Differential signaling
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Differential signaling is a method of transmitting information over pairs of wires (as opposed to single-ended signalling, which transmits information over single wires).
Differential signaling reduces the noise on a connection by rejecting common-mode interference. Two wires (referred to here as A and B) are routed in parallel, and sometimes twisted together, so that they will receive the same interference. One wire carries the signal, and the other wire carries the inverse of the signal, so that the sum of the voltages on the two wires is always constant.
At the end of the connection, instead of reading a single signal, the receiving device reads the difference between the two signals. Since the receiver ignores the wires' voltages with respect to ground, small changes in ground potential between transmitter and receiver do not affect the receiver's ability to detect the signal. Also, the system is immune to most types of electrical interference, since any disturbance that lowers the voltage level on A will also lower it on B.
Some communications protocols that use differential signaling include RS-422, RS-485, PCI Express and USB.
[edit] Differential vs. single-ended signaling
A differential signalling system is unlike the more common technique of single-ended signaling, in which the transmitter generates a single voltage that the receiver compares with a fixed reference voltage, both relative to a common ground connection shared by both ends.
The widely used RS-232 system is an example of single-ended signaling, which uses ±12 V to represent a signal, and anything less than ±3 V to represent the lack of a signal. The high voltage levels give the signals some immunity from noise, since few naturally occurring signals can create that sort of voltage. They also have the advantage of requiring only one wire per signal. However, they also have a serious disadvantage: they cannot run at high speeds. The effects of capacitance and inductance, which filter out high-frequency signals, limit the speed. Large voltage swings driving long cables also require significant power from the transmitting end. This problem can be reduced by using smaller voltages, but then the chance of mistaking random environmental noise for a signal becomes much more of a problem. Another difficulty is the electromagnetic interference that can be generated by a single-ended signaling system which attempts to operate at high speed.
Differential signaling uses the difference in voltage between two wires to signal information. This system has a lower susceptibility to noise, because distant noise sources tend to add the same amount of voltage (called common-mode noise) to both wires, so the difference between the voltages remains the same. Manufacturers can further reduce noise by twisting the two wires of a pair together (as in Cat-3 Ethernet cables or in telephone wires), so that any noise induced in one half-twist tends to cancel the noise induced in the neighboring half-twist.
Other examples include differential ECL, PECL, LVPECL, RS-422 and RS-485. Low voltage differential signaling is currently the only scheme that combines low power dissipation with high speed.
The type of transmission line used to connect two devices (chips, modules) dictates the type of signaling to be used. Single-ended signaling is used with coaxial cables, in which one conductor totally screens the other from the environment. All screens (or shields) are combined into a single piece of material to form a common ground. Differential signaling is used with a balanced pair of conductors. For short cables and low frequencies, the two methods are equivalent, so cheap single-ended circuits with a common ground can be used with cheap cables. As signalling speeds become faster, wires begin to behave as transmission lines.
Differential signaling has to be used in computers to reduce electromagnetic interference, because complete screening is not possible with microstrips and chips in computers, due to geometric constraints and the fact that screening does not work at DC. If a DC power supply line and a low-voltage signal line share the same ground, the power current returning through the ground can induce a significant voltage in it. A low-resistance ground reduces this problem to some extent. A balanced pair of microstrip lines is a convenient solution, because it does not need an additional PCB layer, as a stripline does. Because each line causes a matching image current in the ground plane, which is required anyway for supplying power, the pair looks like four lines and therefore has a shorter crosstalk distance than a simple isolated pair. In fact, it behaves as well as a twisted pair. Low crosstalk is important when many lines are packed into a small space, as on a typical PCB.
[edit] High-voltage differential signaling
High-voltage differential (HVD) signaling uses high-voltage signals, as opposed to low-voltage differential signaling (see LVDS). In computer electronics, "high voltage" normally means 5 volts or more.
Differential circuitry normally allows longer cables than single-ended signaling. This is because the signal on the wires is received not as the difference between a signal wire and a ground wire, but as the difference between two wires not related to ground (hence the term "differential"). Since any interference to the signals coming from external sources is likely to influence the two wires by an equal amount, leading to a 'common mode' signal which is removed by the receiver, the maximum cable length is increased compared with single-ended circuitry.
SCSI-1 variations included a high voltage differential (HVD) implementation whose maximum cable length was many times that of the single-ended version. SCSI equipment for example allows a maximum total cable length of 25 meters using HVD, while single-ended SCSI allows a maximum cable length of 1.5 to 6 meters, depending on bus speed. Note that LVD versions of SCSI allow less than 25 m cable length not because of the lower voltage, but because these SCSI standards allow much higher speeds than the older HVD SCSI.
