Principles Of Operation
Communications Network
Principles of Operation
The 3 module communication networks are:
- High Speed Controller Area Network (HS-CAN)
- ISO 9141
- Standard Corporate Protocol (SCP)
Information is sent to and from individual control modules that each control specific functions. All 3 networks are connected to the Data Link Connector (DLC). The DLC can be found under the instrument panel between the steering column and the audio unit. The ISO 9141 network is for diagnostic use only while the HS-CAN and SCP network allow multiple modules to communicate (transfer data) with each other on a common network.
ISO 9141 Communications Network
The ISO 9141 network is a single wire network. The ISO 9141 network does not permit intermodule communication. When the scan tool communicates to modules on the ISO 9141 network, the scan tool must request all information; the modules cannot initiate communications. The ISO 9141 network operates at a maximum data transfer speed of 10.4 Kbps.
The following fault chart describes the specific ISO 9141 network failures and their resulting symptom:
ISO 9141 Network Fault Chart
Standard Corporate Protocol (SCP) Network
The SCP network uses an unshielded twisted pair cable of data (+) and data (-) circuits. The data (+) circuit is regulated to approximately 0.5 volts and the data (-) circuits to approximately 4.5 volts during neutral or rested network traffic. Voltage on data circuits increases/decreases as bus messages are sent. Multiple bus messages can be sent over the network SCP circuits allowing network modules to communicate with each other. The SCP network operates at a maximum data transfer speed of 41.6 Kbps and remains operational at a degraded level if one of the bus wires becomes open or shorted to ground or voltage.
SCP Network Fault Chart
High Speed Controller Area Network (HS-CAN)
The HS-CAN uses an unshielded twisted pair cable of data (+) and data (-) circuits. The data (+) and the data (-) circuits are each regulated to approximately 2.5 volts during neutral or rested network traffic. As bus 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 bus message is sent. Multiple bus messages can be sent over the HS-CAN circuits allowing network modules to communicate with each other.
Controller Area Network (CAN) Fault Tolerance
NOTE: The oscilloscope traces below are from the Integrated Diagnostic System (IDS) oscilloscope taken using the IDS pre-configured Controller Area Network (CAN) settings. The traces are for both data (+) and data (-) circuits taken simultaneously (2-channel) at 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 in the CAN utilize simultaneous communication of 2 or more message on the same network circuits. The following chart summarizes the messages sent and received on the network.
NOTE: Some HS-CAN faults may result in a no-start condition.
CAN Module Communication Message Chart
