I am always meet and observe some short of people who talks about USB, The USB Speed and other things but don’t know about USB and is types as well as standards.
In this guide, I will tell you everything that I know about USB from history to types and from power to standards.

So, what actually the USB is?

USB, short for Universal Serial Bus, is an industry standard developed in the mid-1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication, and power supply between computers and electronic devices. It is currently developed by the USB Implementer’s Forum (USB IF).

USB was designed to standardize the connection of computer peripherals (including keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters) to personal computers, both to communicate and to supply electric power. It has become commonplace on other devices, such as smartphones, PDAs and video game consoles. USB has effectively replaced a variety of earlier interfaces, such as parallel ports, as well as separate power chargers for portable devices.


A group of seven companies began the development of USB in 1994: Compaq, DEC, IBM, Intel,Microsoft, NEC, and Nortel. The goal was to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater data rates for external devices. The first integrated circuits supporting USB were produced by Intel in 1995.


Release name Release date Maximum transfer rate Information
USB 0.8 December 1994 Prerelease
USB 0.9 April 1995 Prerelease
USB 0.99 August 1995 Prerelease
USB 1.0 RC November 1995 Prerelease
USB 1.0 January 1996 Low Speed (1.5 Mbit/s), Full Speed (12 Mbit/s)
USB 1.1 August 1998
USB 2.0 April 2000 High Speed (480 Mbit/s)
USB 3.0 November 2008 Super Speed (5 Gbit/s) Also referred to as USB 3.1 Gen 1 by USB 3.1 standard
USB 3.1 July 2013 Super Speed+ (10 Gbit/s) Also referred to as USB 3.1 Gen 2 by USB 3.1 standard
USB Type C August 2014 Super Speed+ (10 Gbit/s)

Note: 1Gbit (Gigabit) = 125 MBs (Megabyte) or 0.125 GB (Gigabyte).

See the difference between Gigabit (Gbit) & Gigabyte (GB).

USB 1.x

Released in January 1996, USB 1.0 specified data rates of 1.5 Mbit/s (Low Bandwidth or Low Speed) and 12 Mbit/s (Full Bandwidth or Full Speed). It did not allow for extension cables or pass-through monitors, due to timing and power limitations. Few USB devices made it to the market until USB 1.1 was released in August 1998, fixing problems identified in 1.0, mostly related to using hubs. USB 1.1 was the earliest revision that was widely adopted and led to Legacy Personal Computers.

USB 2.0

USB 2.0 was released in April 2000, adding a higher maximum signaling rate of 480 Mbit/s (High Speed or High Bandwidth), in addition to the USB 1.x Full Speed signaling rate of 12 Mbit/s. Due to bus access constraints, the effective throughput of the High Speed signaling rate is limited to 280 Mbit/s or 35 MB/s.

USB 3.0

USB 3.0 specification was released on 12 November 2008, with its management transferring from USB 3.0 Promoter Group to the USB Implementers Forum (USB-IF), and announced on 17 November 2008 at the SuperSpeed USB Developers Conference.

USB 3.0 defines a new SuperSpeed transfer mode, The new SuperSpeed mode provides a data signaling rate of 5.0 Gbit/s. The payload throughput is 4 Gbit/s (due to the overhead incurred by 8b/10b encoding), and the specification considers it reasonable to achieve around 3.2 Gbit/s (0.4 GB/s or 400 MB/s), which should increase with future hardware advances.

USB 3.1

USB 3.1 was released on 31 July 2013, replacing USB 3.0 standard. USB 3.1 specification takes over existing USB 3.0’s SuperSpeed USB transfer rate, The USB 3.1 standard increases the data signaling rate to 10 Gbit/s (1.25 GB/s), double that of SuperSpeed USB,

The USB 3.1 standard is backward compatible with USB 3.0 and USB 2.0.

USB Type-C

Developed at roughly the same time as the USB 3.1 specification, but distinct from it, the USB Type-C Specification 1.0 was finalized in August 2014 and defines a new small reversible-plug connector for USB devices. The Type-C plug connects to both hosts and devices, replacing various Type-A and Type-B connectors and cables with a standard meant to be future-proof, similar to Apple Lightning Connector and Thunderbolt.


The functionality of USB devices is defined by class codes, communicated to the USB host to affect the loading of suitable software driver modules for each connected device. This provides for adaptability and device independence of the host to support new devices from different manufacturers.

USB mass storage (USB drive)

USB implements connections to storage devices using a set of standards called the USB mass storage device class (MSC or UMS). This was at first intended for traditional magnetic and optical drives and has been extended to support flash drives. It has also been extended to support a wide variety of novel devices as many systems can be controlled with the familiar metaphor of file manipulation within directories. The process of making a novel device look like a familiar device is also known as extension. The ability to boot a write-locked SD card with a USB adapter is particularly advantageous for maintaining the integrity and non-corruptible, pristine state of the booting medium.

