CANBus Control Area Network (Module P6, PA)68C3 Operations Manual North Atlantic Industries, Inc. 12/19/2014Rev: 2014-12-19-0947 www.naii.com Page 73 of 330CANBUS CONTROL AREA NETWORK (MODULE P6, PA)Principle of OperationP6 = CAN A/BPA = CAN J1939CAN is a serial bus system with multi-master capabilities, that is, all CAN nodes are able to transmit data andseveral CAN nodes can request the bus simultaneously. The serial bus system with real time capabilities is thesubject of the ISO 11898 International standard and covers the lowest two layers of the ISO/OSI reference model.CAN is a multi-master broadcast serial bus standard for connecting Electronic Control Units (ECUs).Each node is able to send and receive messages, but not simultaneously. A message consists primarily of an ID(identifier), which represents the priority of the message, and up to eight data bytes.The devices that are connected by a CAN network are typically sensors, actuators, and other control devices.These devices are not connected directly to the bus, but through a host processor and a CAN controller.If the bus is idle which is represented by recessive level (Logical 1), any node may begin to transmit. If two ormore nodes begin sending messages at the same time, the message with the more dominant ID (which has moredominant bits, i.e., zeroes) will overwrite other nodes' less dominant IDs, so that eventually (after this arbitrationon the ID.) only the dominant message remains and is received by all nodes. This mechanism is referred to aspriority based bus arbitration. Messages with numerically smaller values of IDs have higher priority and aretransmitted first.Each node (the P6 or PA CAN module on an appropriate board/system platform provides a node) provides:Host module processing. The host (on-module) processor decides what received messages mean and whichmessages it wants to transmit itself. Sensors, actuators and control devices can be connected to the hostprocessor.CAN controller; hardware with a synchronous clock.Receiving: the CAN controller stores received bits serially from the bus until an entire message isavailable, which can then be fetched by the host processor (usually after the CAN controller has triggeredan interrupt).Sending: the host processor stores ithe transmit messages to a CAN controller, which transmits the bitsserially onto the bus.TransceiverReceiving: it adapts signal levels from the bus to levels that the CAN controller expects and hasprotective circuitry that protects the CAN controller.Transmitting: it converts the transmit-bit signal received from the CAN controller into a signal that is sentonto the bus.Bit rates up to 1 Mbit/s are possible at network lengths below 40 m. Decreasing the bit rate allows longer networkdistances (e.g., 500 m at 125 kbit/s). The improved CAN (CAN FD) extends the speed of the data section by afactor of up to 8 of the arbitration bit rate.The P6 and PA provide the CAN data link layer protocol as standardized in ISO 11898-1 (2003). This standarddescribes mainly the data link layer (composed of the logical link control (LLC) sublayer and the media accesscontrol (MAC) sublayer) and some aspects of the physical layer of the OSI reference model. All the other protocollayers are the network designer's choice.The P6 or PA CANBus module provides independent, isolated, channels of CAN serial data bus links, andconforms to the ISO 11898 International Standard. Both CAN A & B (P6 module) and J1939 (PA module)protocols are supported. The CAN protocol was developed by Robert Bosch GmbH and is recognized andprotected by patents and licensed by Bosch.
