CAN Communication

Power steering control module, vehicle theft deterrent module, radio, HVAC control module, sensing and diagnostic module, digital radio receiver, remote control door lock receiver, sunroof control module. Wait, there's much more. Heated seat control module, DVD player, garage door opener, body control module, adjustable pedals control module, electronic brake control module, turn signal hazard flasher module. It goes on and on. It's commonplace today to see vehicles with these electronic control units and more.

In order to facilitate all of the new electronics and still keep the amount of hardware, wiring and connectors to a minimum, a wide range of serial data strategies are being used. By using this type of architecture, the total circuitry in a given vehicle can be reduced as much as 75 percent.

Serial Communication

The types of serial communication used by all manufacturers have quickly evolved from low-end, non-emission general purpose communication (UART) to real-time safety-related and emission systems communication (CAN). (Fig. 1) Types of serial communication used over the years include:

 

Fig. 1

·       UART (Universal Asynchronous Receive-Transmit) 

·       E&C (Entertainment and Comfort)

·       SBI (Simple Bus Interface) 

·       SPI (Serial Peripheral Interface)

·       LIN (Linear Interconnect Network)

·       Keyword 81, 82 and 2000

·       Class 2

·       CAN (Controller Area Network)

How CAN Works

GM Local Area Network (GMLAN) communication is the latest serial data protocol on GM vehicles. It is a reliable (less susceptible to interference), cost-effective (less complicated network) and modular (virtual networks) way to handle sharing between different electronic control units in the vehicle through the means of a family of serial communication buses using the Controller Area Network protocol.

The GMLAN supports three separate buses: low-speed (33.33 kilobits per second), mid-speed (95.2 kilobits) and high-speed (500 kilobits). Each bit is 2 microseconds, compared to 5-6 milliseconds on a Class 2 line. High-speed serial data is transmitted on two twisted wires to reduce interference. When active, one line goes high (3.5 volts) and the second line goes low (1.5 volts). Modules that need real-time communication are attached to the high-speed network.

In most vehicles, the BCM is the gateway that translates the serial data messages between the GMLAN high-speed bus and other types of serial data communication. (Fig. 2)

 

Fig. 2

CAN protocol calls for a label to be assigned to every message, giving it a unique "identifier." The identifier classifies the content of the message, such as engine speed. Each module processes only those messages whose identifiers are stored in the module's acceptance list. This is CAN's form of message filtering.

The identifier labels both the data content and the priority of the message being sent. When more than one module starts to translate simultaneously, the message with the highest priority is assigned first access.

For the Tech 2 scan tool to read GMLAN data, a CANdi (Controller Area Network diagnostic interface) module is needed.

Power Mode Master

In previous communication systems, modules required ignition input to serve as a wake-up. As a way to reduce the amount of wiring, connectors and electrical load on the ignition switch, a node such as the BCM or DIM is assigned as the Power Mode Master (PMM). The role of the PMM is to receive the ignition switch inputs and send messages related to the ignition switch position to other modules on the serial data line. These messages include:

·       Off-Asleep - No activity on the serial data circuits, modules asleep, minimum power usage

·       Off-Awake - Activity on the serial data circuits, modules awake, expecting inputs

·       RAP (Retained Accessory Power, if equipped) - Modules that have functions enabled in RAP are fully operational, other modules in Off-Awake

·       Accessory - Modules that have functions enabled in Accessory are fully operational, other modules in Off-Awake

·       Run - All modules are fully functional

·       Crank - Those modules that have no function critical to engine starting are off to provide maximum power for cranking and starting operation

The best way to check for a power moding problem is to look for DTCs. A DTC set in a module may let you know if there is a problem with a module or a basic power mode circuit fault. Some scan tools have a menu for checking power mode operation: select Diagnostic Circuit Check, then Power Mode. (Fig. 3)

 

Fig. 3

Networking

Networking is also known as multiplexing; a method of transferring data among distributed electronic control modules via a serial data bus. Without networking, dedicated, point-to-point wiring would result in bulky, expensive and even more complex wiring harnesses.

The wiring harnesses of an average-size vehicle is one mile long, weighs 200 pounds, has 300 mating connectors and 2,000 pins.

One common type of network is a ring link network, which connects each control module to the next in line by a serial data line. (Fig. 4) To isolate control module faults, each module must be disconnected from the network. Each vehicle has main network connectors that can be disconnected so that the portions of the network can be isolated and checked for faults.

 

Fig. 4

Another common type of network is a star link network. (Fig. 5) This network has a serial data line that exits from each control module and a serial data line is connected to a central point. The central point is referred to as a splice pack. Some vehicles use two or more splice packs to tie all the modules together. In most applications, the bus bar used in each splice pack can be removed. A special diagnostic tool can be installed in its place, allowing each control module to be isolated and checked for a possible fault.

 

Fig. 5

A third type of network is a ring/star hybrid, which uses both techniques to connect control modules through a serial data line. Knowledge of exactly how each control module connects to the network is necessary to know which technique to use and when to use it.

The next generation of high-speed vehicle communication may be fiber optics. Fiber optic cable is lightweight, very immune to electromagnetic interference and capable of handling data speeds of 25 megabits per second. Some Saab models feature a fiber optic communication system referred to as Media Oriented System Transfer (MOST). The cables are 2-3 millimeters thick. The transparent inner core is approximately one millimeter thick and is the master of the bus in a ring configuration.

When handling and replacing fiber optic cable, the minimum bend radius is 25 millimeters and the flashing light emitted from a disconnected cable should not be viewed at close range.

For more information about the latest communication protocols and networking systems, several ACDelco training courses are available. Visit www.acdelcotechconnect.com and click on the Training tab.

- Thanks to Mike Militello