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Camera-to-PC Interfaces

New interface options promise bandwidth for high-resolution, high-speed imagers without taking up more space.

By Kelly Davis, Matrox Imaging

Reprinted with permission from Photonics Spectra, January 2000, p. 120.
Copyright Laurin Publishing Co. Inc.

Machine vision providers are realizing the benefits of digital cameras with high resolution and frame rates, and they are migrating toward these higher-performance alternatives. As this happens, the need for more bandwidth to transfer the cameras' data is critical.

It is already clear to imaging professionals that transmitting digital data directly from a camera to a personal computer is beneficial because it does not require the conversion from digital to analog and back to digital again. Now the focus is on transmitting more data at faster rates, using smaller and fewer wires and connectors, at a lower overall cost.

Increased bandwidth through a smaller interface is the trend for camera-to-PC connections. For example, Pulnix's TM-6710 camera left) has a traditional 8-bit low-voltage differential signal connector, but the TMC-1000 outputs 24 bits of data and four control signals through a smaller National Semiconductor Channel Link interface.

These needs are driving camera-to-PC interface technologies now and will continue to do so. Digital serial interfaces are emerging to replace classic parallel interface technologies that are cumbersome, expensive and labor-intensive.

Simplifying cables

The IEEE-1394 multimedia standard is mainly geared toward consumer electronic devices and computer peripherals largely because of its simplified cabling and networking capabilities. Supporting transfer rates of 100, 200 and 400 Mb/s, this interface provides video data transmission from camera to PC, communications from PC to camera and power to the camera over a single cable - eliminating the need for a specialized frame grabber and separate power source.

Today, it provides enough bandwidth to handle the video transfer demands at typical image resolutions and frame rates, prompting digital camera manufacturers such as Sony and BASLER-VC to employ it in new products.

Chris Seymour, industry market manager for Basler's Machine Vision Components Div., said IEEE-1394 offers the benefits of a digital camera with the space and price performance of older interface options such as RS-170. He noted, however, that bandwidth limitations prevent it from being useful in high-frame-rate, high-resolution applications.

The future of IEEE-1394 in industrial and scientific markets depends, of course, on its acceptance in the consumer market.

Intel has announced its support of the fledgling USB 2.0 digital interface, which also was designed to enable high-speed peripheral connections. Nevertheless, IEEE-1394 has a jump on USB, with a product "explosion" forecast this year for silicon running at 800 Mb/s. Its use in machine vision imaging applications will increase if the consumer market embraces it and if its standard cable length (currently 4.5 m) increases to better meet industry needs.

Hybrid link

For more demanding imaging requirements, National Semiconductor has developed a technology that it has trademarked under the name Channel Link. The technology was originally aimed at digital communication applications. However, the company has recently been promoting it for a digital flat-panel display bus.

Essentially, Channel Link is a variation of traditional low-voltage differential signal technology, which serializes parallel data to reduce the wire numbers, size and shielding materials.

With conventional low-voltage differential signal technology, transmitting 28 bits of data requires 56 conductors just for the data. Channel Link can carry 24 bits of data plus four bits for control in as few as 11 conductors. This enables transmission rates of 2.3 Gb/s.

Because Channel Link is essentially a "hybrid" of low-voltage differential signal and digital serial interfaces, it provides an easier step than IEEE-1394 for machine vision providers who need higher performance and a smaller connector footprint.

Channel Link does not require sophisticated communication software to handle the links, while IEEE-1394 more closely resembles Ethernet in software requirements.

Channel Link technology benefits frame grabber manufacturers, allowing them to design boards with less-expensive connectors and smaller form factors. Channel Link receivers also are inexpensive and adapt readily to existing designs, Seymour said.

Channel Link will carve out a market niche only if cabling costs drop and major frame grabber companies incorporate it. Matrox is supporting IEEE-1394 and will support Channel Link when it is sufficiently standardized.

For example, the imaging industry must define how to use each signal in the set of 28, then adopt a connector/cabling standard for high-speed digital cameras and their associated parts.

We believe these standards will be set this year as Pulnix and BASLER-VC develop a common camera interface platform and other camera manufacturers consider the technology. Pulnix has already adopted Channel Link as a camera interface, said Rebecca Grosklaus, the company's corporate business development manager.

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