This relatively high-speed bus with the ability to transmit different types of information packets can be found in both machine vision and motion control applications.
According to Tony Iglesias, project manager for National Instruments, the Austin, Texas manufacturer of automation and data acquisition products, the 1394 bus has gone through several incarnations over the years. “Right now,” he says, “the most popular is 1394A. There are six wires on the cable—four are communication conductors and two carry power to the devices. This ability to provide power in one cable is a critical differentiation for this bus.”
Firewire allows two protocols, or methods of communicating, on the cable at the same time. One is called asynchronous, the other isochronous. Iglesias reports, “Asynchronous transmission guarantees delivery of the packet, but, like the popular Ethernet protocol transmission control protocol/Internet protocol (TCP/IP), it does not guarantee bandwidth, that is, a guaranteed time of delivery. It is based on the theory of retries. A device tries to send a packet of information. If it senses network traffic, then it waits for a period of time and tries again. Meanwhile, isochronous communication guarantees bandwidth (time of delivery), but not necessarily delivery of all packets. This latter is ideal for vision applications. The bus supports both protocols on a device-by-device basis.”
When a device is plugged into the bus, it negotiates with the controller, telling it whether it wants asynchronous or isochronous communications and what amount of bandwidth. The controller evaluates the situation to determine that there is sufficient bandwidth available.
One thing propelling 1394 acceptance in machine vision applications has been development of a camera standard, which allows cameras to self-describe themselves to 1394 controllers. Standard information includes name, model number, frame rate, resolution—whether color or black-and-white—and supported features such as triggering modes, shutter speeds and gain. “By using the standard,” Iglesias says, “a compact vision system can use a wide variety of cameras. And the number and type of cameras supporting this standard is growing every day. Even infrared cameras are appearing, allowing expansion of machine vision into even more diverse applications.”
Ideal for motion
Vision is not the only industrial application that can benefit from Firewire. The same flexible characteristics plus the inherent speed of the network lends itself well to distributed motion control.
Keith Bailey, director of OEM business at Adept Technology Inc., the Livermore, Calif. manufacturer of robotic and vision systems says, “We wanted to develop a distributed motion control architecture. Our system at the time was based on the VME bus computer standard. This entailed multiple computer boards, processors, input/output (I/O) cards, vision cards, motion cards and more. It was a centralized system with all wires routed back to the central controller.”
Adept started to evaluate alternatives to the centralized computer architecture, but it still needed speed and guaranteed delivery of information packets to devices.
“We looked for a new bus and realized Microsoft had a push on to put Firewire in PCs,” reports Bailey. “We wanted to get on a consumer-related architecture rather than industrial, since those don’t have the volumes of Firewire and USB (universal serial bus) that drive down costs. USB had too many limitations at the time, but Firewire was fast enough and pushed by major players. So we went that direction.”
Bailey concludes, “One of the beauties of Firewire for servo applications is that it has both asynchronous and isochronous data transfer. Servo control requires the guaranteed time of arrival for data. Typical servo rates run at 1 kHz, while high performance servos may be at 8 kHz. The built-in isochronous clock runs at 8 kHz, so its very nature fits servo extremely well.”
Gary Mintchell, [email protected]
