Speedy buses suit vision tasks

Camera and frame-grabber suppliers now offer products that operate on one of two high-speed digital buses. These buses simplify camera-to-computer connections so integrators and developers can concentrate on their machine-vision applications rather than on configuring a system. Each bus—IEEE 1394 and Camera Link—offers characteristics that may suit one type of application better than another. Learning more about these buses will help you choose the one best able to work in a new vision application

The need for high-speed and high-resolution images has pushed equipment suppliers to look at new ways to get data from a camera to a PC. Digital buses offer a flexible form of communication because they simply deliver streams of 1's and 0's. This digital data can represent whatever the equipment manufacturers wish—they're no longer locked into older broadcast-video standards such as NTSC, PAL, and SECAM.

Of course, the use of a digital bus means the analog-to-digital conversions of image information takes place in the camera, not in the attached computer. In general, digital cameras provide higher resolutions, greater pixel depths, and a wider range of frame rates than analog cameras, but as a result, they cost more than traditional cameras. Digital cameras offer another benefit, too: The transmission of digital signals instead of an analog signal reduces the chances for signal corruption on transmission lines.

Early digital cameras meant for machine-vision applications provided parallel RS-422 differential connections to a computer. (Manufacturers still offer a variety of such parallel-digital cameras.) The sheer bulk of the parallel connections caused problems: A cable had to provide two wires for each data bit or timing signal. So, a typical two-channel, 10-bit camera required at least 46 connections. Trigger and other camera-control lines could increase this number to 50 or more.

Thus, machine-vision applications required thick, stiff cables that cost much more than traditional coaxial-cable video connections. To make things worse, most camera families required custom cables that matched camera connectors to the connectors on frame-grabber or interface cards in a host computer. If you switched to a different camera type, you needed to buy a new, expensive cable.

Two standard high-speed buses—IEEE 1394 and Camera Link—specify fewer electrical connections and, thus, use smaller connectors than those used on digital cameras with proprietary interfaces. The use of standard connectors lets suppliers and developers buy less-expensive, off-the-shelf cables to connect cameras to computers.

In 1995, Apple Computer introduced the Firewire bus, the predecessor to the IEEE 1394 bus, mainly to let users easily connect peripherals to the company's line of computers. Since its introduction, Firewire has served primarily as a bus for high bandwidth applications such as communications between a PC and a consumer-grade camcorder. The electronics industry took note of this high-speed bus and standardized it as IEEE 1394. The 1394 Trade Association (www.1394ta.org) oversees the standard for the electronics industry.

An IEEE 1394 bus can operate at up to 400 Mbps over either a four- or six-wire cable (Figure 1). A four-wire interface lacks the two power lines provided by a six-wire interface. The different connector types prevent the mixing of four-wire and six-wire equipment. Devices on an IEEE 1394 bus operate in a peer-to-peer mode, which means a camcorder can communicate with a VCR without involving a host PC in the communication.

The machine-vision industry thought the IEEE 1394 bus looked promising as a high-speed path between a camera and a computer, and several companies now offer machine-vision cameras that come with a standard IEEE 1394 bus connection. Although communications over an IEEE 1394 bus can use two data-transfer protocols—asynchronous (which guarantees delivery), and isochronous (which guarantees bandwidth)—the 1394 Trade Association's Instrumentation and Industrial Digital Camera (IIDC) specification states that machine-vision cameras must use the isochronous protocol.

Isochronous transfer requires a bus manager that can exist within a device on the bus or, more commonly, on a controller card in a host PC. The manager receives bandwidth requests from devices on the bus and apportions the available bandwidth accordingly. The manager also controls power on the bus.

Every 125 µs, an IEEE 1394 bus manager sends a cycle-start packet to devices on the bus. The standard guarantees devices that have requested isochronous bandwidth a predetermined number of data packets in each 125-µs period. After the isochronous devices send all their data packets, any remaining time can accommodate waiting asynchronous data transfers (Figure 2). Because the recipient of an isochronous transfer does not acknowledge properly received packets, the isochronous protocol provides no guarantee of data integrity. But the isochronous protocol does guarantee the arrival of data packets in the proper sequence. Although the isochronous transfers lack any error checking, in most cases (particularly when only two devices exist on a bus), receptions of data take place successfully. Thus, the tradeoff of speed for the time-consuming error checking in an asynchronous transfer is worth the risk of an occasional error.

The IEEE 1394 Trade Association's IIDC spec provides for standard image formats and camera specifications. The image format for color, for example, includes compression standards for YUV (8, 12, 16, or 24 bits/pixel) and RGB (24 bits/pixel). The spec also defines standard operations that can control a camera's trigger, gain setting, and shutter. Frame rates can range from 1.875 frames/s to 60 frames/s. A 640x480-pixel monochrome camera can operate over the IEEE 1394 bus at 30 frames/s, and a 1280x960-pixel monochrome camera can operate at 7.5 frames/s. Thus, you can trade resolution for image-acquisition speed.

