USB
From MobileRead
Universal Serial Bus, USB, is a serial bus standard to interface devices to a host computer.
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[edit] Overview
USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve the plug-and-play capabilities by allowing hot swapping, that is, by allowing devices to be connected and disconnected without rebooting the computer or turning off the device. Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.
USB is intended to replace many legacy varieties of serial and parallel ports. USB can connect computer peripherals such as eBook Readers, mice, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal media players, and flash drives (memory). For many of those devices USB has become the standard connection method. USB was originally designed for personal computers, but it has become commonplace on other devices such as PDAs and video game consoles, and as a bridging power cord between a device and an AC adapter plugged into a wall plug for charging purposes. As of 2008, there are about 2 billion USB devices in the world.
[edit] Master Slave
Unlike earlier serial ports available on computers the USB serial port is implemented as a master slave. What this means is that the implementation of a USB port on your device is specific to one type of connection. A given port will either be a master port or a slave port. This has ramifications as to what you can talk via a USB port. A master port only talks to slave ports and slave ports can only talk to master ports but not to each other. This prevents, for example, a PDA having a slave port to sync to a computer being able to add a keyboard using the same port.
[edit] USB-OTG
The master slave limitation that has caused problems for many people and so the designers of USB have developed a new port called a USB-On the GO or USB-OTG. An on to go port is special in that it can be switched to be either a maser or a slave as needed to talk to another device. However there are limitations in this ability. For example a typical master port can talk to a wide range of devices and the device manufacturers are very interested in providing drivers for master devices so that their devices can talk to the master device. An USB-OTG master typically has a very small subset of drivers that are included and a user should not expect that this specific list will be expanded.
Two USB-OTG devices talking to each other can actually switch roles if need be. The master always initiates the connection so this can be important. USB-OTG devices a usually small portable units like PDAs and do not have sufficient power to provide much power to run external devices. Typically no more that 100 mA will be available.
[edit] Adapters
There have been a few hardware adapters that are designed to allow a slave to talk to another slave device, for example a GPS to a PDA. The idea is the hardware in the adapter is a master that can translate the data from the GPS to the PDA. One problem is that a slave device supplies no power so the adapter has to have external power ability.
There is also an adapter that can plug two masters together. It includes drivers that must be installed on both master computers and is designed to permit any disk transfers between the devices.
[edit] Hubs
Hubs are designed to provide expansion capability for Computers with a limited number of USB ports. Hubs require specific implementation support and all devices may not work through a hub. In addition a USB-OTG device will lose the ability to switch device types. A hub might still be useful to provide external power to a device that needs it but there are no guarantees that this will work.
[edit] Performance
The initial implementation of USB provided a "full speed" capability of 12 Megabits/second. They also provided a lower speed mode (1.5 Mbits/s) for very lower power devices like a keyboard or mouse. Version 1.0 was quickly replaced with version 1.1.
Version 2 of the USB specification added the ability for a "Hi Speed" mode that could achieve 480 Megabits/second. This permitted disk drives and other high speed devices to be attached via USB. This change was backwards compatible and devices could still attach to USB 1.1 ports with slower performance.
Version 3 of the USB specification added a "SuperSpeed" mode that can achieve 5 Gigabits/second. The connector was changed such that an older device can use the new USB port (running at slower speeds) but a device designed for USB 3.0 cannot be plugged into an older USB port.
[edit] Cable connectors
The Computer end of the USB connector has been standardize to one size. However, there are several connectors available for the device end of the USB capable device. There are devices with custom connects (mostly PDAs) but most devices use one of the defined USB connectors. As devices continue to get smaller and thinner the connectors have gotten smaller as well.
- Mini-USB plugs are 6.8mm (+0.04/-0.05) wide by 3mm (+0.04/-0.05) high.
- Micro-USB plugs are 6.85mm (+0.02/-0.06) wide by 1.8mm (+0.02/-0.06) high.
