Information Bus: Description and Operation
VEHICLE COMMUNICATIONThe Programmable Communication Interface multiplex system (PCI bus) consists of a single wire. The Body Control Module (BCM) acts as a splice to connect each module and the Data Link Connector (DLC) together. Each module is wired in parallel to the data bus through its PCI chip set and uses its local ground as the bus reference, The wiring is a minimum 20 gage.
The bus is designed to handle a maximum number of nodes to meet all of the bus load and timing requirements. The bus resistance and capacitance is designed to provide a maximum level of filtering without distorting the bus symbols. This is why extra capacitance and resistance should not be added to the PCI Data Bus circuitry.
In order for an electronic control module to communicate on the PCI data bus, the module must have bus interface electronics consisting of a transmitter/receiver (or transceiver) and Logic control, but termination for loading and filtering the bus, and wiring which interconnects the system.
Because of the unique coding scheme used in the PCI bus, each module is required to have logic control. The data moving on the bus is called symbols. The interface logic performs the following functions: encode and decodes binary data into symbols, synchronizes all bus symbol timing and edges, controls and the sending and receiving of messages, monitors for bus arbitration, monitors for corrupted symbols due to noise and controls the transceiver.
The bus logic control also provides the transceiver with the information it needs to transmit symbols on the bus. The transceiver controls the waveshaping of each symbol that it transmits by using internal voltage controlled current drivers. When the bus transceiver is turned ON, the bus driver ramps up the voltage to the set range and maintains this voltage until the driver is turned OFF; at which time the voltage is ramped back down to the low voltage, ground. The transceiver is designed to allow for arbitration between modules. If more than one module is trying to access the PCI bus at one time, the code determines the message that has the higher priority, and is then allowed to access the bus first.
The PCI bus requires a resistance and capacitance termination load to ground to operate. Because each bus transceiver can only source current to the bus, the resistance load is required to sink this current and pull the bus to ground on the falling edge of the symbols. The bus resistance is minimum of 315 ohms to a maximum of 2 K ohms. The termination capacitance is required for noise filtering and to help in the symbol wave shaping. The bus capacitance shall not be greater than 12,000 pf.
Each module on the PCI bus has a small termination load of a parallel resistance and capacitor to make up part of the overall bus termination load. One or two modules on the PCI bus may have a higher load termination to provide stabilizing influence over the variations of the vehicle builds. These modules, called dominant modules, may very for car line to car line. The Powertrain Control Module (PCM) and the body control module (BCM) are the dominant nodes for this vehicle.
Each module provides its own bias and termination in order to transmit and receive messages. The bus voltage is at zero volts when no modules are transmitting and is pulled up to about seven and a half volts when modules are transmitting. The bus messages are transmitted at a rate averaging 10800 bits per second. Since there is only voltage present when the modules transmit, and the message length is only about 500 milliseconds, it is ineffective to try and measure the bus activity with a conventional voltmeter. The preferred method is to use the DRB III lab scope. The 12 V square wave selection on the 20-volt scale provides a good view of the bus activity. Voltage on the bus should pulse between zero and about seven and a half volts. Refer to the following figure for some typical displays.
The PCI Bus failure modes are broken down into two categories. PCI Bus Communication Failure and individual module no response. Causes of a PCI Bus communication failure include a short to ground or to voltage on any PCI Bus circuit. individual module no response can be caused by an open circuit at the BCM or at the module, or an open battery or ground circuit to the affected module.
Symptoms of a complete PCI Bus communications failure would include but are not limited to:
- All gauges on the MIC stay at zero
- All Telltales on MIC illuminated
- MIC backlighting at full intensity
- No response received from any module on the PCI Bus through the DRB III (Except the PCM)
- No start (if equipped with sentry key immobilizer system)
Symptoms of individual module failure could include any one or more of the above. The difference would be that at least one or more modules would respond to the DRB III
Diagnosis starts with symptom identification. If a complete PCI Bus failure is suspected, begin by identifying which modules the vehicle is equipped with and then attempt to get a response from the modules with the DRB III. If any modules (except PCM) are responding, the failure is not related to the total bus, but can be caused by one or more modules PCI Bus circuit or power supply and ground circuits. The DRB III may display "BUS +/- SIGNAL OPEN" or "NO RESPONSE" to indicate a communication problem. These same messages will be displayed if the vehicle is not equipped with that particular module. The CCD error message is a default message used by the DRB III and in no way indicates whether or not the PCI Bus is operational. The message is only an indication that a module is either not responding or the vehicle is not equipped with that particular module.
NOTE: COMMUNICATION OVER THE BUS IS ESSENTIAL TO THE PROPER OPERATION OF THE VEHICLES ON-BOARD DIAGNOSTIC SYSTEMS AND THE DRB III. PROBLEMS WITH THE OPERATION OF THE BUS OR DRB III MUST BE CORRECTED BEFORE PROCEEDING WITH DIAGNOSTIC TESTING. IF THERE IS A PROBLEM, REFER TO THE COMMUNICATIONS CATEGORY.
Bus Failure Message
Odometer Displays "No Bus" - The Mechanical Instrument Cluster (MIC) cannot communicate over the bus and does not know why.