I've got a view from a link about RS422:
http://sangoma.com/support/tutorials/rs232_rs422_and_v35.html
this post has mentioned about terminator (end resistors) , i haven't been clear why we need terminator ?
hope someone can help me ! ^~^

 

Hello,

I hope the following page will make the use of termination resistors more clear:
http://hw-server.com/rs-485-rs-422

This is a link of the EDUCYPEDIA page on RS232-RS485 technology:
http://educypedia.karadimov.info/computer/rs232485.htm

Bertus

 

When an electrical signal is sent down a conductor, any conductor, whether it is a piece of wire, twisted pair or coax cable, the signal reaches the end of the conductor at a speed close to the speed of light and is reflected back along the conductor to the transmitting end.

This signal propagation can continue back and forth until the energy is absorbed as heat in the resistance of the conductor.

This effect, commonly known as "transmission line reflection" can be detrimental to the operation of the circuit and system and signal integrity.

To prevent reflection, the receiving end of the transmission line must be terminated with a resistance that is equal to the impedance of the transmission line.

Every piece of wire is a transmission line. When the length of the line is short in comparison with the wavelength of the signal the effect is tolerable and proper termination can be ignored. When the length is long, and this might be 10cm in very high frequency circuits, this can become a problem.

 

Although my following question that seems quite stupid but i still would like to ask you that:"Why it is not good if there is signal reflected from the receiving end to the transmitting end ?" .

 

The reflected pulses can corrupt the signal levels causing data loss / corruption.

 

some experienced people said that "when the transmitting distance increases, the transmitting rate must decrease ".Hope someone can give me an explaination ! ^~^

 

some experienced people said that "when the transmitting distance increases, the transmitting rate must decrease ".Hope someone can give me an explaination ! ^~^

The transmission rates relationship to distance is related to the ratio of noise to the signal (in a broadcast mode). If your signal is much stronger than the noise level then distance only becomes a limiting factor when that ratio becomes small. The nature of that "noise" is a complex subject but in general it's the limiting factor on distance because you can regenerate the original signal in a 'repeater' if it's received without errors from noise.

 

some experienced people said that "when the transmitting distance increases, the transmitting rate must decrease ".Hope someone can give me an explaination ! ^~^

The capacitance of the cable is also a factor. As the cable length increases, so does capacitance which attenuates the signal more and more to the point where the signal is unusable.
So, decreasing the transmission frequency allows a higher capacitance or longer cable.

 

The capacitance of the cable is also a factor. As the cable length increases, so does capacitance which attenuates the signal more and more to the point where the signal is unusable.
So, decreasing the transmission frequency allows a higher capacitance or longer cable.

sorry, but could you mind explaining more clearly for example you can explain more about impedance or give mathematic equations etc.
if you just said that "so does capacitance which attenuates the signal more and more to the point where the signal is unusable." .it's difficult for me to understand how this happen or theriotical base !

 

The capacitance is a factor but it is included in the determination of the impedance of the cable.
The impedance is constant, independent of the length of cable.

 

Impedance of a cable is based on both inductance and capacitance, and it's expressed in ohms. The bizarre thing about it, if you aren't used to the concept, is that the impedance of a cable is the same regardless of how long the conductor is. To get the best performance from a cable, you have to minimize changes in impedance, because those cause distortions or reflections in the signal. Think of shouting down a long tube--it will work best if the tube has no kinks or changes in diameter.

The obvious problem with a cable is that eventually it comes to an end. The best way to handle this is to put a termination on it which is equivalent to a cable of infinite length, and the most common termination is just a resistor equal to the cable's characteristic impedance.

50 ohm cable is common in video systems. But don't pick up a length of it and measure the resistance with an ohmeter and then complain to the vendor that it's defective--"You sell it as 50 ohms and it's a short circuit along its length and infinite resistance between the conductors!"

http://en.wikipedia.org/wiki/Electrical_impedance