Detailed description: C-cable: Notes
WARNING: This page is about a different car, the 2005 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.
A C-cable is a communications lead, which in actual fact is a single-wire bus for slow communication between two modules. Such a lead has a maximum transfer rate of 10 kbit/s. Communication is serial and bi-directional. The diagnostic tool is connected to the bus, as are all control modules. All the same, there are a number of C-cables.
Most often, one or both of the modules use a pull-up resistor to raise the voltage on the lead to close on B+. Transmission and reception then take place in accordance with a predetermined protocol. Component manufacturers use the protocol as a standard for message structure to avoid misunderstandings.