GigE Vision offers choice of software drivers
-- Test & Measurement World, 10/23/2007 8:21:00 AM
Machine-vision systems based on the GigE Vision standard can employ software drivers to perform the tasks once handled by a frame grabber. Dwayne Crawford, product manager for Matrox Imaging, explained some of the available types of software drivers.
"A filter driver exploits the standard Windows protocol stack, identifying GigE Vision data for processing but also allowing other protocols to pass through unaltered," said Crawford. "This approach requires only a single cable and a single network interface card to manage many types of devices."
A filter driver will work with all makes of network cards, although it runs on some better than others because cards offer different feature sets. Crawford noted that Intel cards offer three key features that many others don't—jumbo packets, interrupt throttling, and receive descriptors.
"Jumbo packets are exactly what they sound like," he said. "Larger data packets reduce the number of packets and consequently the number of interrupts for the host. The larger packets can increase the latency required for transmission, however—a drawback if you are acquiring images in real time. This problem is particularly acute with high data rates in noisy environments that can cause a significant number of packet resends.
"Interrupt throttling groups the interrupts, processing them only every so many milliseconds. It's like when you are getting ready for a party, but you have to stop constantly when the doorbell rings. You could open the door less often, but then your guests will be standing longer on your doorstep.
"Receive descriptors assure that the system has enough memory packets and sufficient interrupt moderation to both capture and buffer all of the data, thereby preventing lost packets."
With another type of driver, the stack-replacement driver, a custom stack completely replaces the Windows-supplied version. This special stack accepts only GigE Vision data, dropping everything else. Motion-control and other protocols require a separate cable running across the factory floor and a separate network card, increasing the complexity, inconvenience, and cost of the overall system.
Proponents of this approach contend that it requires less work from the protocol stack because the stack doesn't have to determine whether a data packet contains GigE Vision or something else. "In reality, that 'advantage' saves only a few steps out of the thousands that the protocol requires," commented Crawford. "We have compared our filter stack with the stack-replacement equivalent and found comparable performance.
"Besides," he continued, "replacing the standard stack means designing your own. Intel and Microsoft have already spent enormous resources developing and optimizing the standard Windows stack. Machine-vision companies can't dedicate the resources necessary to approach their level of performance. Also, a stack-replacement driver can't work with any network interface card. It has to already know the architecture of the card it will work with."
Crawford explained that his company's Matrox Solios GigE interface card represents a third type of driver. "It works similarly to the filter driver," he said, "but it offloads the GigE Vision protocol onto special hardware that reconstructs an entire image from the hundreds of data packets, passing the resulting image to the computer host. This approach permits large images and high frame rates, freeing up significant host resources and avoiding the interrupt loading on the host."
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