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CAN (Controller Area Networks)

Is a high-integrity serial data communications bus for real-time control applications

Operates at data rates of up to 1 Mega bits per second

Was originally developed for use in cars, Is now being used in many other industrial automation and control applications

Fields in a CAN Frame
A Standard CAN Frame consists of seven different bit fields:

  • A Start of Frame (SOF) field - which indicates the beginning of a message frame.
  • An Arbitration field, containing a message identifier and the Remote Transmission Request (RTR) bit. The RTR bit is used to discriminate between a transmitted Data Frame and a request for data from a remote node.
  • A Control Field containing six bits:
    * two reserved bits (r0 and r1) and
    * a four bit Data Length Code (DLC). The DLC indicates the number of bytes in the Data Field that follows
  • A Data Field, containing from zero to eight bytes.
  • The CRC field, containing a fifteen bit cyclic redundancy check code and a recessive delimiter bit
  • The ACKnowledge field, consisting of two bits. The first is the Slot bit which is transmitted as recessive, but is subsequently over written by dominant bits transmitted from any node that successfully receives the transmitted message. The second bit is a recessive delimiter bit
  • The End of Frame field, consisting of seven recessive bits.

    Following the End Of Frame is the INTermission field consisting of three recessive bits

  • Data Rate vs Bus Length

    The rate of data transmission depends on the total overall length of the bus and the delays associated with the transceivers. For all ISO11898 compliant devices running at 1Mbit/sec speed, the maximum possible bus length is specified as 40 Metres, For longer bus lengths it is necessary to reduce the bit rate. To give some indication of this the following numbers are from the DeviceNet features list:

  • 500 K bits per second at 100 metres (328 ft)
  • 250 K bits per second at 200 metres (656 ft)
  • 125 K bits per second at 500 metres (1640 ft)
  • Inter Integrated Circuits (I2C)
    • two-wired bus originally to interact within small num. of devs (radio/TV tuning, ...)
    • speeds:
    – 100 kbps (standard mode)
    – 400 kbps (fast mode)
    – 3.4 Mbps (high-speed mode)
    • data transfers: serial, 8-bit oriented, bi-directional
    • master/slave relationships with multi-master option (arbitration)
    • master can operate as transmitter or receiver
    • addressing: 7bit or 10bit unique addresses
    • two-wired bus
    – serial data line (SDA)
    – serial clock line (SCL)


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