Byteflight
byteflight
Introduction
The permanently increasing complexity of in-car electronics and the rapidly growing amount of sensors, actuators and electronic control units, places higher demands on high-speed data communication protocols.
Safety critical systems need quick "thinking" protocols with fault-tolerant behavior. The need for on-board diagnostics calls for flexible use of bandwidth and an ever-increasing number of functions.
None of the communication solutions available on the market until now have been able to fulfill all these demands.
To solve these problems, BMW together with several semiconductor companies have developed byteflight a new protocol for safety-critical applications in automotive vehicles.
The byteflight is the bus system used exclusively for the ISIS system.
Design
The Intelligent Safety Integration System (ISIS) consists, depending on equipment level, of up to 11 satellites (control units), the Safety Information Module (SIM) and the Central Gateway Module (ZGM). Seven of the eleven satellites contain crash detection sensors.
In the event of an accident, the necessary restraint systems have to be activated within a fraction of a second in order to provide the maximum amount of protection for the vehicle's occupants.
The system also has to ensure that the restraint systems are not deployed accidentally. In order to ensure that the ISIS meets the requirements placed upon it, large volumes of data have to be exchanged between the control units. This is achieved by means of an extremely high data transmission rate.
The data is exchanged via a bus system. In order to protect the signals traveling along the bus system from interference caused by electrical or magnetic fields, the ISIS system does not use copper bus wires but optical fibers.
The layout of the ISIS takes the form of a star pattern. The Safety Information Module (SIM) is at the centre of the system. Arranged radially around it are the satellite modules. Each of the eleven satellites is connected to the SIM by a fibre-optic cable. Each of the satellites contains a transmitter/receiver module.
The SIM contains twelve transmitter and receiver modules. All information from each of the satellites is made available to every other satellite by the SIM. Each individual fibre-optic cable in the byteflight bus system is used for bi-directional data transmission.
Function
Transmitter/Receiver Module (SE)
The transmitter/receiver module is a module that can convert electrical signals into optical signals and transmit them via optical fibers. Every satellite has an electrical-optical transmitter/receiver module.
The transmitter/receiver modules are connected individually via the byteflight to the Intelligent Star Coupler in the SIM. The SIM also contains a transmitter/receiver module for each satellite.
All information transmitted across the byteflight is in the form of data telegrams sent by means of light pulses. The transmitted receiver modules in the SIM receive the light pulses from the various satellites. The Intelligent Star Coupler sends the data telegrams to all satellites. Data transmission is possible in both directions.
The transmitter/receiver (SE) module contains the LED for the driver circuit and the receiver amplifier for converting the optical signals into digital signals. It also has an integral transmission quality monitoring circuit.
If one of the following faults occurs on one of the fiber-optic cables, the satellite concerned is shut down:
- No optical signal received over a pre-defined period.
- Transmitter diode sends a continuous light signal.
- Attenuation on the fiber-optic cable too great.
The permissible degree of attenuation is stored within the system. If the attenuation exceeds the specified maximum level, one of the following faults may be the cause:
- Kinking in the fiber-optic cable.
- Compression of the fiber-optic cable.
- Stretching of the fiber-optic cable.
- Broken fiber-optic cable.
- Damaged fiber-optic cable.
Data Transmission
The ISIS has a number of sensors positioned at strategic points throughout the vehicle. They are located in the satellites that are connected to the SIM via the byteflight. All sensors are scanned continuously and the data distributed to all satellites.
Diagnosis
Diagnosis of the byteflight is carried out using the Diagnosis Program. Since the byteflight is a star structure and not a continuous ring like the MOST there is a test module "Data transmission to the satellites" that can check communication between each satellite and the SIM module.
Any satellite that does not respond must then be checked individually using a separate Test Module.
Repairs to the byteflight fiber-optic cable are not permitted. However, complete cables between the SIM and the affected satellites may be replaced.