Information Bus: Description and Operation
COMMUNICATIONS NETWORKNOTE: The smart junction box (SJB) is also known as the generic electronic module (GEM).
Vehicle communication utilizes the International Standards Organization (ISO) 9141 network, the medium speed controller area network (MS-CAN), and the high speed controller area network (HS-CAN). 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 control module (ACM).
The 3 module communication networks are:
- ISO 9141
- Medium speed (MS) CAN
- High speed (HS) CAN
All 3 networks are connected to the DLC. This makes diagnosis and testing of these systems easier by allowing one scan tool to be able to diagnose and control any module on the 3 networks from one connector. The DLC can be found under the instrument panel between the steering column and the ACM.
Network Topology:
ISO 9141 Network Operation
The ISO 9141 communication network is a single wire network, used for diagnostic purposes only.
The ISO 9141 communication network is used for the following module:
- RCM
MS-CAN Network Operation
The MS-CAN network 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 medium speed communication network used for the following modules:
- IC
- Audio control module (ACM)
- SJB
- Satellite digital audio receiver system (SDARS) module (if equipped)
HS-CAN Operation
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 high speed communication network used for the following modules:
- Instrument cluster (IC)
- PCM
- ABS module (if equipped)
Network Termination
The CAN uses a network termination circuit to improve communication reliability. The network termination of the CAN bus takes place inside the termination modules by termination resistors. Termination modules are located at either end of the bus 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 a 120 ohm resistor across the positive and negative bus connection in the termination module. With 2 termination modules on each network, and the 120 ohm resistors located in a parallel circuit configuration, the total network impedance (total resistance) is 60 ohms.
Network termination improves bus message reliability by:
- stabilizing bus voltage.
- eliminating electrical interference.
Gateway Module
The IC is the gateway module, translating HS-CAN to MS-CAN and vice versa. This information allows a message to be distributed throughout both networks. The IC is the only module on this vehicle that has this ability.