USB Brings Cost and Connectivity Advantages to Imaging

As published in Quality Magazine
March

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USB is the most prevalent method for connecting computers to peripheral devices. A brief survey of my desk reveals several devices: a smartphone, headphones, a camera, a mouse, and a keyboard, all relying on USB connections.

Thanks in part to its ease of use, USB dominates consumer-to-computer connectivity applications and is rapidly being adopted in other markets. For instance, home-based medical devices that track general health or more complex systems monitoring a patient's response to therapy benefit from the plug-and-play simplicity of USB.

Frequently, these products store data in memory until plugged into a PC. This data can be analyzed locally—like how a fitness tracker motivates us to move more—or transferred to a physician’s office for evaluation. For most of these applications, USB is a straightforward method to support the bandwidth needed to transmit data-intensive information. However, there’s no real-time requirement for data delivery. The few seconds it takes to transmit data from the device to the computer does not affect a patient's or physician’s ability to monitor health or treatment response.

The scenario is different in machine vision, where automated decisions must be made within milliseconds or even faster. Consequently, the imaging industry was initially slow to adopt USB due to earlier transmission limitations. However, as the USB standard has evolved and bandwidth capabilities have improved, new opportunities for the technology are surfacing in the traditional imaging market. Moreover, as machine vision extends into new markets, USB is a familiar and user-friendly interface for less technical end-users.

USB and Machine Vision

While USB interfaces have been widely adopted across other markets, earlier versions of the standard did not support sufficient bandwidth for imaging applications requiring uncompressed, raw data for real-time image analysis. This changed with the introduction of USB 3.0, or SuperSpeed USB, providing ten times the bandwidth of USB 2.0.

With this bandwidth boost, the vision industry established the USB3 Vision standard, providing designers with a framework to meet performance, cost, and usability requirements for real-time imaging systems. Data is transmitted directly to existing ports on a computer, achieving sustained throughputs nearing 3 Gbps; this performance surpasses that of Camera Link Base configurations, rivalling Medium configurations, without the need for specialized frame grabbers to capture data at endpoints. By removing the necessity for frame grabber cards, designers gain access to a broader range of computing options, including laptops, tablets, and single-board or embedded processors.

As the imaging industry embraces USB 3.0, the USB Implementers Forum (USB-IF) continues to evolve the technology. With the trend of thinner computing devices, USB-C is becoming a common connector to transmit data and power to new laptops. The USB 3.1 Gen 2 protocol offers 10 Gbps data transfer and 100 watts of power. Recently, the USB-IF announced USB 3.2, boasting a theoretical maximum data transfer rate of 20 Gbps.

Advantages and Challenges of USB3

Although GigE Vision currently leads the machine vision interface market, USB 3.0 quickly reaches deployment levels rivaling more established solutions. Bandwidth has been a significant driver behind the initial adoption of USB3 Vision, but manufacturers should not overlook its cost and design advantages.

The thinner, lighter USB 3.0 cable is easier to route than bulky Camera Link cables and connects with "plug-and-play" ease, facilitating quicker setup and teardown of inspection stations. To enhance reliability, the USB3 Vision standard specifies locking connectors for the cables.

With video, control data, and power transmitted over a single cable, cabling complexity is reduced, and the need for an external power supply for cameras is eliminated, leading to lower costs and a smaller system footprint. USB 3.0 reduces system overhead and CPU usage by using asynchronous signaling instead of the polling mechanism seen in USB 2.0, along with direct memory access (DMA) transfers. Consequently, more processing power is available for the vision application.

USB3 Vision is solidifying its market position in applications that value high-bandwidth point-to-point video transmission. While the intrinsic networking and multicasting of GigE Vision is advantageous in systems with multiple image sources and endpoints, several USB 3.0 cameras can operate in parallel on a single bus using an off-the-shelf USB 3.0 hub. In comparison, each Camera Link camera requires its dedicated cable and frame grabber.

Distance presents the last challenge for USB 3.0 interfaces. Many vendors now offer active cabling solutions, extending the potential distance between imaging sources and processing units well beyond the initial three to five-meter range.

