USB 3.0 vs USB 2.0: Reference at a glance(2)

Bandwidth differences
USB 3.0 has improved upon the bulk data transfer mechanism of USB. The effective bandwidth available via the bulk transfer method is around 400 MB/s; approximately 10 times that of USB 2.0. This important transfer mechanism has enabled machine vision camera vendors to build high-throughput USB 3.0 cameras. This has created significant cost-saving opportunities for integrators as well as improving the overall system speed and efficiency. Users can now use fewer cameras while still covering the same imaging area with large resolution USB 3.0 cameras. The higher bandwidth also allows for faster frame rate, increasing the performance of the system.

Power delivery
USB 3.0 also provides more efficient power management and increased power delivery over USB 2.0. The amount of current draw for USB 3.0 devices operating in SuperSpeed mode is now 900 mA, resulting in an increase in total power delivery from 2.5 W to 4.5 W (at 5 V).

Communication architecture differences
USB 2.0 employs a communication architecture where the data transaction must be initiated by the host. The host will frequently poll the device and ask for data, and the device may only transmit data once it has been requested by the host. The high polling frequency not only increases power consumption, it increases transmission latency because the data can only be transmitted when the device is polled by the host. USB 3.0 improves upon this communication model and reduces transmission latency by minimising polling and also allowing devices to transmit data as soon as it is ready.

Power consumption and capacity
USB 3.0 has been designed to reduce power consumption while increasing its capacity to support and deliver more power. The introduction of USB Battery Charging 1.2 specification allows up to 7.5W of power to be supplied to USB 3.0 devices. USB 3.0 also offers an improved mechanism for entering and exiting low-power states, depending on whether a device is active or not, and eliminates power-consuming polling.

Cable length: USB 3.0 vs USB 2.0
The standard maximum cable length is 5m for USB 2.0 devices. The USB 3.0 standard does not specify a standard length; the maximum distance currently supported in USB 3.0 is 3m.

Timestamp enhancements
Unlike USB 2.0 cameras, which can range in accuracy from 0 to 125 us, the timestamp originating from USB 3.0 cameras is more precise, and mimics the accuracy of the 1394 cycle timer of FireWire cameras.

PHY register & network topology visibility
It is possible to view the network topology of USB 3.0 cameras on the bus. However, PHY node information is not available. USB 2.0 cameras do not provide an interface for viewing either topology or PHY node information.

USB 3.0 Vision
Several machine vision standards exist today for popular interfaces such as IIDC for FireWire and GigE Vision for Ethernet. The standards provide a common way to access and control machine vision cameras, increasing the ease of use and allowing interoperability between different hardware and software vendor.

While no camera control standard exists for USB 2.0 cameras, a new standard called USB3 Vision was has been ratified in 2013 for USB 3.0 cameras. USB3 Vision builds upon the popular GeniCam standard and defines USB 3.0 related requirements, device identification and control interfaces, data streaming mechanisms, mechanical requirements, and testing frameworks.

Conclusion
USB 3.0, or Super-speed USB, overcomes key limitations of other specifications 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 with 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.

About the author
Abhishek Gupta is a business analyst for Cypress Semiconductor.每He has a B.E. in Electronics & Communications from Maharishi Dayanand University, Haryana, India.每He has worked with Agilent Technologies as a Logistics Coordinator (RoHS Specialist).