CAMERA LINK DELIVERS HIGH SPEED AND PERFORMANCE
The demand for high-speed serial buses such as Camera Link in the machine-vision industry comes from a need for digital camera connectivity.
By Jason Mulliner
Vision Product Marketing Manager
Austin, TX, USA; www.ni.com
The demand for high-speed serial buses such as Camera Link in the machine-vision industry comes from a need for digital camera connectivity. Digital cameras provide flexibility because they are not bound by the same resolution and speed specifications as are analog video cameras. They also offer higher resolutions, greater pixel depths, and more variable frame rates. In addition, digital technology delivers improved signal-to-noise ratio. Moreover, the variety of digital camera features enables end users to select the camera that best meets the needs of their application from a range of choices.
The first generation of digital cameras incorporated a parallel-type output bus connection. However, this approach proved to be challenging. When designing industrial or scientific machine-vision systems, connectivity is often perceived as a lower-tier detail and does not get the same attention as selecting the major components. Overlooking this detail often causes inspection systems to exceed the cost budget.
In a parallel digital platform, the cable must contain a single wire for each bit of data and each timing signal. This method means that a typical two-channel, 10-bit camera with an RS-422 output requires at least 46 connections. Trigger and camera-control lines easily raise this number to greater than 50 connections. The result is a thick, inflexible, and expensive cable. An additional expense is that a custom cable must be designed for each permutation of camera and frame grabber because no standard connector exists for the digital camera-to-frame grabber connection. If connectors could be standardized, then cable costs and lead times would decline.
In addition, high-pin-count connectors are inherently large in size. Machine builders and manufacturing engineers are constantly looking for platforms that minimize usage of factory floor space. Advances in camera technology allow the design of small, nonintrusive cameras; but in some cases, the size of the digital camera connector limits how small the cameras can be. By implementing a reduced pin count, a serial interface would allow for smaller connectors and cables that would be less expensive and easier to manipulate.
The Camera Link interface was designed and implemented with these design restraints in mind—retain all the benefits of digital technology, but minimize the connectivity challenges.
The Camera Link interface standard is based on National Semiconductor Channel Link technology, which is used in flat-panel displays. In 2000, a group of camera and frame-grabber companies launched the Camera Link camera-to-frame-grabber interface specification. This specification greatly improves the customer experience when integrating digital cameras and frame grabbers by including features that specifically address the needs of the machine-vision industry, such as standard cabling, data formats, triggering, high-speed acquisition, and camera control.
The Camera Link specification further defines the behavior of the timing and data signals to provide a consistent protocol. In addition to better defining the protocol for image data transfer, control signals were included to fit the specific needs of the machine-vision industry. Four camera-control lines allow the frame grabber to control various camera features, such as exposure time and frame rate. In addition, the specification contains provisions for communications between the image-acquisition device and the camera through low-voltage-differential-signaling (LVDS) pairs based on the RS-232 protocol. LVDS minimizes the effect of noise on data integrity, and low-voltage swings of 350 mV mean that high speeds can be achieved because of low rise- and falltimes.
Camera Link defines a variety of tap configurations and modes for vision-system design. Its Base, Medium, and Full configurations define the number of Channel Link chips and the number of connectors required for each system. These configurations also define the amount of bandwidth that can be accommodated. The Base configuration offers the most flexibility in selecting the number of camera taps (see table), but the bandwidth reaches a maximum at 255 Mbytes/s. The Medium configuration offers fewer choices for camera taps, but its maximum throughput is 382 Mbytes/s. For more demanding applications, the Full configuration currently delivers 680 Mbytes/s of bandwidth at the camera tap currently supported. The theoretical limit of the Full configuration rises above 1 Gbyte/s.
Camera Link provides several major installation benefits for machine-vision users. For one, the standard 3M MDR 26-pin cable provides easy connectivity. For another, the cables are more flexible and smaller than the parallel digital ones currently in use. In addition, the potential of higher throughput when using the Full configuration ensures that Camera Link should be able to solve the high-speed imaging challenges of both the near and the long term.
Camera Link is well suited for high-speed performance applications. Despite the relative newness of the technology, many cameras are available that cover the spectrum from low cost, low resolution, and low speed to high cost, high speed, and high resolution. For example, applications that require higher speeds and precise camera controls include electronics inspection and particle velocimetry. In another case, a manufacturer of medicinal-spray-delivery systems might use a high-speed camera operating at hundreds of frames per second to characterize the particles jettisoned by an asthma inhaler. Many more applications are expected to benefit from the higher speeds and resolutions of Camera Link, especially as machine-vision products are used more and more in continuous process control loop applications such as automated vehicle control.
Whereas Camera Link has many advantages, it can still be improved. Unlike IEEE 1394 (FireWire), Camera Link is not a plug-and-play technology. Machine-vision engineers must still configure their image-acquisition device to accept inputs from a Camera Link camera. Even though camera electronics has steadily been reduced in size, the current size of the connector (1.55 × 0.51 in.) is a limiting factor in designing smaller cameras. The Camera Link connection provides a relatively short standard cable length. However, several vendors have recently supplied Camera Link fiberoptic cables and connections that extend the interface length to hundreds of meters.
Camera Link is well poised to be incorporated into high-speed applications, since it already supports high data-transfer rates with its Medium and Full configurations. However, other interface methods also seek to capture a portion of the machine-vision market. FireWire provides a relatively lower bandwidth than Camera Link does, but its thin cabling, ease of configuration, and numerous camera options make it appealing for a range of straightforward machine-vision applications. Recently, Gigabit Ethernet and USB 2.0 interfaces have emerged as potential contenders, but they both suffer from a lack of available industrial-grade cameras and standardized software.
For high-speed and high-performance applications, Camera Link has proven to deliver the required bandwidth to solve present applications. As more frame grabbers and cameras come to market offering Medium and Full configurations, higher speed imaging applications will be solved with the Camera Link interface standard.
J. Mulliner,Vision Systems Design, September 2003, Camera Link Special Report II, p. S12; www.vision-systems.com.
Vision Systems Design, May 2002, Camera Link Special Report, www.vision-systems.com.