USB3 Vision in Imaging Applications

Nearly every camera manufacturer now offers USB 3.0 products, with external frame grabbers also available to convert images from existing cameras into a USB3 Vision video stream. USB3 imaging devices are being integrated into various applications requiring high resolution and frame rates, such as document scanning and print verification.

Barcode print verification systems assist in lowering costs, enhancing productivity, and minimizing errors by providing detailed quality analysis to ensure high read rates in automated quality inspection and warehouse processes. If printing errors are not identified promptly, and automated reading systems fail to recognize the product, barcodes may require manual entry into supply chain systems or necessitate the destruction of unverified products. This results in disrupted manufacturing processes and additional expenses.

Print verification systems utilize machine vision hardware and software to ensure barcodes meet industry readability standards in automated processes. USB3 Vision imaging products can help reduce these system costs by identifying unreadable barcodes early in the manufacturing process.

In this application, the portable barcode verification unit employs a CMOS image sensor to capture a high-resolution image of the barcode. The video feed is converted into a USB3 Vision image stream and transmitted over a standard USB 3.0 cable directly to a computing platform where barcode readability is confirmed. By forgoing the need for a PCIe frame grabber to capture image data, system designers can utilize lower-cost computing platforms, including laptops, for barcode analysis and verification. Furthermore, power over USB simplifies cabling and reduces component costs.

Ease of use is another significant advantage of USB 3.0 video connectivity, especially in applications where end-users may not be vision experts. Microscopy systems for medical quality inspection often utilize a Camera Link camera to transmit images to a computer for analysis and display. However, the Camera Link video interface requires bulky, specialized cabling and a PCIe frame grabber to capture images at the computer, leading to more complex systems, higher costs, and limited component selection.

Instead, external frame grabbers can convert the Camera Link image feed into USB3 Vision-compliant video. The uncompressed video is transmitted with low, consistent latency directly to an existing USB 3.0 port on a laptop used for analysis and display, allowing for faster setup and teardown of inspection stations.

The Future of USB in Vision

The vision market increasingly leverages expertise developed in other sectors to deliver advantages in imaging applications. Technologies such as Gigabit Ethernet, USB 3.0, and more recently NBASE-T are bringing cost, performance, and usability benefits to automation systems.

As imaging manufacturers and system integrators implement USB 3.0 in their products, we will closely observe how rapidly the consumer market adopts higher-bandwidth USB interfaces. As more devices on our desks begin to support USB 3.x and beyond, we can anticipate the next wave of high-bandwidth imaging devices for the automation market.

Silicon Imaging
USB 2.0 Cameras: 

Setting the Imaging world on fire!

The high-speed image capture and connectivity market is about to undergo a revolution. The need for purchasing and installing a custom frame grabber or IEEE interface card will soon be a distant memory. The new standard will permit you to connect your high-speed megapixel machine vision and line scan cameras with a single 4-wire cable directly into a 480 Mbit/sec (megabits per second) USB 2.0 port, which will soon become standard in every new motherboard produced with Intel P4 chipsets. These new USB 2.0 vision cameras will transmit precision 8 or 12-bit digital grayscale or color image data, eliminating the sampling jitter that traditional analog RS-170 or NTSC systems experience, at speeds 40 times faster than the predecessor USB 1.1 and 20% faster than FireWire (aka. IEEE-1394 or Sony i-link) devices. This interface will also provide bi-directional serial communication for camera setup and control, triggering, strobing, and other I/O signaling. One of the most convenient benefits, especially for those imaging executives and sales engineers traveling with the latest lightweight laptops, is not having to carry an additional power supply; these cameras will be powered through the same USB cable. For the vision system end-user, the benefit will be a lower system cost than previous camera and frame grabber solutions due to plug-and-play installation.

MegaCamera-2.0: See the light!

