FIREWIRE OFFERS HIGH-SPEED SERIAL NETWORKING
By Jerry Fife
Senior Product Manager
Visual Imaging Products
Sony Electronics
Park Ridge, NJ, USA www.sony.com/videocameras
The FireWire digital camera interface standard, also known as IEEE 1394, is a high-speed serial bus that meets the needs of a variety of machine-vision applications. Its key features are high bandwidth to support high-speed image capture, deterministic operation, flexible serial network topology that supports numerous devices, small standard connector and cable, and an open protocol environment that can support different types of devices on a network at the same time.
Deterministic performance is critical for demanding machine-vision environments. FireWire provides two methods of transferring imaging data to support "guaranteed bandwidth" and "guaranteed delivery." Isochronous transfers are used for delivering time-critical video and audio data in a broadcast or "one-to-all" mode. To guarantee bandwidth, the desired bandwidth on the bus is pre-allocated to a device (for example, a camera) as an "iso-channel" and it is always available to that device. All other devices on the bus (for example, a computer) monitor the iso-channel for information they want to use.
Asynchronous transfers are used to deliver content-critical data such as commands, status, and files. To guarantee delivery, the transmitting device sends the data to a specific device in a "one-to-one" mode and receives a confirming message from the receiving device. Asynchronous transfers are similar to computer file transfers in that the transfer goes to a specific location and the transfer will retry if necessary until completed. FireWire uses both transfer methods simultaneously to address the needs for both guaranteed bandwidth and delivery.
High bandwidth
FireWire was designed to handle high-bandwidth data such as video. The initial version of FireWire, IEEE 1394a, supports transfer rates of 100, 200, and 400 Mbits/s; the highest rate translates into approximately 40 Mbytes/s of sustainable throughput. Because isochronous transfers have priority on the bus, 80% of the bus bandwidth is available for isochronous transfers. This allocation ensures that there is always bandwidth available for delivery of asynchronous transfers. The isochronous throughput of 32 Mbytes/s is sufficient to support most imaging requirements. For example, a monochrome VGA camera running at 30 frames/s needs only 9.2 Mbytes/s of bandwidth. Multiple cameras can be transmitting simultaneously on the bus and the bandwidth can be dynamically allocated as needed.
The new FireWire standard, IEEE 1394b, is now coming to market and provides data transfers of 800 Mbits/s and even supports higher rates of 1600 and 3200 Mbits/s. Disk drives and interfaces boards are now available that support 800 Mbits/s, and industrial cameras will soon be available at 800 Mbits/s.
Flexible topology
The flexible topology of FireWire allows users to organize a network of devices in different configurations, such as daisy chain, tree, and star. As many as 63 devices can share the network. Because FireWire is a peer-to-peer network, even multiple computers can be connected to the same network and devices can communicate directly without the need for a master computer.
The network is easy to set up because it dynamically configures itself based upon the connected devices; no identification switch or addresses have to be set up. In addition, devices can be hot-plugged and unplugged in "plug-and-play" mode without disrupting other activities on the bus. There is no requirement to reboot the system.
Standard cables and connectors are used in FireWire-based networks. This means that many different devices can share the cables. The six-pin connector is the most common connector used in machine-vision applications because it carries power to end devices such as cameras and hubs. Latching and hardened versions of the six-pin connector are available to support the more demanding requirements of some industrial environments.
The four-pin connector used on many battery-powered devices like camcorders and laptops can also be used on the FireWire bus but this connector does not transport power. Users can select the connector and cable that best suit their needs.
Common myths
A common myth about a FireWire-based system is that cable lengths are restricted. The IEEE 1394a specification calls for a 4.5-meter maximum length of a single cable, and up to 16 cables and hubs can be connected between any two devices. This specification therefore allows for up to a 72-meter cable run to be used between any two devices. Cables up to 10 meters long are frequently used for small networks or to reduce the number of intermediate hubs. Fiberoptic connectors and cables are available for hub use that can extend the network distance up to 500 meters. FireWire networks can be configured to support the cable distances of most machine-vision environments. There is no distance limit on cable length for IEEE 1394b.
Another myth about FireWire is that more bandwidth is needed to support multiple cameras. FireWire allows as many as 63 devices to be connected to the serial bus and they can all be communicating simultaneously using asynchronous transfers. Image data transfers are typically done using isochronous transfers for guaranteed bandwidth.
The number of FireWire cameras that can be configured for simultaneous transfers is typically limited to the four available iso-channels. (The IEEE 1394 OHCI spec defines four to 32 iso-channels, but most chip sets support only the minimum of four iso-channels.) The 32 Mbytes/s of isochronous bandwidth available can be shared as needed by the iso-channels. Because most vision applications do not involve imaging from all the cameras simultaneously, the available bandwidth and number of iso-channels are usually sufficient. In addition, the cameras using the isochronous channels and bandwidth can be dynamically changed as needed by the application. If more bandwidth and iso-channels are required, users can just add additional IEEE 1394 interface cards.
Other interfaces
Other interface-standard methods are available to connect digital cameras to computers, such as Camera Link and Universal Serial Bus (USB; see table). FireWire- and USB-based networks both connect multiple devices together and share the available bandwidth. USB comes in two versions: the original USB 1.0, which delivers about a 1-Mbyte/s (11-Mbit/s) throughput, and the later USB 2.0, which delivers about a 40-Mbyte/s (480-Mbit/s) throughput. Camera link provides throughputs to 680 Mbytes/s.
The IEEE 1394a version of FireWire provides about 40 Mbytes/s (400 Mbits/s) of bandwidth, and IEEE 1394b offers about 80 Mbytes/s (800 Mbits/s). The USB and FireWire interfaces offer approximately the same built-in capabilities for cable length, number of devices, interface availability, and computer connectivity. However, FireWire offers the significant advantage of sufficient power on the cable to run high-performance cameras and peer-to-peer networking of devices. In addition, FireWire IEEE 1394b increases the bandwidth to 80 Mbytes/s or more as well as handles longer cable lengths.
Camera Link is a point-to-point connection between the camera and the interface board. This interface continues the analog camera model of connecting the camera through a dedicated cable to a frame-grabber board, which offers the advantage of a higher bandwidth. Its disadvantages are the need for larger connectors and cables and a limited cable distance. Moreover, the specialized Camera Link interface requires the addition of an expensive interface board to the computer. Using FireWire as a backbone for a system reduces overall system cost because it calls for fewer interface boards and lower-cost cables. In addition, standard notebook or small slotless computers can be used with FireWire-based cameras for more flexibility and lower cost.
After evaluating the merits and detriments of the three common camera interfaces, FireWire seems to offer the best combination of cost-performance features to satisfy a range of vision and imaging system environments: high bandwidth, deterministic data transfers, flexible dynamic topology, many connected devices per network, small connector and cable, and open protocol.
REFERENCES
J. Fife,Vision Systems Design, February 2003, p. 19.
Vision Systems Design, Sept. 2002, "FireWire Special Report," www.vision-systems.com.