What is the fourth wire used for?

1, The fourth wire in USB interface: the key to data transmission and control
In the USB (Universal Serial Bus) interface, the standard USB 2.0 interface consists of four wires: VCC (power line, usually positive 5V), GND (ground line), D+(data line positive), and D - (data line negative). The fourth wire here, the D-wire, together with the D+wire, forms the data transmission channel of the USB interface. In USB communication, D+and D-lines use differential signal transmission, which can effectively resist electromagnetic interference and improve the stability and reliability of data transmission.
In addition to data transmission, the D-line also cooperates with the D+line to achieve the control function of the USB interface. For example, when a USB device is connected to a host, the voltage changes on the D+and D - lines will trigger the host's enumeration process, allowing the host to recognize and configure the connected device. Therefore, the fourth wire (D-wire) is not only responsible for data transmission in the USB interface, but also participates in the identification and control process of the device, and is a key component of the normal operation of the USB interface.
2, The fourth wire in Ethernet: increasing bandwidth and redundant backup
In Ethernet communication, especially in networks with gigabit Ethernet and above speeds, four pairs (eight wires) are usually used for data transmission. Although theoretically only two pairs of wires (four wires) are needed to achieve full duplex communication (one pair for transmission and one pair for reception), in practical applications, using four pairs of wires can bring many benefits.
Firstly, using four pairs of wires can increase the bandwidth of the network. In Gigabit Ethernet, although only two pairs of wires are used for data transmission and reception, the additional two pairs of wires can be used for other purposes such as link aggregation, thereby improving the overall bandwidth and throughput of the network.
Secondly, the fourth pair (eighth wire) can also be used as a redundant backup in Ethernet. In some cases, if a pair of lines in the network fails, another pair of backup lines can be used to continue data transmission, thereby improving the reliability and stability of the network. Although not all Ethernet devices use four pairs of wires for data transmission and redundant backup due to cost and complexity considerations in practical applications, the existence of a fourth pair of wires undoubtedly provides network designers with more choices and flexibility.
3, The fourth wire in serial communication interface: achieving full duplex and multi device connection
In serial communication interfaces such as RS-232, RS-422, and RS-485, the purpose of the fourth wire may vary depending on the interface standard and specific application scenario. Taking RS-422 as an example, this interface standard supports full duplex communication and multi-point connection. In the RS-422 interface, there are usually two transmission lines (A and B) and two reception lines (Y and Z), but some implementations may add additional lines such as ground or signal ground (SG) to improve signal quality and anti-interference capability. In this case, if we consider these additional lines as the 'fourth line' or more, they play a crucial role in improving the reliability and stability of communication.
In addition, in serial communication interfaces, the fourth wire (or more wires) can also be used to achieve multi device connections. For example, in the RS-485 interface, polling communication between multiple devices can be achieved by adding additional control lines (such as enable lines), thereby constructing a complex serial communication network.
4, The fourth cable in audio and video interfaces: improving sound quality and image quality
The fourth wire also plays an important role in audio and video interfaces. Taking audio interfaces as an example, some balanced audio cables (such as XLR cables) consist of three wires: positive, negative, and ground. However, in some advanced audio devices, additional wires (such as shielded wires or extra ground wires) may be added to improve sound quality and reduce noise interference. These additional lines can be considered as the 'fourth line' or more lines, which are crucial for ensuring the clarity and purity of audio signals.
In video interfaces such as HDMI, DisplayPort, and other high-definition video interfaces, although the physical number of wires for each interface may vary, they usually contain multiple wires for data transmission, clock signals, power, and ground wires. In these interfaces, the fourth wire (or more wires) may be used to transmit additional control signals, audio signals, or provide additional power support. These additional cables not only enrich the functionality of the interface, but also improve the quality and stability of video transmission.