Camera Link becomes de facto vision interface

By Andrew Wilson, Editor

Attendees to the VISION 2002 Show in Stuttgart, Germany, and the Vision Show West in Santa Clara, CA, last November, have no more interface-standard doubts. Based on its widespread incorporation into products, the Camera Link interface has become the de facto machine-vision interface for high-speed cameras and frame grabbers. Since the publication of our last Camera Link Special Report (see Vision Systems Design, May 2002), more than 20 new and established vendors have added Camera Link cameras, frame grabbers, interfaces, converters, or cables to their product lines.

This represents an approximate 25% increase in the number of such standard products in a period of less than one year (see tables, p. S5). And, while serial bus-based protocols such as FireWire and USB continue to gain acceptance in their own right, they are nowhere near the widespread acceptance of the point-to-point Camera Link standard.

Easy, simple, inexpensive

The reasons for the success of Camera Link are numerous. First, it is a relatively simple interface to design. Based on the Channel Link chip set from National Semiconductor (Santa Clara, CA), it was originally developed as a parallel-to-serial interface for flat-panel-display applications, and the cost of implementation was low. Second, the parallel-to-serial nature of the design replaces the bulky cabling and large connector associated with previous LVDS-based designs. Accordingly, CCD and CMOS camera vendors can develop very-small-form-factor cameras while retaining a high-speed data output. Third, the nature of the Camera Link interface allows for easy expansion.

With Camera link, developers can choose from Base, Medium, or Full configurations that allow up to 533 Mbytes/s of image data to be transferred from camera to frame grabber. As expected, all the vendors who have endorsed the standard offer Base configuration products, a few offer the Medium configuration, and fewer offer the Full configuration. The reasons are, of course, market driven. Machine-vision cameras with VGA resolution that operate at 30 frames/s do not transfer data at very high speeds and, therefore, only require Base configuration connectors and compatible frame grabbers.

At the high end, of course, even the Camera Link Full configuration does not provide enough bandwidth for some applications. And many companies offering such products are pushing for further Camera Link developments to support their megapixel multiple-tap cameras with even higher bandwidths. How and when this occurs are interesting issues to watch.

Expanding Camera Link

Certainly, the modular Base, Medium, and Full configurations of the Camera Link standard could be expanded to include extra bandwidth. But this expansion necessitates extra numbers of Channel Link chipsets, a new "extended data mode," and an extra connector. At present, the number of Camera Link Full configuration implementations can be found only in a handful of cameras and frame grabbers.

To accomplish expansion, these products must incorporate three MDR connectors in their designs. To extend the bandwidth further to support ultrahigh-speed megapixel multiple-tap cameras would necessitate the use of four or more connectors. Unfortunately, many camera and frame-grabber vendors are already unhappy about the size of the single Base connector.

With camera manufacturers offering products with sizes that are often less than two cubic inches, the size of the current connector is beginning to limit the size of their designs. Frame-grabber vendors, too, often prefer to port their PCI-based designs to PMC-based mezzanine modules; they are experiencing the same size problem. The size of the connector is limiting the number of Camera Link configurations that can be placed on a single card.

Past experiences

Interestingly, the characteristics of the Camera Link interface and its limited 30-m operating distance may prove to be the salvation for vendors craving higher bandwidth. Currently, a number of vendors offer Camera Link-to-fiber/fiber-to-Camera Link converters that extend the operating distance of the camera-to-frame-grabber interface to hundreds of meters. At the same time, these products offer electromagnetic interference protection of data signals so often required in factory-floor, military, and medical applications.

Such products are reminiscent of the days when RS-170 and NTSC were the only "standard" interfaces for frame grabbers. Back then, camera vendors encoded their clean digital-camera signals into a messy RS-170 format. Frame-grabber vendors could then offer products that decoded the analog information back to its digital format. Because of this design redundancy, today's digital cameras and digital frame grabbers were born. By eliminating the encoding and decoding functions at the camera and the frame grabber, respectively, camera and frame-grabber vendors could provide products that captured images with less noise and greater accuracy.

Today, the Camera Link community faces the same problem. To increase bandwidth and reduce the size of the connector, the Camera Link committee, under the auspices of the Automated Imaging Association (Ann Arbor, MI), should move rapidly to implement a fiber-based point-to-point version of the standard. This could be accomplished relatively inexpensively, by using such chipsets as the GigaStar from Innova Semiconductor (Munich, Germany). By doing so, the size of the connector would never become a limiting factor since it would always remain a small fiber connector and would never have to be redesigned. Better still, the bandwidth offered by such a fiber implementation would offer a means to support every conceivable megapixel, multipletap camera.

Already, a handful of camera vendors have built cameras that offer direct fiber data output for proprietary data interfaces. Because of the success of the Camera Link standard, however, proprietary standards would limit the acceptance of such products. An AIA-endorsed Optical Camera Link standard would do much to promote the acceptance of the technology for both camera and frame-grabber vendors and increase the size of the market.