The Silicon Imaging MegaCamera-2.0 family offers a broad range of models based on desired resolution, frame rates, capture method, and cost, each optimized for full USB 2.0 speed performance. All cameras provide 12-bit precision digitizing, sampling rates of up to 50 MHz, software programmability for exposure, gain, clock, 8/12 bit transfer, and triggering modes, along with a very small size and bus-powered capability. The SI- is the world’s first 3.2 Megapixel USB 2.0 camera, capable of capturing images at 15fps or x HDTV resolution at 24fps. An 8x digital pan/tilt/zoom function allows a specific region of the image to be read out at video rates and moved through the scene for video conferencing and surveillance applications. For lower-cost applications, the SI- provides x resolution at 30fps. For scientific analysis, medical imaging, and stop-motion machine vision applications, the SI-F has x resolution up to 30fps with large 12um pixels, full-frame shutter, binning, adaptive readout, and windowing. For high-speed motion capture and sports analysis, the SI-320F can capture and record 320x240 resolution video at over 500 frames per second. All models are available in either monochrome or Color RGB Bayer formats, with products starting under $995. Yes, this includes a cable.

USB Background

The Universal Serial Bus (USB) standard was originally developed to minimize the number of ports at the back of PCs. The primary goal of USB was to define an external expansion bus that makes adding peripherals to a PC low cost and as simple as hooking up a device to a wall jack. USB featured a maximum bandwidth of 1.5 Mbit for low-speed devices, such as mice and keyboards, and a maximum bandwidth of 12 Mbit for higher-speed devices like webcams, printers, scanners, and external CD-RW drives. Frustrated by Apple's royalty fees on FireWire devices, in April, seven industry-leading companies—Compaq, Hewlett Packard, Intel, Lucent, Microsoft, NEC, and Philips—published the specifications for USB 2.0. It has taken approximately two years for USB 2.0 to become a mainstream replacement for USB 1.1. In just a few short months, USB 2.0 will be the high-speed PC peripheral connectivity choice over IEEE-1394.

Introduction to USB 2.0

The USB 2.0 specification extends the maximum speed of the connection from 12 Mbps on USB 1.1 up to 480 Mbps (60 MBytes/sec). This enables the transfer of x images at 24fps (frames per second) for high-definition video conferencing or 320x240 images at 500fps for high-speed video motion analysis. Transitioning to USB 2.0 will be seamless, as USB 2.0 is both forward and backward compatible with USB 1.1. Older peripherals will simply plug into new USB 2.0 capable PCs and hubs, as the USB connectors and cables are identical. It is even possible for a high-speed USB 2.0 device to connect to a legacy USB 1.1 port and operate at reduced throughput. Both Hi-Speed USB 2.0 and original USB 1.1 peripherals can operate on a computer simultaneously. The new USB 2.0 expansion hub design manages the transition of data rates between the high-speed host and lower-speed USB peripherals while maintaining full bandwidth utilization. Up to 127 USB peripherals with five hub levels can be connected to a single USB host controller. With five meters (16.4 feet) of cabling between devices, a network of cameras, sensors, data acquisition, and I/O devices can physically extend up to 30 meters (98 feet) from the PC. A peripheral can either be self-powered or bus-powered, consuming up to 500mA. To meet the needs of low-power embedded and portable computer applications, a power-management mechanism is also incorporated.

Getting Started - Enumerate it!

A USB device can be plugged in anytime, even while the PC is powered on. The installation process is genuinely plug-and-play with no hassles regarding jumpers, IRQ settings, interrupts, or incorrect drivers. To achieve this almost magical connection process, the PC and peripheral, here exemplified by the camera, undergo a detailed electrical handshaking and data setup procedure. Initially, a high-speed camera attaches to the PC USB controller as a classic full-speed (12Mb/sec) device. The camera then signals its high-speed capability. The host controller confirms its USB 2.0 capability and begins high-speed communication. When the PC operating system detects that a USB device has been connected, it assigns a unique address to the peripheral. All further communication with that device utilizes this address, allowing it to be distinguished from other devices on the bus. The PC subsequently learns the device's capabilities by requesting its "descriptors." This information contains the power and bandwidth requirements as well as the necessary driver. Based on this data, the PC automatically loads the device's driver into the operating system, making it ready for use. This initialization process is termed Enumeration.

