USB headers make front-panel USB ports work.
USB headers are connectors on a computer's motherboard that provide a USB signal for the ports located on the front panel of many desktop computer cases. A typical header will have nine pins, enough to provide a full USB four-wire connection to two different ports. The ninth pin connects to a shield wire that carries no data but protects the USB connection against electromagnetic interference.
Vcc Pin
One of the four key connectors on a USB header is the Vcc pin. This pin, also referred to, at times, as Vbus or V+, carries a 5-volt signal that provides up to 500 milliamps of power for connected USB devices. While the "V" in Vcc refers to the voltage that it provides, the "cc" is not short for anything. The "cc" refers to a positive voltage connected to the collector terminal of a transistor. Double-letter codes for voltages start with bb and run through ss depending on the type of voltage and the place where it is to be connected.
+D and -D Pins
These two pins carry upstream and downstream data connections. As a serial connection, USB transfers one bit at a time sequentially. Data speeds vary depending on the flavor of USB. USB 1.1 transfers data at 12 megabits per second while USB 2.0 has a 480 mbps transfer rate. SuperSpeed USB 3.0 is capable of speeds as fast as 5 gigabits per second.
Ground Pin
The ground pin provides a neutral "ground" connection. This pin provides a place for the current from the Vcc connection to flow back, completing the circuit. A ground pin is necessary to prevent electric shock in devices using large quantities of power. In low-voltage USB connections, it not only completes the circuit but also can reduce electromagnetic interference.
The Purpose of USB Headers
Motherboards provide USB headers to allow cases to have front-mounted USB ports. Motherboards must be designed to work in a wide range of cases, and headers provide a place for the case's ports to connect to the board. Offering front-panel connections also lets motherboard builders save money by putting fewer USB ports on the back panel, which they build.
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