Windows-compliant driver speeds 1394 cameras
Although the standard Windows FireWire bus driver works well with devices such as cameras and hard drives equipped with the IEEE 1394a interface, the driver has not been updated to take advantage of newer functions introduced with IEEE 1394b.
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 6/1/2009 2:00:00 AM
 Jens Hashagen Software product manager Allied Vision Technologies |
Although the standard Windows FireWire bus driver works well with devices such as cameras and hard drives equipped with the IEEE 1394a interface, the driver has not been updated to take advantage of newer functions, especially higher data speeds, introduced with IEEE 1394b. Jens Hashagen, software product manager for Allied Vision Technologies, commented on how this affects engineers designing 1394b-compliant machine-vision systems running Windows, and he discussed the pros and cons of alternative 1394b bus drivers.
Q: What are the speed limitations that the original Microsoft FireWire bus driver places on 1394b-equipped cameras and other devices used in machine vision?
A: The native OHCI 1394 bus driver for Microsoft Windows XP, which was included in Microsoft's SP2 (Service Pack 2) release, was designed to operate primarily with 1394a-compatible devices. If you connect a 1394a camera to a 1394b bus using this driver, the maximum data-transfer rate on the bus decreases to 100 Mbps. The same is true if you connect a 1394b device. So, even though the 1394a maximum speed is specified at 400 Mbps, you need to do some tricks to reach that speed on a 1394b bus. And 400 Mbps is the maximum transfer rate you can reach with this driver, even for devices with a 1394b interface.
Q: How have designers of machine-vision systems coped with this situation?
A: The original 1394 driver of Windows XP SP1 (Service Pack 1) allowed 800-Mbps data-transfer rates, so for XP systems, you can run 1394b cameras at that speed. However, there is no such option if you're running Windows Vista.
The only other alternative is to use a proprietary bus driver from a camera vendor if it supports 800-Mbps data-transfer rates, as most of these drivers do, since that's why they were written. But most of these drivers are also noncompliant with the Windows 1394 standard driver. This means that other FireWire devices connected to the PC, such as hard drives, may not operate correctly.
Q: What other issues associated with the original 1394 driver affect industrial imaging?
A: There are stability issues that are unacceptable for industrial applications with high reliability demands. For example, with the Microsoft 1394 driver, an invalid packet sent in asynchronous mode could lead to an interruption of the communication between the PC and the camera, which could cause problems such as corrupt data. There's no mechanism in place to prevent that; you would have to restart the system.
Q: What has Allied Vision Technologies done to help improve this situation?
A: Besides fault-tolerant operation and low CPU load, the speed limit is the main reason we developed the AVT 1394 bus driver package. Since the driver conforms fully to the IEEE 1394 standard and complies with Microsoft's OHCI 1394 bus driver, it works with 32-bit versions of Windows, including Vista, XP, and 2000. It lets our 1394b-compliant AVT cameras transfer image data at 800 Mbps. Our bus driver can be used with the CMU (Carnegie Mellon University) 1394 camera driver, and with 1394 camera drivers based on the Microsoft bus driver, which are provided in imaging software from A&B Software, Cognex, Matrox, and National Instruments.
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