System Operation: Operation

Network Termination 

The Controller Area Network(CAN) uses network termination to improve communication reliability. Termination modules are located at both ends of the network. As network messages are broadcast in the form of voltage signals, the network voltage signals are stabilized by the termination resistors.

Each termination module has an internal 120 ohm resistor that bridges across the positive and negative bus connection. With two 120 ohm resistors located in a parallel circuit configuration, the total network impedance, or total resistance, is 60 ohms.

Network termination improves bus message reliability by:

• stabilizing bus voltage.
• eliminating electrical interference.

Gateway Module 

The Body Control Module(BCM) and Instrument Panel Cluster(IPC) are the gateway modules. The BCM translates messages on the HS-CAN to the MS-CAN and vice versa. The IPC translates messages on the HS-CAN to the I-CAN and vice versa. This allows a message to be distributed through all three networks. The BCM and IPC are the only modules on this vehicle that have this ability.

High Speed Controller Area Network (HS-CAN) 

The HS-CAN operates at a maximum data transfer speed of 500 Kbps and is designed for real time powertrain information transfer and control.

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

Medium Speed Controller Area Network (MS-CAN) 

The MS-CAN operates at a maximum data transfer speed of 125 Kbps and is designed for general information transfer.

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

Infotainment Controller Area Network (I-CAN) 

The I-CAN operates at a maximum data transfer speed of 500 Kbps and is designed for real time audio and multimedia information transfer and control.

Modules on the I-CAN communicate using bussed messages. I-CAN uses an unshielded twisted pair cable, data bus (+) and data bus (-) circuits. The I-CAN allows sharing of information between all modules on the network.

The I-CAN is connected to the DLC, but the I-CAN modules communicate with the scan tool over the HS-CAN. The IPC is used as a gateway module for the messages to transfer between the scan tool and the modules on the I-CAN.

Controller Area Network (CAN) Fault Tolerance 

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 in illustration. Compare any suspect readings to a known good vehicle.

Normal CAN Operation 

Courtesy of FORD MOTOR COMPANY

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 

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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 

Courtesy of FORD MOTOR COMPANY

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 

Courtesy of FORD MOTOR COMPANY

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 

Courtesy of FORD MOTOR COMPANY

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 

Courtesy of FORD MOTOR COMPANY

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 

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Rhythmic oscillations, inductive spikes or random interference can disrupt 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.

Communication Message Chart