Understanding Smart Sensors - Nomads.usp

Communications for Smart Sensors 129characteristics to define the requirements. (Bit timing rules in the CAN specificationmust be met.) Accepted physical media include, but are not limited to,twisted pair (shielded or unshielded), single wire, fiber-optic cable, and transformercoupled to power lines. RF transmitters are also being developed bysome users for CAN systems. Acceptable transmission rates range from 5 Kbpsto 1 Mbps. The most widely used implementation to date is a twisted-pair buswith NRZ bit formatting. Twisted-pair CAN drivers that simplify systemdesign are available [7].6.4.3 Other Automotive ProtocolsAs shown in Table 6.1, other automotive protocols have been developed, someof which are covered by SAE specifications. The implementation of those alternativesrequires a volume user that obtains benefit by using the alternativeprotocol, as well as a semiconductor manufacturer willing to implement theprotocol in silicon hardware. For comparison, the Automotive Bit-SerialUniversal Interface System (A-bus) and the vehicle area network (VAN) arediscussed.A-bus was developed by Volkswagen AG [6]. Error detection is by bitonly. Both single wire and fiber optics can be used for the transmission media.The maximum bus length is not specified but is typically 30m. The maximumdata rate of 500 Kbps is considerably higher than the rate specified in SAEJ1850.VAN, developed by French automaker Renault, is being considered as anISO standard. The transmission medium is twisted pair. The maximum datarate is user definable. A maximum of 16 nodes is possible with a maximum buslength of 20m. Bit encoding is one of two variations on Manchester coding.The Manchester coding technique encodes a 1 with a high level for the first halfof the bit time, anda0isencoded with a low level for the second half of the bittime. Consequently, a maximum of two transitions per bit time is possible.The Technical University of Wien in Austria has developed a timetriggeredprotocol (TTP). The protocol provides information about the future(i.e., a priori knowledge) behavior of the system through timing algorithms. Allsystem activities are triggered by the progression of real time. The architecturerequires clock synchronization by a global time base. TTP supports high-speeddistributed control systems, where fault-tolerant operation is critical.A consortium of German and French automakers have developed anoperating system (OS) standard for automotive electronics. The OSEK (Germanacronym for Open System for Automotive Electronics) OS is an extensionof the front end of the MCU hardware [9]. It is intended to provide a structureddesign with additional higher software levels encapsulating lower levels to