In the last 14 years, the Universal Serial Bus (USB) has become the standard interface to connect devices to a computer. Whether it’s an external hard drive, a camera, the mouse, a printer, or a scanner, the physical connection to transfer data between devices generally is a USB cable. The interface is indeed universal.
USB technology has been under development since 1993. The first official definition, USB 1.0, was introduced in 1996. It provides a Low-Speed transfer rate of 1.5 Mbits/s for sub-channel keyboards and mice, and a Full-Speed channel at 12 Mbits/s. USB 2.0, which came in 2001, made a leap to Hi-Speed transfer rates of up to 480 Mbits/s. In 2010, USB 3.0 finally hit the market.
USB 3.0 or SuperSpeed USB
USB 3.0 is the third major version of the Universal Serial Bus (USB) standard for computer connectivity. Among other improvements, USB 3.0 adds a new transfer mode called “SuperSpeed” (SS), capable of transferring data at up to 5 Gbits/s (625 MB/s), which is more than ten times as fast as the 480 Mbit/s (60 MB/s) high speed of USB 2.0. Beside different connectors used on USB 3.0 cables, they are also distinguishable from their 2.0 counterparts by either the blue color of the ports or the SS initials on the plugs.
A successor standard named USB 3.1 was released in July 2013, providing transfer rates up to 10 Gbits/s (1.25 GB/s, called “SuperSpeed+”), which effectively put it on par with the first version of Thunderbolt.
USB 3.0 Specifications
A number of changes have been implemented in USB 3.0 to satisfy the increased demands of external devices. Here is a quick overview of USB technology:
This new Super Speed interface provides realistic transfer rates of around 3,200 Mbits/s or 3.2 Gbits/s. The theoretical top signaling rate is 4.8 Gbits/s.
USB 3.0 introduces full duplex data transfer. Two of five lanes are reserved for transmitting data, while another pair is dedicated to receiving data, meaning that USB 3.0 can read and write data simultaneously at full speed. Previous USB specifications did not support bi-directional data transfer.
The “SuperSpeed” bus provides for a transfer mode at a nominal rate of 5.0 Gbit/s, in addition to the three existing transfer modes. Accounting for the encoding overhead, the raw data throughput is 4 Gbit/s, and the specification considers it reasonable to achieve 3.2 Gbit/s (0.4 GB/s or 400 MB/s) or more in practice.
All data is sent as a stream of eight bits (one-byte segments) that are scrambled and then converted into a 10-bit format, using what is known as 8b/10b encoding. This helps to reduce electromagnetic interference (EMI). The inverse process is carried out at the receiving end. Scrambling is implemented using a free running linear feedback shift register (LFSR). The LFSR is reset whenever a COM symbol is sent or received.
The unit load has been increased to 150 mA and a configured device can draw up to six unit loads, which adds up to 900 mA. This exceeds USB 2.0 by 80% and leads to faster recharging or powering of more than four devices from a single hub. In addition, the minimum device operating voltage was dropped from 4.4 V to 4 V, which saves energy.
USB 3.0 suspends device polling, which is replaced by interrupt-driven protocol. As a result, idle devices won’t experience a power drain since a signal from the device is required to initiate data transfer. With USB 2.0, the host controller used to look for active transfers, slowly draining power. USB 3.0 supports idle, sleep, and suspend states, as well as link-, device-, and function-level power management
USB 3.0 can support up to 8m of passive cable compared to 5m for USB 2.0 and 4.5m for FireWire. It is almost comparable to Camera Link in terms of cable length (10m) but much shorter than Gigabit Ethernet (100m).
Additionally, a USB 3.0 cable is capable of delivering 4.5W of power, enough to power a machine vision camera or a surveillance camera without any additional power supply.
Plug-and-Play Ready and Vision standards
The USB Video Class (UVC) standard provides a readily available USB driver for video camera software applications that reads data from an image sensor and sends control information to an image sensor controller. It is plug-and-play compatible with all personal computers and is commonly used in video capture applications like PC webcams.
However, the UVC class restricts images to YUV format, thereby limiting the choice of image sensors and control signal options for the industrial vision market. The Automated Imaging Association (AIA) has come up with a new ‘USB 3.0 Vision’ standard which allows more image sensors that stream non-YUV images, supports more camera control features, and allows customer-driver implementations.
The USB3 Vision standard allows reuse as many blocks as possible from existing standards like GigE Vision and Coax Press to significantly reduce design cycles. This helps eliminate the need to develop custom drivers and adds flexibility by supporting more sensors and controls to make Fit-to-purpose solutions for exact application requirements.
The frame grabber less camera interfaces like Gigabit Ethernet, USB 3.0 and Firewire have differences in CPU Load. For the same image size transfer, there is a low CPU load with FireWire and USB 3.0, because both interfaces work with DMA (Direct Memory Access).
The CPU load with GigE is considerably higher, because GigE requires copy processing. Say at a data rate of 85MB/s, the CPU load for USB 3.0 would be around 1%, while GigE will require around 6% CPU load. Thus, camera implementations using GigE adds the overhead of CPU load optimization on the designer. This adds more design cycle time as well.
USB 3.0 is backwards compatible with USB 2.0. So whether you get a new USB 3.0 device or a new computer that supports USB 3.0, your old device will be able to communicate with the new interface. Naturally, it will do so at the old USB 2.0 speed. However, you won’t be able to use a USB 3.0 cable to connect a USB 2.0 device.
The devices that will benefit most from USB 3.0 are those that already out speed USB 2.0, including HD webcams, Blu-Ray drives, or some external hard drives.
USB 3.0 or Super-speed USB overcomes these limitations with six (over IEEE 1394b) to nine (over USB 2.0) times higher bandwidth, better error management, higher power supply, longer cable lengths and lower latency and jitter times. These advantages, coupled with the fact that USB 3.0 has become a standard in the consumer market and has a lot of hardware supporting native USB 3.0, has made this interface a de facto choice for cameras in a relatively short period of just a year, post the official ratification of USB 3.0 Vision standard in January 2013.