Acquisition of color images over an IEEE 1394 bus can run into a limit caused by both bus bandwidth and software. The software that obtains a color image encoded as YUV data must convert the image to the RGB data needed by most image-processing and display software. Today, few PCs have the processing power to achieve 30 frames/s when converting a 640x480 image from YUV to RGB.

The Camera Link standard takes a slightly different approach and provides for serial transfers, but over several parallel lines (Figure 3). Camera interface circuits that conform to the Camera Link standard use low-voltage differential signaling (LVDS) ICs developed by National Semiconductor (Santa Clara, CA) for what the company calls Channel Link communications (www.national.com/appinfo/lvds/). The unidirectional differential signals in a Channel Link circuit operate with a differential voltage of about 350 mV and at a data rate of up to 1.6 Gbps. The Camera Link spec defines the electrical signals over which a camera and a computer communicate (Ref. 1). The Automated Imaging Association (www.machinevisiononline.org) now maintains the Camera Link standard.

The Camera Link interface can operate at three levels: base, medium, and full. The base configuration provides 28 data lines—three bytes for video and four bits for timing information. Interface circuits serialize and transmit the 28 data bits over four parallel differential lines. The medium level adds 28 more data lines (three bytes and four timing bits), and the full level adds another 20 data lines (two bytes and four timing bits). The three levels let interfaces accommodate the wider data paths required by multi-tap cameras (those with more than one video-output stream) and allow for future expansion. The full Channel Link configuration includes three Channel Link receivers and achieves an effective bandwidth of 4.8 Gbps.

Each Camera Link level also includes a clock signal, a serial port, and four unidirectional camera-control signals that go from the computer to the camera. The benefit of the Camera Link specification centers on its definition of standard electrical signals for frame grabbers and cameras. The standard also specifies connector and cable types, and it includes specs for camera triggering and other real-time camera controls.

Although a Camera Link cable contains many more conductors than an IEEE 1394 cable, Camera Link transmissions take place quickly, and data transfers do not involve multiple devices on a single bus. Each Camera Link camera connects to a PC through a separate interface circuit.

Camera Link's maximum throughput accommodates high-bandwidth applications such as the high-speed inspection of automotive parts. The Camera Link cameras currently on the market offer 1024x1024-pixel resolution and can operate at 50 frames/s. The Camera Link standard includes hardware for real-time control of a camera. The IEEE 1394 spec, due to its origins in the audio-visual PC market, lacks that hardwired capability, although a PC can use a standard data packet to send control information over the IEEE 1394 bus to a camera (Table 1).

To acquire an image from either an IEEE 1394 or a Camera Link camera, you need driver software provided by the image-acquisition device vendor. Most operating systems released during the past two years include an IEEE 1394 driver and simple application software that support consumer-grade cameras. But operating systems require a special driver to acquire image data from industrial-grade cameras, decode the data, and format it for use by machine-vision software. The IEEE 1394 driver included with Windows XP will detect industrial-grade cameras and acquire images from them, but this rudimentary driver lacks many of the features provided in drivers available from IEEE 1394 interface suppliers and machine-vision vendors.

Although the Camera Link standard provides an electrical specification, different camera models may employ different methods to send image data to a host computer, and thus they may need different drivers. The methods vary depending on whether the camera operates at the base, medium, or full level and on the arrangement of the pixels coming out of the camera. A multi-tap camera, for example, can provide several simultaneous bit streams of image data. Thus, a Camera Link driver must contain configuration data for each specific type of camera so the driver knows how to arrange the image bits as they arrive from a camera. Instead of saving all the configuration data in the driver, an IEEE 1394 driver will retrieve specific configuration information from a camera on the bus.

In many machine-vision systems, commands often flow from a host PC to a camera. Software may need to change camera parameters such as shutter speed, trigger mode, frame rate, and so on while a camera is in use. The Camera Link standard provides for special control lines and a serial port for just such communication. The driver software controls the use of these lines. Systems based on IEEE 1394 change camera parameters by sending command packets to devices on the bus.

Cameras based on the IEEE 1394 and on the Camera Link standards each have their merits, and each has a place in the machine-vision market. Cameras based on the IEEE 1394 standard will inherit applications traditionally served by analog cameras, and Camera Link cameras will dominate applications that require high-speed digital cameras. Camera and frame-grabber companies continue to invest in both technologies to give users choices and to make machine-vision equipment easier to use by people in the manufacturing-test community.