USB2.0 Speed Throughput

In USB architecture, the PC acts as the master while the peripherals serve as slaves. The PC makes requests, and the peripherals respond. A typical USB transaction involves three stages: First, a Token packet is sent by the PC containing the address of the device and an indication of the desired data transfer direction. If data is being sent to the peripheral, such as camera setup commands, it is categorized as an OUT request. Conversely, if data will be transmitted from the peripheral to the PC, such as image data input from a camera, it is classified as an IN request. The next stage involves data packet transfer. A data packet can be composed of up to bytes accompanied by a CRC value for error checking. The final stage is the handshake. Once data is successfully received, a packet is sent to acknowledge receipt. For streaming data that does not necessitate error checking or handshaking, to reduce overhead, it can be sent in isochronous mode. For guaranteed data accuracy in Bulk mode, packets with errors are retransmitted. Every 125 microseconds, a bus synchronization signal called Start-of-microframe is issued. During each microframe, the PC can perform multiple data transfers. To maximize bandwidth utilization, up to 13 packets containing 512 bytes of data can be transferred during each microframe, translating to over 53 Mbytes/sec throughput (13 packets/microframe * 512 bytes/packet * 8 microframes/ms), including protocol and handshake overhead.

Intel sends USB 2.0 into ubiquity

In May, Intel solidified the success of USB 2.0 connectivity by releasing a family of chipsets named 845E, 845G, and 845GL for both the Pentium-4 and Celeron processors. These chipsets, which incorporate the ICH4 South Bridge chip, feature an embedded USB 2.0 enhanced host controller interface and hub, capable of supporting up to six high or low-speed ports. PC OEMs will no longer need to purchase and incorporate an additional peripheral IC to implement this interface, effectively receiving it for free. Further accelerating market dominance, Intel also released a family of motherboards using this new chipset architecture. The day after its debut, Dell launched its first desktop machine featuring this new chipset, the S, retailing at $789! Yes, that includes a monitor.

Since the ICH4 connects directly to the memory controller (North Bridge chip) over a 266MB/s (32bit/66MHz) hub interface, it can simultaneously transfer data from the USB 2.0 connection at maximum rates without compromising the bandwidth capacity of the 32bit/33MHz PCI bus. For applications requiring multiple high-speed simultaneous captures, additional USB 2.0 PCI adapter cards can be integrated into the system without saturating the bus. This new PC architecture enhances digital video capture similar to the improvements displayed by the AGP bus for increased display throughput. The result is improved input options, speed, resolution, or frame rate at reduced costs.

Microsoft not in the Driver Seat?

Earlier this year, there was some industry speculation surrounding Microsoft's inadequate support of the emerging USB 2.0 standard, as the supportive drivers weren’t included with the launch of Windows XP. The main problem was that USB 2.0 drivers were incomplete at Windows XP's launch time, alongside a lack of peripherals available for testing. This delay was merely a precaution against potential incompatibilities, reminiscent of early FireWire issues, aimed at ensuring clean installations and operations for the new generation of peripherals. Since then, Microsoft has released a driver for Windows XP and plans for upgrades for Windows Me and Windows 2000. However, they have stated they will not provide USB 2.0 driver support on Windows 9x or earlier operating systems. Adaptec, a well-known leader in USB 2.0 adapter cards and hubs, has sought to fill this market void by developing and delivering drivers for their USB 2.0 host products for Windows 98, XP, Me, and 2000. The Linux community has also recognized the anticipated growth of USB 2.0 and has already released driver support in their latest kernel.

CameraLink still the king of speed

CameraLink, a connectivity standard for industrial digital cameras and frame grabbers, will remain the interface of choice for the fastest imaging applications. In its base configuration, CameraLink can transfer data at up to 1.6 Gb/sec. In Full configuration, utilizing two cables, the rate extends up to 4.8 Gb/sec. This exceptionally high throughput has allowed the Silicon Imaging SI- CameraLink camera to achieve sustained real-time 30fps 12-bit video at 3.2 Megapixel resolution, or 1.2 Gb/sec peak transfer speeds. To manage the increased transmission capacity of this interface, CameraLink frame grabber manufacturers have recently expanded their offerings to include 32bit/66MHz and 64bit/66MHz PCI cards with multiple channel input capability. At the entry level, 32bit/33MHz PCI cards deliver price/performance advantages with products starting as low as $695. No, this price does not include the cable.

