what is differential signal?

[tran]

Hi all,
I begin study network, and I meet some difficulties. I see in some document people say that they can tranfer differential signal, but I don't understand what is called differential signal. If we tranfer differential signal, we use two line , one for + and one for -, example we use "RxD+" & "RxD-" to tranfer 'receive data'. But I don't understand : What is differential signal?
Does differential signal mean that the voltage between pin "RxD+" to "Ground" and voltage between pin "RxD-" to "Ground" and we need a different pair circuit at receiver to receive this signal, we must connect signal "Ground" (that we tranfer from transmitted station) to ground node of our circuit and "RxD+" &"RxD-" to the inputs of our circuit ? or Does differential signal mean that voltage between "RxD+" and "RxD-" and at the receiver we only connect "RxD+" and "RxD-" to the inputs of our different pair, do not connect "Ground" to ground node of our circuit, the signal "Ground" dose not use? in this situation.
Can anybody give me some ideal about this problem?
Thank you very much.

 

[nanoware]

the benefit of diff signaling is in canceling noise
its idea comes from differential amplifiers in electronics, where the amp, amplifies difference between two signals and removes comman signal
bcuz noise has an identical effect on both wires, by suppressing common signal, the effect of noise is reduced
so diff signalling is used in most recent data transfer techs such as usb, ethernet, isdn and more...

for more info, see **broken link removed**

 

[kishorekumar_ms]

differential signal is one in which there should be no suddent changes.. say step signal changes rapidly at some point. And at the point of this transmission from one level to another level the wave form is discontinous that is it cant be differentiated at this point... so waves signals without such discontinous can be considered as differential signals.. Step signal is differentialble because apart from the transmission point we can differential it.....

 

[budhy]

BALANCED (DIFFERENTIAL) DATA TRANSMISSION
Balanced data transmission requires two conductors per signal. In balanced communication two lines are switched.
The logical states are referenced by the difference of potential between the lines not with respect to ground . This fact makes differential drivers and receivers ideal for use in noisy environments.
Differential data transmission nullifies the effects of coupled noise and ground potential differences. Both of these are seen as common mode voltages (seen on both lines) not differential and are rejected by the receivers. In contrast to unbalanced drivers, most balanced drivers feature fast transition times allowing for operation at higher data rates

 

[tran]

Thank you very much.
But I don't understand if I transmit differential signal, do I need to transmit 'ground' signal? Example, If I want to transmit 'Receive signal', I use two line one for 'RXD+' and one for 'RXD-', do I need to use one line to transmit ground signal or I only use two line? At receiver we only need to receive 'RXD+' and 'RXD-' and we can receive 'Receive signal' accurately we don't need to receive ground signal from transmitter, do us. If two line is enough, then I don't understand why in electrical connector (use to transmit differentia signal) there is ground pin. We still receive signal accurately without ground signal from transmitter why we need ground pin?
I don't understand this, that is my problem. Can anyone give me some ideas about this?
Thank you very much.

 

[asuprash]

No U dont need to transmit ground signal if u transmit a differential signal.
A differential signal is basically a signal wherein u transmit two equal and
opposite signals wrt a common mode such that the difference gives u the
actual signal.
eg:-
If We need to transmit Vin
then over differential lines we will transmit
( Vcm + Vin /2 ) and ( Vcm - Vin /2)

 

[tran]

Thank you very much.
If I transmit differential signal, I only need two wire to transmit signal and its opposite signal. But in many electrical connector used to transmit differential signal, there is also a pin for ground signal,that means we still use a wire to transmit ground signal so I don't understand why we need this wire. Why we need transmit ground signal when we transmit differential signal? Dose it help us to improve noise immunity?
Can you give me some ideal about this?
Thank you very much.

 

[budhy]

in many electrical connector used to transmit differential signal, there is also a pin for ground signal,that means we still use a wire to transmit ground signal so I don't understand why we need this wire.

There are potential voltage different between two device which connected together with defferential signal lines, this potential different can easily burning the interface chip between two device.

The ground wire is used to eleminated the potential different.