Communications Network: Notes

WARNING: This page is about a different car, the 2009 Ford Flex. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: The Smart Junction Box (SJB) is also known as the Generic Electronic Module (GEM).

Multiplexing is a method of sending 2 or more signals simultaneously over a single circuit. Multiplexing is used to allow 2 or more electronic modules (nodes) to communicate simultaneously over a twisted-wire pair [data (+) and data (-)] network. The information or messages that can be communicated on these wires consists of commands, status or data. The advantage of using multiplexing is to reduce the weight of the vehicle by reducing the number of redundant components and electrical wiring.

The vehicle has 2 module communication networks which are connected to the Data Link Connector (DLC), located under the instrument panel:

High Speed Controller Area Network (HS-CAN)
Medium Speed Controller Area Network (MS-CAN)

The HS-CAN and MS-CAN are defined by various ISO documents:

ISO 11898 specifies the electrical, mechanical, and procedural interface for CAN serial communication technology.
ISO 15765 specifies the protocol of CAN-based network systems and specifies implementation of a common set of unified diagnostics.
ISO 14229 specifies the vehicle diagnostic architecture and changes the way PIDs, DTCs, and active commands are processed and displayed on the scan tool.