloscope traces below are from the IDS oscilloscope taken using the IDS pre-configured CAN settings. The traces are for both data (+) and data (-) taken simultaneously (2-channel) at a sample rate of 1 mega-sample per second (1MS/s) or greater. Traces below are viewed at 500mV per division (vertical axis) and 20 microseconds (20�s) per division (horizontal axis). Readings taken with a different oscilloscope vary from those shown. Compare any suspect readings to a known good vehicle.

Normal CAN Operation





The data (+) and data (-) circuits are each regulated to approximately 2.5 volts during neutral or rested network traffic. As messages are sent on the data (+) circuit, voltage is increased by approximately 1.0 volt. Inversely, the data (-) circuit is reduced by approximately 1.0 volt when a message is sent.

Successful communication of a message can usually be identified by the slight spike at the end of a message transmission. Any signals that are significantly different than the normal CAN waveform may cause network DTCs (U-codes) to set or may cause a complete network outage.

CAN Circuits Shorted Together





In the event that the data (+) and data (-) circuits become shorted together, the signal stays at base voltage (2.5V) continuously and all communication capabilities are lost.

CAN (+) Circuit Shorted To Ground





In the event that the data (+) circuit becomes shorted to ground, both the data (+) and data (-) circuits are pulled low (0V) and all communication capabilities are lost.

CAN (-) Circuit Shorted To Ground





In the event that the data (-) circuit becomes shorted to ground, the data (-) circuit is pulled low (0V) and the data (+) circuit reaches near-normal peak voltage (3.0V) during communication but falls to 0V instead of normal base voltage (2.5V). Communication may continue but at a degraded level.

CAN (+) Circuit Shorted To Battery Voltage





In the event that the data (+) circuit becomes shorted to battery voltage, the data (+) circuit is pulled high (12V) and the data (-) circuit falls to abnormally high voltage (above 5V) during communication and reaches battery voltage (12V) for peak voltage. Communication may continue but at a degraded level.

CAN (-) Circuit Shorted To Battery Voltage





In the event that the data (-) circuit becomes shorted to battery voltage, both the data (+) and data (-) circuits are pulled high (12V) and all communication capabilities are lost.

CAN Circuit Signal Corruption





Rhythmic oscillations, inductive spikes or random interference can corrupt the network communications. The corruption signal source may be outside electrical interference such as motors or solenoids or internal interference generated from a module on the network. In some cases, an open in either the data (+) or data (-) circuit to a network module may cause the module to emit interference on the one circuit which is still connected. The trace shown is an example of a "sawtooth" pattern transmitted from a module with one open network circuit.

Other corruptions may be present when a module is intermittently powered up and down. The module on power up may initiate communication out of sync with other modules on the network causing momentary communication outages.


Controller Area Network (CAN) Multiplex Messages

Modules on the CAN utilize simultaneous communication of 2 or more messages on the same network circuits. The following chart summarizes the messages sent and received on the network.

CAN Module Communication Message Chart