Linux+: Hardware Part 10 – Universal Serial Bus (USB)

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  1. Jarret W. Buse

    Jarret W. Buse Well-Known Member Staff Writer

    Jul 3, 2013
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    Linux+: Hardware Part 10 – Universal Serial Bus (USB)

    The Universal Serial Bus (USB) is an external bus in which the USB standards define the cables, connectors and communication protocols for it.

    There are basically three versions of USB as follows:

    1. USB Version 1.0
      • USB Version 1.1
    2. USB Version 2.0
    3. USB Version 3.0
      • USB Version 3.1

    NOTE: The update for versions 1.0 and 3.0 were considered updates and not a major version revision.

    Cables and Speed

    Cabling for USB 1.x and 2.0 use twisted pair. Twisted pair cabling takes two or more wires, in this case four, and twists them around each other. One wire is usually the electrical positive and the other negative (+ and -). With USB 1.x and 2.0 the other two wires are for data transfer (D+ and D-). By twisting the wires, noise and crosstalk is reduced. The method of twisting the cables allows for external Electro-Magnetic Interference (EMI) to be cancelled out. Signals sent on nearby wires will not cause interference (crosstalk) with the USB cable.

    USB 3.x standard cables have nine wires in a twisted pair configuration. The extra wires are to support the SuperSpeed.

    NOTE: SuperSpeed is the name given to signify USB 3.x. There is a USB 3.0 connector which has ten wires.

    USB 1.1 allows for a maximum cable length of 3 meters operating at full speed (12 Mbit/second) and 5 meters when operating at low speed (1.5 Mbit/second).

    USB 2.0 allows for a maximum cable length of 5 meters at Hi Speed (480 Mbit.second).

    USB 3.0 does not specify a maximum cable length limit, but it is accepted that 3 meters should be the maximum length. The speed is 4.0 Gbit/second for data throughput.

    USB 3.1 allows for a data throughput of 10 Gbit/second. USB 3.1 is sometimes referred to as SuperSpeed+.

    USB cables can be color coded. USB 1.x are usually white cables, while USB 2.0 are usually black but can be white. USB 3.x cables are usually blue.


    USB provides some power to the USB devices connected to the external bus. If the power requirement of the USB device is too much the device is required to be connected to a self-powered hub.

    NOTE: A self-powered hub is one which is plugged into its own power outlet instead of getting power only from the USB connection on the Personal Computer (PC). The USB ports on a PC are referred to as the Root Hubs since these are the base, or root, of the USB connections.

    USB 1.0 supplies 5 volts with a current of 150 mA. USB 2.0 also supplies 5 volts but a current of 500 mA. USB 3.0 is also 5 volts but a higher current of 900 mA.

    NOTE: If too many devices are placed on a single Universal Serial Bus there may not be enough power to allow all devices to function normally. Devices may act erratically. If devices do not work correctly, move some to their own bus line. Each USB Root Hub can support 127 devices from one Interrupt Request (IRQ).


    There are four common connector types for USB 1.x and 2.0. The USB Standards provided for two connector types: Type A and B. Originally, Type A was for connecting to a USB Hub or the USB connector on a PC. Type B was for connecting to devices.

    NOTE: In a Type A device the Data connections are recessed to allow the power connectors to make contact first. Remember that each type has a Male and Female connection type.

    For smaller devices Mini-A, Mini-B and Mini-AB connectors were created. The Mini-AB allowed for either a Mini-A or Mini-B to be connected. The Mini types are not too widely used and have been replaced with the Micro Types.

    The Micro-A, Micro-B and Micro-AB are more widely used and also part of the USB Standards.

    The various connectors are shown in Picture 1.

    Hardware 10 - Picture 1.JPG
    PICTURE 1​

    USB 3.0 introduced a new connector, the Micro-B, which has an extra set of connections to the side as shown in Picture 2.

    Hardware 10 - Picture 2.JPG
    PICTURE 2​


    USB communications consist of packets. There are three types of packets used in USB communications:

    1. Handshake – response to data being sent. Either data was received, not received or the device has failed.
    2. Token – packet sent by host to a device to transmit data.
    3. Data – actual data sent in packets.


    USB 2.0 devices can be used on a system with USB 1.x Root Hubs. The USB 2.0 devices will operate at USB 1.x speeds.

    A USB 2.0 Root Hub can have USB 1.x devices running on it. The USB 1.x devices will operate at the slower USB 1.x speed. If a USB 2.0 device is connected to a USB 1.x Root Hub, the 2.0 device will operate at the slower USB 1.x speed. USB 2.0 devices should be placed closest to the USB 2.0 Root Hub.

    Systems with USB 3.x can operate USB 2.0 devices at a slower speed. Again, newer USB 3.x devices should be placed closest to the Root Hub.

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