Camera Link promises connectivity
In the past, industrial vision and imaging applications used a variety of cable connections to transfer analog data between a camera and a frame grabber.
CHRISTOF ZOLLITSCH is chief technical officer and PATRICK GAILER is product manager vision hardware at Stemmer Imaging GmbH, Puchheim, Germany; www.stemmer-imaging.de.
In the past, industrial vision and imaging applications used a variety of cable connections to transfer analog data between a camera and a frame grabber. Now, however, a new digital interface standard—Camera Link—is emerging and capturing market share away from analog interfaces. Camera Link technology can transfer higher data rates at less cost over a standard cable.
The unique advantage of this new technology is that competitive manufacturers of cameras and frame grabbers mutually banded together to form a consortium with the goal of developing a vision-industry standard that would benefit their customers. The new Camera Link standard is based on Channel Link technology that was designed by National Semiconductor (Santa Clara, CA; www.national.com) for an easy and inexpensive interface for display cards and flat-panel displays to satisfy the needs of the PC mass market.
The result is CameraLink technology that is optimized for interfacing digital cameras and frame grabbers in industrial vision and imaging applications. It can be applied to accommodate different data-transfer rates based on three system setup configurations: Base, Medium, and Full. But its most important attribute focuses on reducing the cost of the cable connecting the camera to the frame grabber. Camera Link cables are inexpensive because their pinouts, lengths, and construction have all been standardized. With numerous technical and economical advantages, Camera Link is expected to be broadly accepted in the vision and imaging marketplace.
From a technical point of view, Camera Link technology offers several key benefits:
- Camera Link offers a higher bandwidth than other interfaces. In theory, it specifies a data-transfer rate of 255 Mbytes/s in the Base configuration and 680 Mbytes/s in the Full configuration.
- Many frame-grabber and camera manufacturers have agreed to support this common cable connection standard.
- The commonly used RS-232 serial interface for camera control is included.
- Camera Link offers a bidirectional communications camera setup
- There is no limitation on resolution and modes (see figure).
In Channel Link technology, 28 bits of data are packed into a transmitter device and transfered into a serial protocol using five parallel channels. In Camera Link technology, the 28-bit data consist of a maximum of 24 bits of image data and four control bits. These data are serialized in a ratio of 7:1 by the transmitter and transfered over four of the low-voltage-differential-signaling lines in the standard cable to a receiver device. This receiver converts the serial data back into parallel format. The fifth channel is used for the clock signal. The 26-pin cable also includes wires that carry control bits for directing the transfer of information from the frame grabber to the camera.
To be used in the vision/imaging industry, the Channel Link protocol has been modified: Camera Link includes four control bits; Camera Link uses a supplemental RS-232 serial interface; and Camera Link describes three data-transfer configurations: Base, Medium, and Full. In the Base, or basic, configuration of the Camera Link standard, the transfer rate is 255 Mbytes/s. Transceiver components can be added to the camera or the frame grabber to increase the Base configuration data rate to that of the specified Medium or Full configuration. These data rates can be double or triple the Base configuration rate and also allow the use of 4- and 8-tap cameras.
Available transmitters are operating at 85 MHz. Unfortunately, this speed cannot yet be used by a standard PC because of the limitation of the PCI bus. This bus sustains a maximum data transfer of about 120 Mbytes/s. Therefore, many Camera Link frame grabbers use a 40- to 66-MHz receiver.
To overcome this limitation, some frame-grabber manufacturers are developing products using PCI-bus specification 2.2, which allows the transceiving of 64-bit data at 66 MHz, or 528 Mbytes/s. These frame grabbers can work with a 85-MHz Camera Link receiver and capture data from a camera at a transfer rate of 680 Mbytes/s. The rate difference between the Camera Link interface and the PCI bus must be buffered on the frame grabber's image-memory board.
Interfacing different cameras to frame grabbers is much easier with Camera Link. This interface allows easier cabling and camera setup. After installation, the camera and frame grabber are anticipated to be easily exchangeable. Users will be able to switch parts, depending on the desired price/performance, without having to make changes to the application. Standard transmitters, receivers, and cables manufactured in high quantities should result in lower parts prices.
Ease of use of this technology offers small manufacturers or manufacturers for niche markets to build competitive products by achieving a faster product time to market. Camera Link lowers the barrier into the industrial vision market for these manufacturers because of low cost, ease of use, and increased flexibility. Many tranceiver components are still being manufactured with higher data rates because of the demand for high-resolution displays in the PC market. Therefore, Camera Link has room to grow in the future.
An important requirement of the vision/imaging industry is longtime availability. Therefore, other alternative technologies must be evaluated in selecting a new camera-to-frame grabber interface. Two alternatives to Camera Link are IEEE 1394, also known as FireWire, and the Universal Serial Bus (USB), version USB 2.0.
IEEE 1394 is a bus system that was developed for serial data transfers in multimedia applications. FireWire allowes the interface of 63 peripheral devices in a tree-topology-structured system. Today, in theory, its data rate is 50 Mbytes/s. However, just 80%, or about 40 Mbytes/s, can be used for data transfers because of control overhead. For most vision applications, this low data rate is inadequate.
A major benefit of the IEEE 1394 standard is the fact that a frame grabber is not needed in a vision application. The camera can be directly connected to an interface card that transfers the camera data into PC host memory. This benefit plus the easy cabling method result in low pricing.
However, FireWire has not yet expanded its market niche. One reason is that FireWire will not be included in the new chipsets scheduled for release from Intel Corp. (Santa Clara, CA; www.intel.com). Therefore, FireWire is projected to be not widely used in the mass PC market. This issue and the fact that users are limited to a low defined image resolution also support the theory of low FireWire expansion into the vision market.
The USB is a standard in PC technology that directs communications between a PC and peripheral devices. Like FireWire, USB allows users to set up a tree-topology-structured system. Its most important benefit is hot-plug capability, which means that a system is configured automatically after startup and when a peripheral device is connected or removed from the system.
Today, USB specifies data transfers of up to 1.5 Mbytes/s. However, USB version 2.0 specifies a data transfer rate of 60 Mbytes/s. For vision applications, this rate is not fast enough. Because of a large transfer overhead and no mechanism to handle data losses, USB is not expected to be used in many vision applications. On the other hand, if Intel decides to include USB 2.0 in its new chipsets, PC users will be able to connect several low-resolution cameras to their vision system for applications that are not critical with regard to time or security.
According to company officials at Stemmer Imaging GmbH (Puchheim, Germany; www.stemmer-imaging.de), Camera Link has begun and will continue to win market share in the digital interfacing sector of vision applications. More than 50 camera, frame-grabber, and board manufacturers have Camera-Link products available or under development. This dominant force will establish Camera Link as a viable standard. Compared to alternative interfaces, Camera Link offers higher performance and flexibility and lower component and cable prices. The future of the vision market will be based on a few interfacing standards, but there is still room for an adequate interface for a special application.