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Testing Drives for Ever-Faster High-Speed Digital I/O

Tue, 06/19/2007 - 7:37am
Suppliers continue to introduce new high-speed data acquisition products that often exceed their users’ requirements for data streaming.

New high-definition TV (HDTV) products and a host of LCD (liquid crystal display) imaging applications continue to drive the technologies for faster high-speed data acquisition (DAQ) systems. PXI, a rugged PC platform for measurement and automation systems, has become one of the preferred testing systems for these applications due to their high performance, large array of modular available products, rugged industrial form-factor, and standardized acceptance.

National Instruments offers three high-speed PXI Express instruments, from left, the PXIe-5122, PXIe-6537, and PXIe-1065. Image: National Instruments
National Instruments (NI), Austin, Texas, a sponsoring member of the PXI Systems Alliance (www.pxisa.org), recently introduced the industry’s first PXI Express high-speed instruments, along with the industry’s first 18-slot PXI Express chassis. NI’s offerings in this arena include the NI PXIe-5122 100 MS/sec, 100 MHz dual-channel digitizer and the NI PXIe-6536/6537 25/50 MHz 32-channel digital I/O modules. NI’s PXIe-1065 18-slot chassis offers up to 1 GB/sec per-slot dedicated bandwidth and a combination of both PXI and PXI Express slots. Besides video-based testing applications, these devices are also usable for signal intelligence, spectral monitoring, protocol emulations, semiconductor chip characterization, automotive crash testing, and aerospace systems monitoring—pretty much anywhere where large volumes of streaming data are generated.

The 50-MHz PXIe-6537digital I/O board has selectable voltages from 2.3 to 3.3 V and per-channel directional control of 32 digital lines. The board is based on NI’s DAQmx driver software and compatible with NI’s digital waveform generator so that users can interact with, create, edit, and import digital waveforms.

The PXIe-5122 high-resolution digitizer is a x4 PXIe module that can stream the full data rate of 400 MB/sec to the PC memory, hard disk, or RAID. It has a large dynamic range, a software-selectable input, and the ability to acquire more than 1 million waveforms in onboard memory. More than 50 measurement and analysis functions are programmed directly from the driver and can be expanded to more than 400 with NI’s LabVIEW and LabWindows software systems.

At 400 MB/sec, the PXIe-5122 is streaming data on both of its channels at its maximum data rate. The dedicated per slot bandwidth available on PXIe enables multimodular systems to obtain higher aggregate data rates. High-speed data record and playback applications are possible using the host computer’s memory or high-end storage solutions.

“PXI Express builds on the commercial PCI Express technologies to expand the applications served by the multivendor PXI standard,” says Eric Starkloff, NI’s director of test product marketing. “The addition of high-speed data streaming capabilities to PXI, while preserving compatibility to the more than 1,400 existing PXI modules from the more than 70 PXI vendor-members, has made PXI a leading platform for modular instrumentation.”

An accepted standard
PXI was developed in 1997 and launched in 1998. It was introduced as an open industry standard to meet the increasing demands of complex instrumentation systems. The organization is governed by PXISA, which is chartered to promote the PXI standard, ensure interoperability, and maintain the PXI specification. PXI systems consist of a chassis, system controller, and peripheral modules. Controller options include remote controllers from a desktop, workstation, server, or a laptop PC, and high-performance embedded controllers with either a Microsoft operating system or a real-time OS, such as NI’s LabVIEW Real-Time.

PXI Express (PXIe) is an integration of the PCI Express standard into CompactPCI and PXI. PXIe specification work was begun by the PXISA in mid-2005 and released in June 2006. PXIe increased the PXI backplane bandwidth from 132 MB/sec to 6 GB/sec, a more than 45 times improvement in bandwidth while maintaining software and hardware compatibility with PXI modules.

Click to enlarge.
This improvement was very important for large data streaming applications. Previously, users had to either slow down their sampling rates so data could be transferred over the bus or sample for shorter periods of time to accommodate the limited onboard instrument memory. Along with the faster buses, NI designers incorporated a signal streaming technology that allows users to now stream data directly to disk, according to Vineet Aggarwal, NI product marketing manager for data acquisition. “This is important for applications where large data streams are generated, such as in wind tunnel testing, automotive crash testing, or even testing of consumer electronics products.”

