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. There is also the historic alternative of ground return, rarely used today.

Single ended signaling is less expensive to implement than differential, but it lacks the ability to reject noise caused by:

1. differences in ground voltage level between transmitting and receiving circuits
2. induction picked up on the signal wire

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. (Differential signaling uses at least 2n wires.) A disadvantage of single-ended signaling is that the return currents for all the signals share the same conductor (even if separate ground wires are used, the grounds are inevitably connected together at each end), and this can sometimes cause interference ("crosstalk") between the signals.

Standards

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

• RS-232 serial communications
• PS/2 mouse and keyboard connectors
• I²C serial bus
• TTL circuits
• CMOS logic circuits
• ECL circuits
• Most parallel computer buses, such as:
  • VMEbus
  • PCI
• VGA video connectors
• SCSI interfaces for hard drives and other peripherals
• Parallel ATA interfaces for hard drives and other peripherals

Some kinds of connectors, though more often used for balanced pairs, are sometimes used for single-ended operation:

• RCA jacks for audio signals
• TRS phone connectors for audio signals

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. 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.