By Joseph Sgro
Chief Executive Officer
Alacron Inc.
Nashua, NH, USA; www.alacron.com
Several camera-interface protocols have emerged in recent years, the most popular of which are Camera Link, FireWire (IEEE 1394), Universal Serial Bus (USB) 2.0, and Gigabit Ethernet. Frame-grabber boards and other machine-vision system-integration devices depend on such protocols to interface cameras with PCs. The number of interfaces available to camera users has grown as a result of two recent market trends. The first is the standardization of peripheral buses, which was spurred by the desire of OEM PC manufacturers to add peripherals, including machine-vision cameras, to their PCs without the need for expert assistance. The second trend, which results in increased choices for camera users, is the rapid escalation of CPU and memory data rates, which increasingly permit native PC-based real- or nearly real-time processing.
As part of its frame-grabber design program, Alacron recently decided to examine the comparative merits of the various interface protocols and their probable impact on the frame-grabber and machine-vision markets. It concluded that the USB 2.0 interface standard meets more of the criteria important to camera buyers than the other competing interface protocols. USB 2.0 provides significant user benefits in the form of adequate performance and simplicity of operation, which in many cases outweigh the disadvantage of the current lack of upgrade paths. This interface also offers standards compatibility, broad industry support, plug-and-play installation, and cost-effectiveness.
During 2003, millions of 2.4- to 3.0-GHz motherboards containing built-in USB 2.0 ports at no additional cost will be shipped worldwide. This proliferation indicates that USB 2.0 should eventually become the de facto standard for host-based vision cameras and high-speed image processing. USB 2.0 is expected to be the preferred connection for most PC peripherals, and it should coexist with the FireWire interface in audio/visual consumer-electronics devices. Although Camera Link is projected to dominate in high-speed applications, USB 2.0 will achieve faster speeds and lower costs than FireWire. Therefore, it will govern the machine-vision market with some competition from IEEE 1394b and Gigabit Ethernet due to their low cost and wide availability. For machine-vision system end users, the primary USB 2.0 benefits will be lower cost than previous camera and frame-grabber platforms together with plug-and-play installation.
USB principles
The USB standard was originally developed by computer manufacturers in 1995 to minimize the number of ports in the back panel of a PC. The major goal of USB was to create an external expansion bus that makes adding peripherals to a PC more convenient and transparent to users. USB 2.0 is the 2000 update of the original USB 1.1 standard that is incorporated into most PC motherboards.
The USB 2.0 specification extends the maximum speed of the connection from 12 Mbits/s for USB 1.1 to 480 Mbits/s (60 Mbytes/s). This speed enables the real-time transfer of data for high-definition video conferencing or 320 × 240-pixel images at 500 frames/s for high-speed video motion analysis. USB 2.0 also provides bidirectional serial communications for camera setup and control, triggering, strobing, and I/O signaling. All of the USB 1.1 and 2.0 connectors and cables provide the same 5-m length between devices. A connected peripheral can either be self-powered or bus-powered by up to 500 mA. A USB device can be plugged into a powered system at any time.
During USB operation, the PC is the master and the peripherals are slaves. The PC makes requests, and the peripherals respond. For maximum bandwidth, USB 2.0 transfers as many as 13 packets containing 512 bytes of data during each microframe in isochronous mode. This translates to a sustained data rate of more than 53 Mbytes/s.
As a result of the growing need for direct interconnectivity between devices, the USB 2.0 specification was recently supplemented with "On-The-Go" (OTG) capability. An OTG peripheral has built-in host capability and enables direct data transfer, peer-to-peer, to another USB or OTG peripheral, without the need for a PC.
Vendor support
Intel Corp. (Santa Clara, CA, USA; www.intel.com) has released a family of chipsets starting with the 845 family for the Pentium-4-based family of processors. These chipsets, which include the ICH4 South Bridge chip, all have an embedded USB 2.0 enhanced host controller interface and hub, which are capable of supporting up to six high- or low-speed ports. Similar support chips for other vendor processors and bridges also incorporate the USB 2.0 standard. Currently, more than 80% of all new motherboards incorporate USB 2.0, resulting in its domination of the interface market.
Because the ICH4 chip connects directly to the memory controller (the North Bridge chip) over a 266-Mbyte/s (32-bit/66-MHz) hub interface, it can simultaneously move data from a USB 2.0 port at maximum rates without reducing the bandwidth capacity of the 32-bit/33-MHz PCI bus. Therefore, more USB 2.0 PCI adaptor cards can be added into the system without affecting the bus.
