Understanding Smart Sensors - Nomads.usp

126 Understanding Smart SensorsTable 6.3Automotive Network ClassesClass Type Data Rate LatencyA Low 1 Kbps to 10 Kbps 20–50 msB Medium 10 Kbps to 100 Kbps 5–50 msC High 10 Kbps to 1 Mbps 1–5 msJ1850 is supported by a number of other SAE specifications, which are referencedin J1850, such as the message strategy document, J2178.In addition to the SAE bit-encoding techniques specified in Table 6.4,other common techniques include 10-bit nonreturn to zero (NRZ), Bit-stufNRZ, L-Man (Manchester), E-Man, and modified frequency modulation(MFM). These techniques differ based on variable synchronizing, arbitration,transitions per bit, maximum data rate, oscillator tolerance, and integrity. SAEJ1850 network access is nondestructive prioritized by bit-by-bit arbitration foreither protocol option [6]. A frame is defined as one complete transmission ofinformation. Within the frame, the header contains information regarding themessage priority, message source, target address, message type, and in-frameresponse. For SAE J1850, each frame contains only one message, and the maximumlength for a frame is 101 bit times. A power reduction or sleep modeoccurs at a node if the bus is idle for more than 500 ms. Wakeup occurs withany activity on the bus.6.4.2 CAN ProtocolCAN, a serial communications protocol developed by Robert Bosch GmbH,was originally designed for automotive multiplex wiring systems, especiallyTable 6.4Protocol Options in SAE J1850Feature 1- and 3-Byte Headers 1- and 3-Byte HeadersBit encoding PWM VPWBus medium Dual wire Single wireData rate 41.7 Kbps 10.4 KbpsData integrity CRC CRC