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USB?Universal Serial Bus?

2017-08-10 20:56  
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This article describes the Principle of Universal Serial Bus. The circuit  is very simple but effective utility, worth Duokanjibian then master the principles of this principle. A USB system has an asymmetrical design, consists of a host controller and a plurality of devices are connected in a tree-like fashion hub using special equipment. There is an upper limit to five horizontal branch hub / controller. 127 devices can be connected to a single host controller and the count must include the center of the device. Modern computer may have several host controllers so the total number of useful devices is beyond a reasonable connection can be connected to a single computer. Do not need a terminator on any USB bus like SCSI and some others. Designed to eliminate the need to add a separate USB expansion card to a computer "s ISA or PCI bus, allowing the device to improve through the Plug and Play feature is hot swap or added to the system without having to restart the computer when the first plug a new device, the host it lists and loading device drivers needed to run it. USB peripheral devices can be connected such as mice, keyboards, scanners, digital cameras, printers, hard drives, and network components. multimedia devices, such as scanners, digital cameras, USB has become the standard connection method for printers, USB has become increasingly popular, it began to replace the parallel port because the USB printer simply add more than one computer.

USB (Universal Serial Bus) was developed by Compaq, Digital Equipment Corp, IBM PC Co., Intel, Microsoft, NEC and Northern Telecom.

4 pin USB A or USB B jack4 pin USB A or USB B jack?connector? at the controller

4 pin USB A / USB B / MINI-USB port4 pin USB A / USB B / MINI-USB port?connector? at the peripherals

PinNameDescription
1VCC5 VDC
2D-Data -
3DData
4GNDGround

USB signals are transmitted on a twisted pair of data cables, labelled D and D-. These collectively use half-duplex differential signaling to combat the effects of electromagnetic noise on longer lines. Contrary to popular belief, D and D- operate together; they are not separate simplex connections.

USB supports three data rates. A Low Speed rate of 1.5 Mbit per second that is mostly used for Human Input Devices (HID) such as keyboards, mice, joysticks and often the buttons on higher speed devices such as printers or scanners.

USB has a Full Speed rate of 12 Mbit per second. Full Speed was the fastest rate before the USB 2.0 specification and many devices fall back to Full Speed. Full Speed devices divide the USB bandwidth between them in a first-come first-serve basis and it is not uncommon to run out of bandwidth with several isynchronous devices. All USB Hubs support Full Speed.

USB 2.0 added a Hi-Speed rate of 480 Mbit per second. Not all USB 2.0 devices are Hi-Speed. A USB device should specify the speed it will use by correct labeling on the box it came in or sometimes on the device itself. The USBIF certifies devices and provides special marketing logos for each speed.

Hi-Speed devices should fall back to the slower data rate of Full Speed when plugged into a Full Speed hub. Hi-Speed hubs have a special function called the Transaction Translator that segregates Full Speed and Low Speed bus traffic from Hi-Speed traffic. The Transaction Translator in a Hi-Speed hub (or possibly each port depending on the electrical design) will function as a completely separate Full Speed bus to Full Speed and Low Speed devices attached to it. This segregation is for bandwidth only; bus rules about power and hub depth still apply.

The USB connector provides a single 5 volt wire from which connected USB devices may power themselves. A given segment of the bus is specified to deliver up to 500 mA. This is often enough to power several devices, although this budget must be shared among all devices downstream of an unpowered hub. A bus-powered device may use as much of that power as allowed by the port it is plugged into. Bus-powered hubs can continue to distribute the bus provided power to connected devices but the USB specification only allows for a single level of bus-powered devices from a bus-powered hub. This disallows connection of a bus-powered hub to another bus-powered hub. Many hubs include external power supplies which will power devices connected through them without taking power from the bus. Devices that need more than 500 mA or higher than 5 volts must provide their own power. When USB devices (including hubs) are first connected they are interrogated by the host controller, which enquires of each their maximum power requirements. The host operating system typically keeps track of the power requirements of the USB network and may warn the computer”s operator when a given segment requires more power than is available (and will generally shut down devices or hubs in order to keep power consumption within the available resource).

Power usage:

Bus-powered hubs:Draw Max 100 mA at power up and 500 mA normally.
Self-powered hubs:Draw Max 100 mA, must supply 500 mA to each port.
Low power, bus-powered functions:Draw Max 100 mA.
High power, bus-powered functions:Self-powered hubs: Draw Max 100 mA, must supply 500 mA to each port.
Self-powered functions:Draw Max 100 mA.
Suspended device:Max 0.5 mA

Voltage:

Supplied voltage by a host or a powered hub ports is between 4.75 V and 5.25 V.Maximum voltage drop for bus-powered hubs is 0.35 V from it”s host or hub to the hubs output port.All hubs and functions must be able to send configuration data at 4.4 V, but only low-power functions need to be working at this voltage.Normal operational voltage for functions is minimum 4.75 V.

Shielding:

Shield should only be connected to Ground at the host. No device should connect Shield to Ground.

Cable:

Shielded:
Data: 28twisted
Power: 28AWG – 20AWG non-twisted

Non-shielded:
Data: 28AWG non-twisted
Power: 28AWG – 20AWGnon-twisted

Power GaugeMax length
280.81 m
261.31 m
242.08 m
223.33 m
205.00 m

Cable colors:

PinNameCable colorDescription
1VCCRed5 VDC
2D-WhiteData -
3DGreenData
4GNDBlackGround

 


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