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The Universal Serial Bus (USB) was conceived as the end-all for I/O devices. It was thought that all peripherals, from keyboards and mice to monitors, would be daisy-chained USB devices, which was pretty ambitious for a bus with a total bandwidth of 12 Mbps. Although it was slow to catch on, as bandwidth increased (USB 3.1 is capable of 10 Gbps), many I/O devices are now USB devices. The original keyboard and mouse connections have been eliminated in favor of USB. USB hard drives have not replaced internal drives, but most external drives are USB. There are even USB-powered lights, fans, and one-can soda coolers. Although there are USB monitors, this is about the only device that is rarely a USB device.
One of the first problems USB was conceived to solve was the limited number of IRQs on the PC. The USB controller requires an IRQ, but the controller can handle up to 127 devices. These devices don't use individual IRQs.
It was initially conceived that USB devices would be daisy chained; one device would plug into the computer, another would plug into the first, another would plug into that, etc. This concept didn't catch on, and when USB devices exceed the number of USB ports on the computer, a USB hub can be added to expand the number of ports.
USB requires a host (a computer) to control access to the bus. Host devices have Type-A connectors (see the chart below for illustrations of the connectors). The specification states that connectors should be oriented so that the USB logo is visible when plugged in, thus making it easy for users to orient plugs correctly (Dell violates this rule). USB peripherals have Type-B connectors. USB cables have plugs on each end, and devices have receptacles for the plugs. The obvious exception is an extension cable with a plug on one end and a receptacle on the other.
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USB also provides power for devices that consume low amounts of power. Keyboards, mice, USB flash drives, etc., don't require external power supplies because the USB port can supply sufficient power for these devices. USB hard disks often require external power. Full-sized hard drives (i.e., 3-1/2 inch) will have a wall wart or power brick to supply external power. However, small-sized drives (2-1/2 inch) will not require external power. USB CD-ROM/DVD readers will usually not require external power. CD-ROM/DVD burners will usually have two USB connectors. The second connector is only required to get extra power from a second port when writing to a CD or DVD, which requires more power than reading.
USB devices are "hot-pluggable". Unlike other devices that must be installed before the computer is turned on, USB devices can be attached at any time. The operating system will recognize the device and take appropriate action. It may ask for a driver disk, but it may install a driver supplied with Windows. Mass storage devices should be stopped with the eject option in Windows Explorer or the "safely remove device" option in the notification area. This will flush the write-behind buffer to the device and close all unfinished writes. Then, you can safely remove the device without the risk of corrupting the file system.
The first generation of USB, USB 1.0, had a maximum speed of 12 Mbps. Shortly after USB was introduced, Apple introduced a 400 Mbps version of FireWire. USB followed with USB 2.0 with a speed of 480 Mbps. Not to be outdone, FireWire jumped to 800 Mbps.
Now, USB is in version 3.0, which can reach 5 Gbps, or version 3.1, which can reach 10 Gbps. You can recognize USB 3.0 receptacles by the blue insert. USB-C is a new type of connector using the USB 3 specification. It is symmetrical (can be plugged in both ways) and can interface with Power Delivery chargers for fast-charging phone batteries, power banks, etc.
USB 3 requires five more wires than previous versions to achieve the rated
speed. This is reflected in the following images showing the extra connections.
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Many mobile devices charge batteries via a USB port. Not all devices can charge by plugging them into a computer. These devices require more current to charge than a USB port on a computer can provide. These will require a power adapter. There are automobile power plugs that convert the 12 V from a car system to a USB charging circuit. Some mobile devices can charge through a USB port but cannot transfer data through the port. Several manufacturers (Sony, Nokia, Motorola, etc.) standardized micro USB for charging phones. However, USB-C is the latest standard.
There are several standards for fast charging via USB. Some are Apple Fast Charging, Qualcomm Quick Charge, Samsung Adaptive Fast Charging, Motorola Turbo Power, and Power Delivery. Power Delivery requires the extra connections provided by USB-C. Anker PowerIQ claims compatibility with all the above systems. Fast charging systems are rated at various powers. Higher power ratings provide faster charging. Power Delivery with USB-C provides up to 240 watts.
Wireless USB was a competitor with Bluetooth. It had a speed of 480 Mbps. Like Bluetooth, it has a short range (Specified at 3 meters) intended to replace the short cables between computers and peripherals. No wireless USB hardware is currently manufactured.
Supports 127
devices per controller
Only one IRQ per controller (127 devices, 1 IRQ)
Supports daisy chaining
Hubs used for expansion
Hot-pluggable
Use the eject or safe removal option before removing mass storage devices.
Speeds
1.0: 5 Mbps or 12 Mbps
2.0: 480 Mbps
3.0: 5 Gbps
3.1: 10 Gbps
Actual
throughput is about half of the advertised speed
Supplies power to low-power devices (mice, keyboards, etc.)
Some devices have two plugs to support extra
power
Some devices require external power supplies
Charging
Some devices can charge over USB but not transfer data
Some devices require
more power than a USB port provides
These require plug-in supplies
There
are several fast-charging protocols
USB-C with Power Delivery provides up to
240 watts for charging.
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