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.
As we can see in above diagram following component made a physical layer CAN network –
Let’s see each component in detail –
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
Twisted pair cable able to –
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.
There are two signal level on CAN bus –
During signal transfer there are two types of line code used –
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.
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.
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.
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 –
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.
As per CAN standard, there are two types of CAN standard follows at physical layer –