It is required to improve noise immunity along with the differential signaling. The value is determined by standards bodies where a wide variety of experienced people come toether and get agreement in tradeoffs of immunity improvement, bus length, power consumption, managing reflections, etc.
Thank you todd . but my question is why we require resistance? what if that resistor is not there?Is there any document do u have to get clear idea regarding this.
The terminatino resistance is based on the characteristic impedance of the transimssion line. When you use the proper valve it will prevent standing waves on the transmission line. These reflected waves create problems - can lead to component failure.
If you unferstand how ethernet works you inderstand why you have to terminate can bus The clue is in the collision detection Think ethernet using co axial cable This will then makes sence Think also multiple access All nodes listen whilst any single node transmitts If you dont cap the line you risk nodes not being able to carry out error correction or retries