Though most computers since mid-2004 can boot from USB mass storage devices, USB is not intended as a primary bus for a computer’s internal storage. Buses such as Parallel ATA (PATA or IDE), Serial ATA (SATA), or SCSI fulfill that role in PC class computers. However, USB has one important advantage, in that it is possible to install and remove devices without rebooting the computer (hot-swapping), making it useful for mobile peripherals, including drives of various kinds (given SATA or SCSI devices may or may not support hot-swapping).

Another use for USB mass storage devices is the portable execution of software applications (such as web browsers and VoIP clients) with no need to install them on the host computer.

Media Transfer Protocol (MTP)

Media Transfer Protocol (MTP) was designed by Microsoft to give higher-level access to a device’s filesystem than USB mass storage, at the level of files rather than disk blocks. It also has optional DRM features. MTP was designed for use with portable media players, but it has since been adopted as the primary storage access protocol of the Android operating system from the version 4.1 Jelly Bean as well as Windows Phone 8 (Windows Phone 7 devices had used the Zune protocol which was an evolution of MTP).

The primary reason for this is that MTP does not require exclusive access to the storage device the way UMS does, alleviating potential problems should an Android program request the storage while it is attached to a computer.

The main drawback is that MTP is not as well supported outside of Windows operating systems.

Human interface devices (HID)

In computing, the USB human interface device class (USB HID class) is a part of the USB specification for computer peripherals: it specifies a device class (a type of computer hardware) for human interface devices such as keyboards, mouse, game controllers and alphanumeric display devices.

Joysticks, keypads, tablets and other human-interface devices (HIDs) are also progressively migrating from MIDI, and PC game port connectors to USB.

The USB HID class describes devices used with nearly every modern computer. Many predefined functions exist in the USB HID class. These functions allow hardware manufacturers to design a product to USB HID class specifications and expect it to work with any software that also meets these specifications.

Device Firmware Upgrade (DFU)

Heard about iPhone DFU mode for flashing a firmware to it? Yes, it the class Device Firmware Upgrade.

Device Firmware Upgrade (DFU) is a vendor- and device-independent mechanism for upgrading the firmware of USB devices with improved versions provided by their manufacturers, offering (for example) a way for firmware bugfixes to be deployed. During the firmware upgrade operation, USB devices change their operating mode effectively becoming a PROM programmer. Any class of USB device can implement this capability by following the official DFU specifications.

In addition to its intended legitimate purposes, DFU can also be exploited by uploading maliciously crafted firmwares that cause USB devices to spoof various other device types; one such exploiting approach is known as BadUSB.


There are various types of USB connectors related to it’s specification as well as standard. You can see each type of USB 2.0 to USB Type-C connector in below chart.

Standard Connectors – Type A and Type B

Because of all these reasons, two USB connectors have been defined for basic use, the USB A connector which must be used on devices which provide power (mostly computers), and the USB B connector used on devices which receive power like most peripheral devices.

he type-A plug has an elongated rectangular cross-section, inserts into a type-A receptacle on a downstream port on a USB host or hub, and carries both power and data. Captive cables on USB devices, such as keyboards or mice, will be terminated with a type-A plug.

USB A connector

USB A connector

USB B connector

USB B connector

In the standard USB A and B connectors specified in the USB 1.1 and USB 2.0 specification, four pins are defined. Two pins are used for power and two pins are used for differential data transmission. If you look carefully at the connector you will see that the pins for the power connection (pin 1 and 4) are slightly longer. This is done on purpose to first connect the power supply when connecting a USB device, and only afterwards establish the data connection. With this sequence the chance that the driver or receiver ports of the data connection receive awkward and possible dangerous voltages is lowered substantially.

Mini USB and Micro USB connectors

The good thing of a USB connector standard is that it is possible to design devices without need to think how that device should be connected to other devices. The USB A and B connectors proofed their usability with devices like printers, modems and scanners, but when the faster USB 2.0 was released and USB became not only a way to connect slow and bulky equipment but also faster and smaller devices like photo camera’s and mobile telephones, especially the standard USB B connector was just too big to fit nicely on these smaller equipment. An update to the USB 2.0 specification was posted with the name Mini-B connector engineering change notice which defined a smaller version of the B connector. There has also existed a mini USB A connector for some time, but as the USB A connector is used on the power sourcing side—mostly a larger piece of equipment like a computer—that connector was withdrawn from the standard and no new devices will receive certification any more if they contain such a connector. In practice you won’t find the mini USB A connector any more.

Mini USB A connector

Mini USB A connector

Mini USB B connector

Mini USB B connector

Besides the size, the main difference between the standard USB A and B connectors and the mini USB A and B versions is the extra pin which is called ID. In the mini connector series this pin is normally not connected. It has been added for future enhancements of the USB standard.

