Communication

VEHICLE COMMUNICATION
The Chrysler Collision Detection multiplex system (CCD bus) consists of a twisted pair of wires. These wires run from one module to another. They 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 the 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 circuit and also provide 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. In the Ram Van, the instrument cluster supplies bus bias and termination and the PCM supplies an additional point of termination.

NOTE: Communication over the bus is essential to the proper operation of the vehicles On-Board Diagnostic systems and the DRB. Problems with the operation of the bus or DRB must be corrected before proceeding with diagnostic testing.

Refer to the appropriate diagnostic procedures for details on how to diagnose these faults using a DRB III scan tool.

Refer to the Body Control System for further Diagnostics. Body Control Module

CCD Data Bus
The Chrysler Collision Detection (also referred to as CCD or C(2)D) data bus system is a multiplex system used for vehicle communications. Within the context of the CCD system, the term "collision" refers to the system's ability to avoid collisions of the data that enters the data bus from various control modules at approximately the same time.

Multiplexing is a system that enables the transmission of several messages over a single channel or circuit. Many vehicles use this principle for communication between the various control modules.

Many of the control modules in a vehicle require information from the same sensing device. Multiplexing reduces wire harness complexity, sensor current loads and controller hardware because each sensing device is connected to only one controller, which reads and distributes the sensor information to the other controllers over the data bus. Because each controller on the data bus can access the controller sensor inputs to every other controller on the data bus, more function and feature capabilities are possible.

A multiplex system allows the information flowing between controllers to be monitored using a diagnostic scan tool. This system allows a control module to broadcast message data out onto the bus where all other control modules can "read" the messages that are being sent. When a module "reads" a message on the data bus that it requires, it relays that message to its microprocessor. Each module ignores messages on the data bus that it doesn't recognize.

With a diagnostic scan tool connected into the CCD circuit, a technician is able to observe many of the control module function and message outputs while; at the same time, controlling many of the sensor message inputs.

In order to successfully transmit and receive binary messages over the CCD data bus, the system requires the following:
- Bus (+) and Bus (-) Circuits
- CCD Chips in Each Electronic Control Module
- Bus Bias and Termination
- mBus Messaging
- Bus Message Coding

Following are additional details of each of the above system requirements.

Bus (+) And Bus (-) Circuits
The two wires that comprise the CCD data bus are the D1 circuit [Bus (+)], and the D2 circuit [Bus (-)]. The "D" in D1 and D2 identify these as diagnostic circuits. Transmission and receipt of binary messages on the CCD data bus is accomplished by cycling the voltage differential between the Bus (+) and Bus (-) circuits.

The two data bus wires are twisted together in order to shield the wires from the effects of any Electro-Magnetic Interference (EMI) from switched voltage sources. An induced EMI voltage can be generated in any wire by a nearby switched voltage or switched ground circuit. By twisting the data bus wires together, the induced voltage spike (either up or down) affects both wires equally. Since both wires are affected equally, a voltage differential still exists between the Bus (+) and Bus (-) circuits, and the data bus messages can still be broadcast or received. The correct specification for data bus wire twisting is one turn for every 44.45 millimeters (1-3/4 inches) of wire.