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CAN Physical Layer

At the physical layer, Multiple nodes are connected at twisted pair cable terminated with 120Ω resistance. This all arrangement is known as CAN Network and the twisted pair cable are CAN_H and CAN_L.

CAN Network

As we can see in above diagram following component made a physical layer CAN network –

  1. CAN Bus
  2. Node
  3. Terminator register

Let’s see each component in detail –

CAN Bus

CAN Bus is made of twisted pair cable.

As we know a variety of transmission medium used for data transmission from one place to another like twisted pair cable, coaxial cable, fiber-optics cable and there use is depending on the system requirement like distance,Cost and transmission rate.

Twisted pair cable able to –

  1. reduce the cross talk
  2. reduce electromagnetic interference.
  3. Allows maximum transmission rate 1mbps over 40m length.
Differential Signalling with noise reduction at receiver end

Analyze above figure: As you can see in the above image input signal has distorted pulse slop because of some noise and it is distributed at both wire CAN_H and CAN_L in equal proportion but opposite polarity and because of this arrangement of differential cable noise is canceled when accumulate at the receiver end.

Note: twisted pair cable increases attenuation(loss of signal) with an increase in the distance so with an increase in CAN Bus length data rate must be lower for valid transmission.

CAN BUS use two signal level i.e Dominant and Recessive

There are two signal level on CAN bus –

  1. Logic 1 is known as a Recessive bit
  2. Logic 0 is known as a Dominant bit

CAN Bus Uses NRZ encoding technique with Bit-Stuffing.

During signal transfer there are two types of line code used –

  1. RZ(Return to zero) Code
  2. NRZ(Non-Return to Zero) code

RZ Line Coding

As name explains itself “return to Zero” means in this line coding signal(1 or 0) drops to zero in between the each pulses.You can say each signal take a rest state between next signal transmission whether consecutive signals are same or different.

RZ Coding Diagram

NRZ Line Coding

NRZ is “Non Return To Zero” means in NRZ line coding there is no rest state in between the pulse(it will not return to zero) as well as no change in polarity until consecutive signals are same.

NRZ Coding Diagram

Why NRZ Line Coding with Bit-Stuffing

As we have seen NRZ line will not do self change in its polarity like RZ until it gets dissimilar signal. Long similar polarity duration can cause synchronization issue in receiver because of this CAN use synchronization check during data transmission by changing polarity of data.

So CAN standard allowed only 5 consecutive bits of same polarity and after that signal must change its polarity to check synchronization between transmitter and receiver. If any transmitter wants to send more than 5 consecutive bits then it must insert an opposite signal bit and it is called bit stuffing and this extra stuff bit is de-stuffed by receiver before data processing.

Stuffed Dominant bit after 5 consecutive recessive bits

CAN Node

In a Vehicle CAN network multiple ECUs are connected on CAN BUS.these ECUs are also known as “Node”.Following components are required to make a CAN Node as shown in figure –

CAN BUS Node Architecture
  1. Micro-Controller unit
    • This unit decides how to deal with received data or data to be transmitted.
  2. CAN Controller
    • This unit applies acceptance filtering as well as it receives whole data until the complete message is not received.
    • This unit sent data on CAN Bus serially when the bus is idle.
  3. CAN transceiver
    • This unit converts transmit signal from CAN controller level to CAN-BUS level and Vice-Versa for the recived signal as par the CAN physical layer architecture.

BUS Terminator

In the Bus topology, terminator register is used whether in CAN or Ethernet or somewhere else to absorb the signal at the end to avoid reflection. Terminator register must be equal to the impedance generated because of signal(V/I).In CAN usually 100-120 ohm resistance used at the end of the differential cable.

High Speed CAN Vs Low Speed CAN

As per CAN standard, there are two types of CAN standard follows at physical layer –

  1. ISO-11898-2 standard for High Speed CAN
    1. End to end termination of CAN bus with 120 ohm to avoid reflection.No termination at each node.
    2. Maximum bit rate allowed up to 1 Mbps for 40 m wire length.Bit rate will be lowered with increase lenght.
    3. Voltage must be within 1.5 v to 3.5 v for dominant signal transmission.
    4. For dominant signal transmission, CAN_H drives towards 5v while CAN_L drives toward 0 while both drop at 2.5 volts for recessive .
    5. Typically the differential voltage is 2 v for the dominant bit while for recessive it is 0.
    6. Widely used in Automobile industry because of its simple architecture.
  2. ISO-11898-2 standard for Low Speed CAN
    1. In this Architecture, Nodes may be connected in star or line topology in a linear manner and each node is self-terminated with minimum 100 ohm resistance.
    2. Each node termination makes this architecture more stable as it can work on the single wire if one is faulted while in high speed CAN communication will stop in case of wire broken.
    3. This standard is also known as fault tolerant CAN as it can work on the single wire if one wire becomes faulted.
    4. Maximum bit rate is 125 kbps on this CAN architecture.
    5. For dominant signal transmission, CAN_H drives towards 5 v while CAN_L drives toward 0.
    6. Typically the differential voltage must not be greater than 2.3 v for the dominant bit while for recessive it is 0.
High Speed CAN

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