CAN Bus ProtocolIntroductionController Area Network (CAN) was initially created by German automotive system supplier Robert Bosch in the mid-1980s for automotive applications as a method for enabling robust serial communication. The goal was to make automobiles more reliable, safe and fuel-efficient while decreasing wiring harness weight and complexity. Since its inception, the CAN protocol has gained widespread popularity in industrial automation and automotive/truck applications. Other markets where networked solutions can bring attractive benefits like medical equipment, test equipment and mobile machines are also starting to utilize the benefits of CAN. The goal of this application note is to explain some of the basics of CAN and show the benefits of choosing CAN for embedded systems networked applications.CAN OverviewMost network applications follow a layered approach to system implementation. This system etic approach enables sinter operability between products from different manufacturers. A standard was created by the International Standards Organization (ISO) as a template to follow for this layered approach. It is called the ISO Open Systems Interconnection (OSI) Network Layering Reference Model. The CAN protocol itself implements most of the lower two layers of this reference model. The communication medium portion of the model was purposely left out of the Bosch CAN specification to enable system designers to adapt and optimize the communication protocol on multiple media for maximum flexibility (twisted pair, single wire, optically isolated, RF, IR, etc.). With this flexibility, however, comes the possibility of interoperability concerns. To ease some of these concerns, the International Standards Organization and Society of Automotive Engineers (SAE) have defined some protocols based on CAN that include the Media Dependents Interface definition such that all of the lower two layers are specified.ISO11898 is a standard for high-speed applications, ISO11519 is a standard for low-speed applications, and J1939 (from SAE) is targeted for truck and bus applications. All three of these protocols specify a 5V differential electrical bus as the physical interface. The rest of the layers of the ISO/OSI protocol stack are left to be implemented by the system software developer. Higher Layer Protocols (HLPs) are generally used to implement the upper five layers of the OSI Reference Model.HLPs are used to:1) Standardize startup procedures including bit rates used,2) Distribute addresses among participating nodes or types of messages,3) Determine the structure of the messages, and4) Provide system-level error handling routines. This is by no means a full list of the functions HLPs perform; however it does describe some of their basic functionality.CAN Protocol BasicsCarrier Sense Multiple Access with Collision Detection (CSMA/CD)The CAN communication protocol is a CSMA/CD protocol. The CSMA stands for Carrier Sense Multiple Access. What this means is that every node on the network must monitor the bus for a period of no activity before trying to send a message on the bus (Carrier Sense). Also, once this period of no activity occurs, every node on the bus has an equal opportunity to transmit a message (Multiple Access). The CD stands for Collision Detection. If two nodes on the network start transmitting at the same time, the nodes will detect the collision and take the appropriate action. In CAN protocol, a nondestructive bitwise arbitration method is utilized. This means that messages remain intact after arbitration is completed even if collisions are detected. All of this arbitration takes place without corruption or delay of the higher priority message.There are a couple of things that are required to support non-destructive bitwise arbitration. First, logic states need to be defined as dominant or recessive. Second, the transmitting node must monitor the state of the bus to see if the logic state it is trying to send actually appears on the bus. CAN define a logic bit 0 as a dominant bit and a logic bit 1 as a recessive bit.A dominant bit state will always win arbitration over a recessive bit state, therefore the lower the value in the Message Identifier (the field used in the message arbitration process), the higher the priority of the message. As an example, suppose two nodes are trying to transmit a message at the same time. Each node will monitor the bus to make sure the bit that it is trying to send actually appears on the bus. The lower priority message will at some point try to send a recessive bit and the monitored state on the bus will be a dominant. At that point this node loses arbitration and immediately stops transmitting. The higher priority message will continue until completion and the node that lost arbitration will wait for the next period of no activity on the bus and try to transmit its message again.Message-Based CommunicationCAN protocol is