Single-ended signaling


Single-ended signaling is the simplest and most commonly used method of transmitting electrical signals over wires. One wire carries a varying voltage that represents the signal, while the other wire is connected to a reference voltage, usually ground. The main alternative to single-ended signaling is called differential signaling where the two conductors carry signals equal in magnitude but of opposite electric polarity.
Single-ended signaling is less expensive to implement than differential, but it has a distinct disadvantage: a single-ended system requires a power supply voltage equal to the maximum amplitude of the signal to be received whereas a differential system only requires a voltage half of the signal amplitude to be received. For a given power supply voltage then, a differential system produces signals of twice the amplitude and therefore has twice as good noise immunity as a single-ended system.
The main advantage of single-ended over differential signaling is that fewer wires are needed to transmit multiple signals. If there are n signals, then there are n+1 wires, one for each signal and one for ground, while differential signaling uses at least 2n wires. A disadvantage of single-ended systems that utilize a common return is that the return currents for all the signals use the same conductor, and this can sometimes cause interference between the signals.

Standards

Single-ended signaling is widely used, and can be seen in numerous common transmission standards, including:

Connectors

A wide range of connectors can be used for single-ended signaling. Some common connectors for domestic and entertainment equipment include;
Some kinds of connectors, though more often used for balanced pairs, are sometimes used for single-ended operation:

Example

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 a voltage of such magnitude. 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.
Historically, electrical telegraph used single-ended signaling with earth return, thus completely eliminating the need to provide a return conductor and substantially reducing the cost of long distance lines. Telegraph is the earliest use of the single-ended transmission line type, but is now obsolete.