Understanding Smart Sensors - Nomads.usp

Communications for Smart Sensors 127high-speed data communications. CAN supports distributed real-time controlwith a high level of security and message integrity. It has also become attractivefor use in lower speed and other distributed control applications.The original CAN specification was announced in the 1980s. A revision,CAN 2.0, was announced in 1991. CAN 2.0 consists of an A part and a B part.Part A is known as CAN 2.0 A, CAN 1.2, and BasicCAN. Part A specifies an11-bit identifier field, includes no specification for message filtering, and has alayered architecture description based on Bosch’s internal model. Part Benhancements include an extended 29-bit identifier field, some message filteringrequirements, and layer description based on ISO/OSI reference model.The 29-bit identifier field allows the automotive standard protocol, SAEJ1850, 3-byte headers to be mapped into the CAN identifier field. However,minimum CAN compliance is established by conformance to CAN 2.0 Aonly [7].CAN is a multimaster protocol that allows any network node to communicatewith any other node on the same network. Any node can initiate a transmissiononce it has determined that the network is idle. CAN propertiesinclude user-defined message prioritization. CAN is actually a nondeterministicsystem, but a guaranteed maximum latency for highest prioritization can becalculated. Lower priorities are determined on a statistical basis. CAN utilizescarrier sense, multiple access/collision resolution (CSMA/CR) for nondestructivecollision resolution. The arbitration technique is bitwise and results in thehighest priority message being transmitted with low latency time. The flexiblesystem configuration allows user options that have been led to CAN-developedsystems in automotive and industrial applications [7].The message format for CAN is a fixed-format frame with a variablenumber of data bytes. Zero to 8 data bytes are permissible. The minimum dataframe length is 44 bit times. Four message types are defined: data frame, remoteframe, error frame, and overload frame. The messages are routed to receivingnodes through the use of message identifiers and message filtering. Functionaladdressing allows multiple nodes to act on a single message. However, nothingin the hardware prevents the user from using physical addressing to achievenode-to-node addressing. Part of the error detection scheme is the acceptanceof every message by all nodes or no nodes. Even if a component on the networkis not concerned with the message that is transmitted, it must still receive themessage, check the cyclic redundancy check (CRC), and acknowledge (ACK)acceptance. The data frame, shown in Figure 6.3, is the most widely usedframe. Priority is established in the identifier field based on the user’s messagingstrategy [7].Data is requested through the use of a remote frame and transmittedthrough the data frame. The remote frame contains no data field and the