A CLEAR WINNER IN CAMERA INTERFACING
A key challenge facing today's camera companies and vision-system designers is deciding how to transfer imaging and control data between cameras and PCs.
By George Chamberlain
President, Pleora Technologies
Kanata, ON, Canada; www.pleora.com
A key challenge facing today's camera companies and vision-system designers is deciding how to transfer imaging and control data between cameras and PCs. For decades, these transfers were handled by proprietary digital technologies or lower-performing analog standards. In the past few years, however, four digital transport standards have come into play, triggering a dramatic shift in the vision-system landscape.
Camera Link, launched in 2000, was first on the scene, followed by IEEE 1394 (FireWire), USB (Universal Serial Bus), and Gigabit Ethernet. Each differs in terms of bandwidth, flexibility, and ease-of-use, but all are helping lower costs, shorten development cycles, simplify implementations, and broaden accessible markets.
Ultimately, application requirements dictate which standard to use. Yet subtle differences in the capabilities of each can have a significant impact on system performance (see table on p. D17).
Camera Link, designed for high-performance vision applications, streams data reliably at very high rates—up to 7.14 Gbits/s—over dedicated point-to-point copper links of 10 meters or less. This short reach limits its usefulness in many applications because PCs are essentially tethered to cameras. Fiberoptic extenders stretch the reach to 500 m, but at significant expense.
Camera Link is also limited on the networking front, with no flexibility for interconnecting multiple cameras or centralizing control and maintenance. It also runs over specialized cable and terminates on PCI frame grabbers, both of which enjoy few economies of scale. Despite its limitations, Camera Link delivers unmatched data rates, and is supported by a range of high-end camera manufacturers.
IEEE 1394b (FireWire) is a consumer standard developed for linking digital camcorders to PCs. It offers "plug-and-play" usability, and uses a readily available, low-cost PC interface. FireWire is based on a bus topology, where 800 Mbits/s is shared by up to 63 devices in a "daisy-chain" network. Devices can be separated by 4.5 m, to a maximum length of 72 m, over twisted pair copper cable.
FireWire sends data over both asynchronous and isochronous channels. Asynchronous links are typically used for latency (delay)-tolerant data, such as control signals, and isochronous channels for latency-sensitive data-like video. Up to 80% of the bandwidth, or 640 Mbits/s, can be allocated to a single camera over an isochronous channel. With the shared bus, however, only one camera can access this bandwidth at a time, which means high-priority data can be delayed and reliability compromised. Moreover, FireWire does not include error-checking for isochronous transfers, so data delivery over these links is not guaranteed.
Since one PC can remotely control multiple cameras, the scaleability and networking flexibility of IEEE 1394b is superior to that of Camera Link. However, even at the maximum rate of 640 Mbits/s, IEEE 1394b data transfers are too slow to support higher-end digital cameras. Many high-speed applications also require real-time PC processing, which is difficult with IEEE 1394b's Windows-based driver, which "hogs" the PC's CPU during data transfers. Some companies have addressed this limitation by developing their own drivers.
Another drawback of IEEE 1394b is the price of its copper cable. Category-5 local-area network (LAN) cable, which costs up to 10 times less, can be used instead, but this limits total available bandwidth to 100 Mbits/s.
Numerous companies support IEEE 1394b, and their cameras are gaining ground in applications where performance requirements are not overly rigorous, such as microscopy, scientific imaging, and process triggering.
Universal Serial Bus
USB 2.0, a consumer standard for connecting peripherals to PCs, has much in common with IEEE 1394b. It leverages a built-in PC interface, uses a shared bus, and supports asynchronous and isochronous transfers. USB 2.0 delivers up to 480 Mbits/s of bandwidth, shared by up to 127 hub-connected devices in a master/slave relationship. Direct PC connections extend up to 5 m. Hubs extend the reach to 30 m, with maximum spans of 5 m between devices.
Like IEEE 1394b, USB 2.0 is best suited for less-demanding applications. Only a few vendors have released USB 2.0 cameras, and the standard is having a relatively low impact on the vision system industry.
The fourth standard, Ethernet, was launched about 25 years ago and has evolved into the dominant global local-area-network technology, covering 97% of installed network connections. Ethernet is flexible, easy to implement and manage, and highly scalable.
On one network, over low-cost Category-5 copper, Ethernet connections operate at 10 Mbits/s, 100 Mbits/s, or 1000 Mbits/s (1 Gbit/s). The top data rate—1 Gbit/s or Gigabit Ethernet (GigE) —is fast enough to support 90% of today's vision-system applications. The next generation of Ethernet, 10GigE, which delivers 10 Gbits/s, is available today over fiber and is expected to run over copper in 2004. Links at all rates interwork seamlessly, allowing users to allocate bandwidth as needed in a multi-pronged network.
Ethernet uses dedicated links, so bandwidth is not shared between cameras, as with IEEE 1394b and USB 2.0. Ethernet supports many connection options, including one camera to one PC, multiple cameras to one PC, one camera to multiple PCs, and multiple cameras to multiple PCs. In configurations with multiple cameras or PCs, interconnections are through full-duplex, inexpensive Ethernet switches. PCs links are through RJ-45 plugs, which are either already on the PC or added via low-cost network interface cards.
Ethernet also goes the distance, supporting individual links of 100 m over Category-5 copper. With switches, the reach is unlimited. This means PCs can migrate out of operations areas, and control and maintenance functions can be centralized in one room. Ethernet's networking flexibility also allows image data to be "multicast," or simultaneously distributed, to multiple PCs. This permits, for example, one PC to display the image, one or more PCs to process it, and another PC to archive it. Furthermore, Ethernet is robust. Most of today's commercial Ethernet equipment supports sophisticated Quality of Service rules, making it suitable for carrying latency-sensitive traffic like video.
Although each camera interface standard has strengths, Gigabit Ethernet delivers a unique combination of distance, bandwidth, networking flexibility, and ease of use that—with its low cost—makes it the clear winner. Numerous 100-Mbit/s Ethernet cameras are already available, and GigE interconnect is rapidly coming on-stream. It will be interesting to watch the market develop over the next few years. ..