USB 2.0 versus Firewire IEEE-1394

The primary difference with IEEE-1394 is its peer-to-peer topology, allowing peripherals, such as a VCR and television, to communicate directly without requiring a master PC. This feature does not offer any value in a system where the PC is responsible for image capture, processing, display, storage, and networking. The USB Master/Slave architecture was selected to minimize peripheral costs. Silicon vendors estimate that peripheral controllers require four to five times more gates to execute the interface than comparable USB 2.0 peripherals. The "smarts" are centralized on the PC side, eliminating the costly intelligence needed for every USB peripheral. Furthermore, USB has maintained a royalty-free structure, making affordable peripherals viable and avoiding licensing burdens.

Category

USB 2.0

IEEE-1394

CameraLink

Topology

Master-slave, OTG

Peer-to-peer

Master-slave

Bit rates

480 Mbits/s

400 Mbits/s

1.6 Gbit/s, 4.8 Gbit/s

Transaction intervals

125 microseconds

125 microseconds

Synchronous

Bus master

Dedicated

Allocated at bus reset

PC

Signaling

Current mode differential

LVDS, optical

Multiplexed LVDS

Cable Distance

5 M, Up to 30M with 5 hubs

4.5 M, more with repeaters

10M max

No. of Wires

4

6

26, 52

Bus Power Consumption

Up to 500mA @ 5V

Up to 1.5A

No

Licensing

No

Licensing agreement and royalties

No

Main applications

PC-centric

Consumer electronics

PC-centric

Devices in network

127

63

1

The base link speed of USB 2.0 at 480 Mbit/sec is 20% faster than Firewire's 400 Mbit/sec rate. For the past two years, proponents have been showcasing a specification for FireWire 800, which supports data rates of over 800 Mbit/sec. Currently, no silicon manufacturer has begun producing these higher data rate devices due to a lack of market demand. When, and if, these devices are introduced to the market, they will face several disadvantages. First, they will likely be primarily aimed at Disk Drive Interfaces rather than imaging. Second, users will still need to purchase a custom interface card, which could saturate the PCI bus, leaving no bandwidth for other devices. Third, this interface will not achieve the higher data rates required for CameraLink connectivity applications.

USB 2.0 OTG is On-The-Go

Given the increasing necessity for direct interconnectivity between mobile devices (e.g., digital cameras, PDAs, cell phones) and computer peripherals (e.g., printers, disk drives, home gateways), the USB 2.0 specification was recently supplemented with "On-The-Go" (OTG) capability. By adding this feature, an OTG peripheral can enable limited host type capabilities, facilitating direct data transfer peer-to-peer to another USB or OTG peripheral without requiring PC intervention. This is another nail in the coffin for IEEE-1394. Nevertheless, both USB 2.0 and IEEE-1394 are expected to coexist in many consumer systems for the foreseeable future.

CONCLUSION

With millions of powerful 2.4 to 3.0GHz motherboards featuring USB 2.0 shipment slated for this year at no additional cost to users, it is undeniable that USB 2.0 will soon establish itself as the de facto standard for vision cameras and high-speed image processing. According to the In-Stat/MDR report titled "USB: The Universal Connection," USB 2.0 enabled devices are predicted to experience rapid market acceptance, with a 220% compound annual growth rate forecasted between now and 2023. USB 2.0 will become the preferred connection for most PC peripherals, while the IEEE-1394 interface will remain in use within audio/visual consumer electronic devices. USB 2.0 is set to achieve faster speeds and lower costs than IEEE-1394. However, CameraLink will still maintain its title as the king of speed. For applications needing direct interconnectivity without a PC, the OTG feature allows for direct connections. An upcoming generation of high-performance digital cameras is likely to further accelerate USB 2.0 adoption in the imaging and machine vision market. This will result in increased resolutions, higher frame rates, reduced costs, more compact designs, enhanced portability, and likely a new generation of products to offer to the market.

For more information on the MegaCamera 2.0, CameraLink, and USB 2.0, visit www.siliconimaging.com!