Universal Serial Bus, or USB, is a serial interface standard for connecting peripherals to computers.
Serial cards with universal serial bus (USB) ports are included on most computers sold today, and USB peripherals are becoming popular, especially in the consumer market.
USB provides higher speeds than traditional RS-232 serial ports and can allow multiple stations to connect to a single port by using USB hubs.
USB also supports “hot plugging” of peripheral devices, which is similar to that supported by PCMCIA cards in notebook computers.
USB makes it easier to connect peripherals to computers and eliminates the need to configure interrupt request (IRQ) settings or dual inline package (DIP) switches. USB peripherals include keyboards, mouse devices, joysticks, scanners, printers, monitors, and digital cameras. USB brings plug-and-play functionality to external peripheral devices and is fully supported by Microsoft Windows.
How Universal Serial Bus (USB) Works
USB uses a tiered star topology to connect up to 127 USB devices to a single USB port on a host. (A USB system can have only one USB host – the computer that has the USB port.)
To set up this topology, you can arrange USB hubs into star and tiered star arrangements. You can physically daisy-chain some USB devices that have built-in USB hubs. Some monitors have this capability for greater flexibility of configuration. This is not true daisy-chaining; it’s simply an extension of the tiered USB topology.
Whatever the arrangement, USB devices can be no more than 5 meters apart or 5 meters from a hub. The entire cabling arrangement is collectively referred to as the USB bus, even if the actual topology itself does not appear bus-like.
The USB controller on the host (the computer with a built-in USB port or a USB serial card) manages communication between the connected USB devices by using a mechanism called transfer queuing, whereby devices make a request to the controller, which queues the request and responds to it in a first-in, first-out (FIFO) manner. Requests made by USB devices are typically one of the following:
- Requests for or reports of status information
- Requests for reading data to or writing data from another device on the bus
All USB requests are handled by the host controller, which obtains and sends the necessary control information or data to the device.
USB cabling is 4-wire cabling that typically uses a 28-gauge twisted pair for data transmission and a 20-gauge untwisted pair for carrying power to the devices. Cabling is available in both shielded and unshielded forms. The following table shows the pinning types for USB cabling. USB supports two speeds, which depend on the type of media used to connect peripherals:
- 1.5-Mbps transmission over unshielded USB cabling
- 12-Mbps transmission over shielded USB cabling
Most USB hubs and serial cards are autosensing and can determine which transmission speed to use from the attached cabling.Pinning Types for USB Cabling
| Pin | Color | Function |
| 1 | Red | +5 V power |
| 2 | White | Data – |
| 3 | Green | Data + |
| 4 | Black | Ground |
Preventing loops in a USB cable
You can prevent loops in a USB cabling configuration by using only USB cables with a Type A (flat) connector at one end and a Type B (square) connector at the other. The flat connector plugs into the USB port on the computer or into the device ports on the hub.
The USB 2.0 standard extends USB to higher transmission speeds of up to 400 Mbps.
USB 3.0
USB 3.0 is the third major version of the Universal Serial Bus (USB) standard for interfacing computers and electronic devices. Among other improvements, USB 3.0 adds the new transfer rate referred to as SuperSpeed USB (SS) that can transfer data at up to 5 Gbit/s (625 MB/s), which is about 10 times faster than the USB 2.0 standard.
Using shielded USB cabling
When you use shielded USB cabling, connect the shield to the ground only at the USB host (computer), not at the USB devices (peripherals).
You can connect an IEEE 1284–compliant printer that has a standard Centronics interface to a USB system by using a Centronics/USB adapter or patch cable.