Micro USB A and B connectors

In the modern world small is never small enough and the mini USB B connector soon was too large for new equipment like cell phones. Therefore in January 2007 the micro USB connector was announced which could be easier integrated in thin devices than the mini USB version. Although the micro USB connector is much thinner than its mini USB brother, it has been especially designed for rough use and the connector is specified for at least 10000 connect/disconnect cycles. One of the reasons is that with mobile devices like cell phones, PDA’s and smartphones the number of mate cycles will be significantly higher than with static equipment like printers and mice. Furthermore the micro USB connector is becoming the de facto standard to charge mobile devices and its use will therefore be even more widespread than of its mini USB counterpart.

Micro A USB Connector

Micro A

Micro B USB Connector

Micro B USB Connector

In the original USB specification there was a strict separation between the host (mostly a computer) which acts as a master device, and the peripherals which have only slave functionality. As mobile devices get smart and often run their own operating system, the separation between the two types of devices has vanished. When connected to a PC a smartphone may be acting as a slave, but it could also be connected to a photo printer directly to print pictures made with the phone. In that case the phone switches from its slave role to a master. To allow this an extension to the USB 2.0 specification has been written which is called USB On-The-Go or more often USB OTG. This supplement provides means for easy switching between the master and slave role of a device.

Because most small devices which can both act as a master and a slave only have one USB connector, additions to the connector definition were necessary to allow a role change with only one type of cable. This is where the mini USB AB and later the micro USB AB connector are defined for. The mini USB AB connector is now officially deprecated, but the micro USB AB connector is replacing its place rapidly. Countries like China are even considering to make this micro USB AB connector mandatory on all new cell phones sold. In this micro USB AB connector the ID pin is used to signal the master of slave function.

Micro USB AB connector

Micro USB AB connector

Micro USB B connector

Micro USB B connector

The pin numbering for the micro USB connectors is the same as for the mini USB connectors. The only difference is that for the micro USB AB connector the ID pin now has a function assigned to it.

USB On The Go (USB OTG) connector

We all use removable drives in our smartphones. Yes, that is called USB OTG. That is not a connector but the standard of USB.

All current USB On-The-Go (OTG) devices are required to have one, and only one, USB connector: a Micro-AB receptacle. Non-OTG compliant devices are not allowed to use the Micro-AB receptacle, due to power supply shorting hazards on the VBUS line. The Micro-AB receptacle is capable of accepting both Micro-A and Micro-B plugs, attached to any of the legal cables and adapters as defined in revision 1.01 of the Micro-USB specification. Prior to the development of Micro-USB, USB On-The-Go devices were required to use Mini-AB receptacles to perform the equivalent job.

USB Type-C connector

USB Type-C is a specification for a small 24-pin reversible-plug connector for USB devices and USB cabling.

The Type-C connectors connect to both hosts and devices, replacing various Type-B and Type-A connectors and cables with a standard meant to be future-proof. The 24-pin double-sided connector is similar in size to the micro-B connector, with a Type-C port measuring 8.4 millimetres (0.33 in) by 2.6 millimetres (0.10 in). The connector provides four power/ground pairs, two differential pairs for non-SuperSpeed data (though only one pair is populated in a Type-C cable), four pairs for high-speed data bus, two “sideband use” pins, and two configuration pins for cable orientation detection, dedicated biphase mark code (BMC) configuration data channel, and VCONN +5 V power for active cables. Connecting an older device to a host with a Type-C receptacle requires a cable or adapter with a Type-A or Type-B plug or receptacle on one end and a Type-C plug on the other end. Legacy adapters with a Type-C receptacle are “not defined or allowed” by the specification, due to their being able to create “many invalid and potentially unsafe” cable combinations.


USB is a serial bus, using four shielded wires for the USB 2.0 variant: two for power (VBUS and GND), and two for differential data signals (labelled as D+ and D− in pinouts). Non-Return-to-Zero Inverted (NRZI) encoding scheme is used for transferring data, with a sync field to synchronize the host and receiver clocks. D+ and D− signals are transmitted on a differential pair, providing half-duplex data transfers for USB 2.0. Mini and micro connectors have their GND connections moved from pin #4 to pin #5, while their pin #4 serves as an ID pin for the On-The-Go host/client identification.

USB 3.0 provides two additional differential pairs (four wires, SSTx+, SSTx−, SSRx+ and SSRx−), providing full-duplex data transfers at SuperSpeed, which makes it similar to Serial ATA or single-lane PCI Express.

See below the pinout of various USB connectors.

Type A and Type B pinout Mini A/Micro A and Mini B/Micro B pinouts
USB Type A and Type B Pinouts USB Mini/Micro-A and -B pinout

Micro-B SuperSpeed plug

1 Power (VBUS, 5 V)
2 Data− (D−)
3 Data+ (D+)
4 ID (On-The-Go)
6 SuperSpeed transmit− (SSTx−)
7 SuperSpeed transmit+ (SSTx+)
9 SuperSpeed receive− (SSRx−)
10 SuperSpeed receive+ (SSRx+)

I have only covered main topics, there are many things that I haven’t covered. I hope you understood and learnt many things as well as many doubts of yours are cleared about USB from this article.

Thank you for reading this guide. Make sure to comment if you have any doubt.

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