Video data hogs
More than most other applications, the testing of video-based applications has become today’s “data hog.” This can include the testing of imaging sensors, video processors, testing of video displays and pattern generation, simulating radar targets, LCD testing, video memory testing, and digital interfacing.

For example, one frame of black and white high-definition video (1,280 x 720 pixels) requires almost 1 MB of data, with a data rate of 30 MB/sec. An RGB (red-green-blue) frame would obviously require three times as many samples, and three times the data rates. Deep memory digital instruments with configurable width/depth features can generally provide more than 1 GB of 16-bit data allowing capture/playback of more than 30 sec of B&W video data, according to Dale Johnson, customer technical support manager at Geotest, Inc., a division of Marvin Test Systems, Irvine, Calif. (Geotest is also a sponsoring member of PXISA.)

An example of a streaming application for the newly introduced NI PXIe products reveals that for the 5122 digitizer with an acquisition rate of 100 MB/sec/channel with 2 simultaneous channels and a 16-bit (2 bytes) transfer per sample, the required bandwidth is: 100 MB/sec/channel x 2 channels x 2 bytes = 400 MB/sec.

For the 50-MHz digital I/O with 32 digital lines and each line being 1 bit, the required bandwidth is: 50 MHz x 32 lines x 1/8 bytes = 200 MB/sec.
A Lexicon of DAQ Buses
GPIB      General purpose interface bus, IEEE-488
Firewire  IEEE-1394 (up to 800 Mb/sec)
PCI          Peripheral component interconnect (up to 133 MB/sec)
cPCI       Compact PCI (Eurocard-based PC with PCI                 backplane)
PCI-X      PCI extended (up to 1 GB/sec)
PCIe        PCI Express (up to 200 MB/sec)
PXI          PCI Extensions for Instrumentation (based on                 CompactPCI)
PXIe        PXI Express

With these fast buses and signal streaming data transfer capabilities, the bottleneck for high-speed DAQ now becomes the hard drive to which the data is directly written. In order to take full advantage of this bandwidth and data streaming capabilities, larger and faster storage solutions like RAID (redundant array of independent drives, or disks) have been developed. There are many RAID schemes, mostly numbered and referred to as levels (i.e., RAID 5—a three-disk minimum set with distributed parity).

Using an 8 x 500 GB drive RAID system, for example, with a capacity of 4 TB, can allow you to capture data at 400 MB/sec for more than 2.5 hr. Application areas that benefit directly from this, include RF/IF data streaming in signal intelligence, data record and playback, and scientific applications.Bottlenecks can be created in other areas as well. There are obviously many variables that must be considered when working with video data captures. Users need to consider the number of channels required for digitizing resolution and the number of channels needed to support arrays. A larger number of channels may split up the new high-speed bandwidth to such a degree that it’s no longer “high-speed.”

Users also may need to determine the number of bits of resolution that are required for each analog channel they record, which may then require multiple digital instruments that are synchronized to one another. Interface logic levels may need to be determined, with programmable levels required to support custom interfaces. Clocking options may need to be determined to synchronize the unit under test to the instrument or vice versa. Software tools and drivers will also need to be selected, controlled, and compared. Libraries may need to be created for comparing the data collected with control patterns or digital video signals.

Oversights in some of these areas can create errors in the data collection. Data errors can be caused by bad physical connections to the measurement hardware, the architecture of the computer bus, and even the driver software. Generally, any system sampling on a regular basis faster than 1 MB/sec should be treated with special care. Errors can arise from delays or distortion in the timing signals, bus delays, noise, and a lack of synchronization.

All in all, while faster bandwidth may solve some problems with large data streams like those from video, other areas like testing methodologies, memory capabilities for digital capture and playback, and even post-processing requirements may cause problems that need new solutions.

—Tim Studt

Geotest, Inc., Irvine, Calif., 949-263-2222, www.geotestinc.com
National Instruments, Austin, Texas, 800-813-3693, www.ni.com
PXI Systems Alliance, www.pxisa.org
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