Microsoft has released a USB 2.0 driver for Windows XP and has upgrades for Windows ME and Windows 2000. However, it has stated that it will not provide USB 2.0 driver support for Windows 9x or earlier Windows operating systems. The Linux software community has also recognized the imminent growth of USB 2.0, and has released driver support in its latest kernel.
Interface comparisons
Current host (native) platforms that depend on the host processor and disk interfaces to process or store image data streams in real-time usually peak at 240 to 320 Mbits/s (30 to 40 Mbytes/s). USB 2.0 is more than adequate for these applications, which constitute the majority of machine-vision installations. At speeds in excess of 400 to 480 Mbits/s (50 to 60 Mbytes/s), either a coprocessor or a more expensive disk array is needed to process or store data in real time. Because system cost and complexity increases significantly for these applications, they form a more expensive and smaller segment of the machine-vision market.
The USB 2.0 interface standard is judged to be superior or competitive with the other common camera interfaces for real-time host processing or storage because it provides adequate speed, low cost, and virtually universal hardware and software support (see table). Whereas USB2.0 is slower than IEEE 1394b (FireWire), Gigabit Ethernet, and Camera Link, its sustained throughput is more than adequate for most machine-vision applications. Motherboard and PC support for USB 2.0 is the highest compared to the other interfaces, and broad software support is available from a range of vendors. The key shortcoming to USB 2.0 is its limited cable length and power compared to the other interfaces, especially IEEE 1394b.
USB pluses and minuses
Performance is a definite plus for USB 2.0 because it is fast. Most machine-vision cameras used for real-time native processing applications or storage only need bandwidth between 200 and 320 Mbits/s. Therefore, the USB 2.0 480-Mbit/s capability provides ample bandwidth. Although some of the other interfaces, such as Camera Link, are faster, the higher speed often is unnecessary due to the limitations of PC host processing and storage.
The current lack of an upgrade path, however, is a disadvantage for USB 2.0. No initiatives to upgrade the speed of USB 2.0 beyond 480 Mbits/s have been undertaken as yet. This compares unfavorably with the FireWire standard, which has definitions for 1600- and 3200-Mbit/s extensions. Similarly, Gigabit Ethernet is already supported by silicon. Camera Link can be extended to Medium and Full configurations and an extended specification using 80 bits running at 85 MHz (that is, 6800 Mbits/s). If the upgrade path or potential upgrade path is important to an application, then USB 2.0 is problematic.
USB 2.0 enjoys broad industry support, and chipset manufacturers are integrating USB 2.0 host controllers into their chipsets. As a result, the system costs of implementing USB 2.0 are incremental—as opposed to the costs of adding a discrete host controller for FireWire. Therefore, USB 2.0 is the preferred connection for most PC peripherals and is installed on nearly all Intel Pentium- and Advanced Micro Devices (San Diego, CA, USA; www.amd.com) Athlon-processor boards. Apple Computer (Cupertino, CA, USA; www.apple.com) has installed USB 2.0, along with IEEE 1394b, in its new Power Mac G5 64-bit machine. However, the FireWire interface will continue to coexist with USB 2.0 in audio/visual consumer electronic devices, mainly because of the support of the Sony Electronics (Park Ridge, NJ, USA; www.sony.com/videocameras) camera line for the IEEE 1394a interface.
Support for the USB 2.0 interface has been growing in the machine-vision community due to the availability of a variety of cameras, add-in boards, and software tools. Because USB 2.0 is relatively recent, its camera support still trails those of IEEE 1394a and Camera Link, but not that of IEEE 1394b or Gigabit Ethernet. In the near term, however, support for USB 2.0 should equal or exceed that of its competitors. One major reason is the relatively slow adoption of the IEEE 1394a standard outside of Sony.
Despite the substantial increase in bandwidth, USB 2.0 is still a user-friendly technology because it uses inexpensive connectors and offers easy installation via plug-and-play. Moreover, only one, easily obtainable, connector style is needed for the PC system, making USB 2.0 similar to IEEE1394a and Gigabit Ethernet. For higher-speed applications, the Camera Link interface needs two connectors.
Most system motherboards come with USB 2.0 built in at no extra cost. Fewer chipsets support IEEE 1394a or Gigabit Ethernet (IEEE 802.3ab), and no motherboard supports Camera Link. Add-in boards for USB 2.0 are similarly priced from $30, which is the cheapest add-in interface available and is only rivaled by IEEE 1394a. Camera Link, however, is faster but also more expensive toadopt.
