Vehicle Communication

VEHICLE COMMUNICATION
The Chrysler Collision Detection multiplex (CCD bus) consists of a twisted pair of wires. These wires run from one module to another. The receive and deliver coded information between the modules. The information is coded to identify the message as well as the importance of the message. When there are multiple messages trying to access the CCD bus at one time, the code determines the message that has higher priority, and is then allowed to access the bus first.

The two wires of the twisted pair that make up the CCD multiplex system are called "bus +" (bus plus) and "bus -"(bus minus) respectively. Each wire has a measurable voltage level of roughly 2.5 volts. In order to maintain the 2.5 volts on each line and provide a means of transportation for the coded messages, there is a "voltage divider network" located in the module that supplies bus bias or voltage. Along with the module that supplies bias, some modules provide termination. Bias is the voltage necessary to make the bus operational. At least one point of termination is necessary, some systems use more than one. Some modules are capable of biasing and terminating the bus by themselves. Termination in the circuitry (a 120 ohm resistor placed across the bus) is required to complete the voltage divider network and also provides some electromagnetic protection for the bus. Without termination, voltage on the bus goes to approximately 5 volts on one wire and 0 volts on the other wire.

NOTE: Communication over the bus is essential to the proper operation of the vehicle's on-board diagnostic systems and the DRB. Problems with the operation of the bus or DRB must be corrected before proceeding with diagnostic testing. If there is a problem 1A Identifying Vehicle Communication Problems

Bus Failure Messages
Short to Battery- Either or both of the bus wires are shorted to the battery potential.
Short to 5 Volts- Either or both of the bus wires are shorted to a 5-volt potential.
Short to Ground- Either or both of the bus wires are shorted to the ground.
Bus (+) & Bus (-) Shorted Together - The two bus wires are shorted together.
No Termination- The bus system has lost connection with all of its terminators.
Bus Bias Level Too Low- Either or both of the bus wire potentials are significantly below their normal 2.5 volts.
Bus Bias Level Too High- Either or both of the bus wire potentials are significantly above their normal 2.5 volts.
No Bus Bias- The bus system has lost connection with all modules that provide bias.
Bus (+) Open- The bus (+) wire has lost connection with termination and/or bias.
Bus (-) Open- The bus (-) wire has lost connection with termination and/or bias.
Not Receiving Bus Messages Correctly- The DRB III cannot communicate over the bus and does not know why.