Electrical Article - CCD (Chrysler Collision Detection) Data Bus

[Mopar1973Man]

CCD (Chrysler Collision Detection) Data Bus

Description

The Chrysler Collision Detection (also referred to as CCD or C2D ) data bus system is a multiplex system used for vehicle communications on many Chrysler Corporation vehicles. Within the context of the CCD system, the term “collision“ refers to the system’s ability to avoid collisions of the electronic data that enters the data bus from various electronic 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 Chrysler vehicles use this principle for communication between the various microprocessor based electronic control modules.

Many of the electronic control modules in a vehicle require information from the same sensing device. In the past, if information from one sensing device was required by several controllers, a wire from each controller needed to be connected in parallel to that sensor.
In addition, each controller utilizing analog sensors required an Analog/Digital (A/D) converter in order to “read“ these sensor inputs. Multiplexing reduces wire harness complexity, the 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. Also, 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.

In addition to reducing wire harness complexity, component sensor current loads and controller hardware, multiplexing offers a diagnostic advantage. A multiplex system allows the information flowing between controllers to be monitored using a diagnostic scan tool. The Chrysler system allows an electronic control module to broadcast message data out onto the bus where all other electronic control modules can “hear” the messages that are being sent. When a module hears a message on the data bus that it requires, it relays that message to its microprocessor. Each module ignores the messages on the data bus that are being sent to other electronic control modules.

With a diagnostic scan tool connected into the CCD circuit, a technician is able to observe many of the electronic control module function and message outputs while; at the same time, controlling many of the sensor message inputs. The CCD data bus, along with the use of a diagnostic scan tool and a logic based approach to test procedures, as found in the Diagnostic Procedures manuals, allows the trained automotive technician to more easily, accurately and efficiently diagnose the many complex and integrated electronic functions and features found in today’s vehicles.

Operation

The CCD data bus system was designed to run at a 7812.5 baud rate (or 7812.5 bits per second). 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

• Bus Messaging

• Bus Message Coding

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

Bus Circuits

The two wires (sometimes referred to as the “twisted pair”) 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 are 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.

CCD Chips

In order for an electronic control module to communicate with the CCD data bus, it must have a CCD chip (Fig. 5). The CCD chip contains a differential transmitter/receiver (or transceiver), which is used to send and receive messages. Each module is wired in parallel to the data bus through its CCD chip.

The differential transceiver sends messages by using two current drivers: one current source driver, and one current sink driver. The current drivers are matched and allow 0.006 ampere to flow through the data bus circuits. When the transceiver drivers are turned On, the Bus (+) voltage increases slightly, and the Bus (–) voltage decreases slightly. By cycling the drivers On and Off, the CCD chip causes the voltage on the data bus circuit to fluctuate to reflect the message.

Once a message is broadcast over the CCD data bus, all electronic control modules on the data bus have the ability to receive it through their CCD chip. Reception of CCD messages is also carried out by the transceiver in the CCD chip. The transceiver monitors the voltage on the data bus for any fluctuations. When data bus voltage fluctuations are detected, they are interpreted by the transceiver as binary messages and sent to the electronic control module’s microprocessor.

Bus Bias And Termination

The voltage network used by the CCD data bus to transmit messages requires both bias and termination. At least one electronic control module on the data bus must provide a voltage source for the CCD data bus network known as bus bias, and there must
be at least one bus termination point for the data bus circuit to be complete. However, while bias and termination are both required for data bus operation, they both do not have to be within the same electronic control module. The CCD data bus is biased to approximately 2.5 volts. With each of the electronic control modules wired in parallel to the data bus, all modules utilize the same bus bias. Therefore, based upon vehicle options, the data bus can accommodate two or twenty electronic control modules without affecting bus voltage.

The power supplied to the data bus is known as bus biasing. Bus bias is provided through a series circuit. To properly bias the data bus circuits, a 5 volt supply is provided through a 13 kilohm resistor to the Bus (–) circuit (Fig. 6). Voltage from the Bus (–) circuit flows through a 120 ohm termination resistor to the Bus (+) circuit. The Bus (+) circuit is grounded through another 13 kilohm resistor. While at least one termination resistor is required for the system to operate, most Chrysler systems use two. The second termination resistor serves as a backup (Fig. 7). The termination resistor provides a path for the bus bias voltage. Without a termination point, voltage biasing would not occur. Voltage would go to 5 volts on one bus wire and 0 volts on the other bus wire. The voltage drop through the termination resistor creates 2.51 volts on Bus (–), and 2.49 volts on Bus (+). The voltage difference between the two circuits is 0.02 volts. When the data bus voltage differential is a steady 0.02 volts, the CCD system is considered “idle.” When no input is received from any module and the ignition switch is in the Off position for a pre-programmed length of time, the bus data becomes inactive or enters the ”sleep mode.” Electronic control modules that provide bus bias can be programmed to ”wake up” the data bus and become active upon receiving any predetermined input or when the ignition switch is turned to the On position.

Bus Messaging

The electronic control modules used in the CCD data bus system contain microprocessors. Digital signals are the means by which microprocessors operate internally and communicate messages to other microprocessors. Digital signals are limited to two states, voltage high or voltage low, corresponding to either a one or a zero. Unlike conventional binary code, the CCD data bus systems translate a small voltage difference as a one (1), and a larger voltage difference as a zero (0). The use of the 0 and 1 is referred to as binary coding. Each binary number is called a bit, and eight bits make up a byte. For example: 01011101 represents a message. The controllers in the multiplex system are able to send thousands of these bytes strung together to communicate a variety of messages. Through the use of binary data transmission, all electronic control modules on the data bus can communicate with each other. The microprocessors in the CCD data bus system translate the binary messages into Hexadecimal Code (or Hex Code). The hex code is the means by which microprocessors communicate and interpret messages. When fault codes are received by the DRBIII scan tool, they are translated into text for display on the DRBIII screen. Although not displayed by the DRBIII for Body Systems, hex codes are shown by the DRBIII for Engine System faults. When the microprocessor signals the transceiver in the CCD chip to broadcast a message, the transceiver turns the current drivers On and Off, which cycles the voltage on the CCD data bus circuits to correspond to the message. At idle, the CCD system recognizes the 0.02 voltage differential as a binary bit 1. When the current drivers are actuated, the voltage differential from idle must increase by 0.02 volt for the CCD system to recognize a binary bit 0. The nominal voltage differential for a 0 bit is 0.100 volts. However, data bus voltage differentials can range anywhere between 0.02 and 0.120 volts.

Bus Failure

The CCD data bus can be monitored using the DRBIII scan tool. However, it is possible for the data bus to pass all tests since the voltage parameters will be in “range“ and false signals are being sent. There are essentially 12 “hard failures“ that can occur with the CCD data bus:

• Bus Shorted to Battery

• Bus Shorted to 5 Volts

• Bus Shorted to Ground

• Bus (+) Shorted to Bus (–)

• Bus (–) and Bus (+) Open

• Bus (+) Open

• Bus (–) Open

• No Bus Bias

• Bus Bias Level Too High

• Bus Bias Level Too Low

• No Bus Termination

• Not Receiving Bus Messages Correctly

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

Bus Failure Visual Symptoms & Diagnosis

The following visible symptoms or customer complaints, alone or in combination, may indicate a CCD data bus failure:

• Airbag Indicator Lamp and Malfunction Indicator Lamp (MIL) Illuminated

• Instrument Cluster Gauges (All) Inoperative

• No Compass Mini-Trip Computer (CMTC) Operation

Wiring Diagrams

 

 

 

View full Cummins article