Host Data Layer 7 Application Interacts with software requiring network communications; identifies partners, resources and synchronization Layer 6 Presentation Formats and encrypts data; unifies syntax and semantics Layer 5 Session Establishes, manages, and terminates connections between computers Segment/ Datagram Layer 4 Transport Provides transparent transfer of data between hosts; end-to- end error recovery and flow control Media Packet Layer 3 Network Provides switching, routing, addressing, error handling, congestion control, and packet fragmentation and sequencing Frame Layer 2 Data Link Encodes/decodes data packets into bits Logical Link Control: handles error in physical layer, flow control and frame synchronization Media Access Control: defines transmission protocol and management Bit Layer 1 Physical Carries bit stream; defines physical characteristics such as voltage/light levels and frequencies Open Systems Interconnect (OSI) Model

Controller Area Network Standard Defines Physical Layer (L1) Defines Data Link Layer (L2) Defines how to Transport (L4) small (8 bytes) datagrams No flow control (L3 and L4) No sequencing and fragmentation (L3) No Session (L5) or Presentation (L6) specs Different Higher Layer Protocols (HLPs) handle the rest

Common CAN HLPs CanKingdom CANopen CCP/XCP DeviceNet SAE J1939 OSEK SDS These define the “Object” Layer (layers not defined by the CAN standard)

CAN Physical Layer – Voltages Open collector (wired-OR); NRZ Dominant bits are logical 0 Recessive bits are logical 1 Provides arbitration free transmission Automatic Collision Detection Transmitter A Dominant (0)Recessive (1) Transmitter B Dominant (0) Recessive (1)Dominant (0)Recessive (1) If A transmits recessive (1) and sees dominant (0) from B, A knows collision occurred and stops transmitting (will retry 6 clock cycles after end of dominant message)

CAN Physical Layer – Timing Each node has its own clock Synchronization done by dividing bit time into four segments Phases 1 & 2 adjusted based on network and node conditions Sample between Phase 1 & 2 Bit Time SyncPropagationPhase Segment 1Phase Segment 2 Clock

CAN Data Link Layer Specifies four message types Data: contains data for transmission Data Request (Remote): requests transmission of a specific identifier Error: transmitted by any node detecting an error Overload: injects a delay between data and/or remote frames

CAN Data Frames FieldBit LengthDescription Start-of-Frame1Signals start of frame transmission Identifier11Unique id for data (embedded priority) Remote Trans. Request (RTR)1Optional, but must be dominant (0) ID Extension bit (IDE)1Optional, but must be dominant (0) Reserved bit (r0)1Reserved, but must be dominant (0) Data Length Code (DLC)4Number of data bytes (0-8) Data Field0-8 bytesActual data CRC15Cyclic redundancy check CRC Delimiter1Must be recessive (1) ACK Slot1Transmit recessive, receiver sends dominant ACK Delimiter1Must be recessive (1) End-of-Frame (EOF)7Must be recessive (1) For Data Request RTR = 1 (recessive) and DLC = 0 (data field empty)

Error Frames Active error generated by transmitter Passive error generated by receiver Error Types – Bit: Send recessive, read dominant – Stuff: more than 5 consecutive bits of same polarity – CRC: computed and received CRCs not equal – Form: invalid bits in field – ACK: no acknowledgement from receiver

Overload Frames Two overload conditions – Internal conditions of receiver – it can’t keep up – Dominant bit detected during expected intermission (interframe space)

Byte Data Link Controller (BDLC) Physical Layer has three forms – 2-wire: 10.4 Kbps, UART, NRZ (Chrysler) – 2-wire: 41.6 Kbps pulse width modulated (Ford) – 1-wire: 10.4 Kbps variable pulse width (GM) High level V; Low level < 3.5 V Buses use weak pull-down, driver pulls it high High signals are dominant High and low values are bit symbols with specific times Logic 0Logic 1 Active 128  s high64  s high Passive 64  s low128  s low

BDLC – Data Link Layer FieldLengthDescription Start-of-Frame (SOF) 200  s high Signals start of frame transmission Header1 or 3 bytes Message priority; IFR required/disallowed; addressing mode; and message type DatavariableActual data CRC1 byteCyclic Redundancy Check for data End-of-Data (EOD) 200  s low Signals end of data; allows receivers to respond In-Frame Response (IFR)variableReceivers may append response here CRC1 byteCyclic Redundancy Check for response End-of-Frame (EOF) 80  s low Signals end of frame SOFHeaderDataCRCEODIFRCRCEOF 3 byte headers contain destination and source addresses