There are other types of automotive networks which use fiber optic cables to broadcast complex network messages using light pulses, notes Ord. For 2007 to 2010 Sprinters, the MOST (Media Oriented Systems Transport) fiber optic network is used in some vehicles for in-vehicle entertainment, communications and navigation systems.
For 2002 to 2005 Sprinters, there is only one network (see Diagram 1): chassis/powertrain CAN (CAN C low speed). For 2007 to 2010 Sprinters, there are four (see Diagram 2): powertrain/chassis M-CAN (ISO CAN C high speed); interior (body) I-CAN (ISO CAN B low speed); diagnosis D-CAN (high speed proprietary MB protocol); and Digital Media Bus for radio/navigation system (MOST).
The networks connect to each other through gateway modules, he says. These gateway control units serve the same purpose as a network hub switch on an office computer network for different rooms or groups. The gateway control units, which also serve as a sort of translator from one type of network to the other, are arranged as follows:
• The instrument cluster (IC) is the interface between M-CAN and I-CAN.
• The electronic ignition switch (EIS) is the interface between M-CAN and I-CAN and between D-CAN and OBD2 diagnostic connector.
• The radio (COMMAND Audio Gateway) is the interface between I-CAN and MOST.
"To begin with, it is important to know that the CAN Data Bus is a bi-directional Bus, that is, each connected control unit is able to send and receive messages," says Ord.
The CAN Data-Bus consists of a twisted-pair cable that is similar to network cables used for computer networks or telecommunications. All control units (computers) that are in the vehicle are connected to these cables. CAN cables are connected together at junction boxes or Z-splices, also called the J-Splice in some factory literature because Mercedes uses a "J" for junction to describe the location. Splices are soldered connections.
CAN-Bus cables express specific voltages when they are active and when they are idle. Each CAN cable is either CAN H, for high, or CAN L, for low. With CAN H cables, the voltage rises when CAN is active; with CAN L cables, the voltage drops when the CAN is active. Depending on the vehicle, type of CAN and speed inherent, voltage levels are a fixed value when sending or receiving.
Data is transmitted over both cables with the Bus levels being a mirror image of each other. In other words, if the level on one cable is 0, the other cable transmits the level 1 and vice versa, as a replacement for on/off signals from analog components in older systems.
"CAN-Bus cables are a twisted pair design. If troubleshooting wiring problems and a pair of cables are found twisted to each other, you may have just found the CAN-Bus network cables. The twisted pair wire concept is used for two reasons: for voltage fault identification and as a redundant safety concept. If a voltage peak occurs on just one line due to interference problems, the receivers can identify this as a fault and ignore the voltage peak.
"The twin cable design is also handy if a short circuit or interruption occurs on one of the two CAN lines. Software in the control units can reroute messages by switching to a single line. The damaged data line is shut down with CAN message speed slowed, but not stopped. Software on networked control units manages how and when the control units on CAN can send and receive messages."
The data flow path is important to understand for effective diagnosis, Ord points out. The 2002 to 2006 Sprinter CAN is set up in order of data flow. The engine control module is the first ECU, the immobilizer control unit (referred to as WSP for Chrysler; SKREEM by Mercedes-Benz) is the last. The CAN terminates at the ECM (CDI/CR control unit) and at the immobilizer control unit with a 120O resistor. In the 2007 to 2010 Sprinter, data flow is different as there are several networks and interfaces between the networks.
Each control unit sends and receives the messages, but only some will use them. Messages are sent along the whole CAN-Bus to all ECUs that are connected, explains Ord. The messages are disregarded and just sent along if it has no function for that ECU. The message will continue until it reaches the intended recipient for processing into a function or change.