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[16 November 2004}: Capturing signals using PC based instrumentation at tens or hundreds of megabytes per second inevitably hits a bottleneck when continuous streaming to the hard disk is required.
It is for this reason that large memory is often installed onto acquisition boards, but where gigabytes of data are involved even this is not sufficient. So is there a way forward? Until recently IDE hard drives for the standard PC and notebook plugged into PATA (Parallel Advanced Technology Attachment) ribbon cable. This interface has been steadily improved in terms of clock speed but hits a problem due to interference between conductors making up the wide data ribbon cables, which is worst at high clock speeds. To combat this development of drives that can use SATA (Serial Advanced Technology Attachment) is seen as a way forward, where serial transmissions run across a single control channel compared to the multiple channels of a parallel interface. This means that at the same clock speeds, the serial line will carry less data, but because the serial method requires fewer wires, less interference is generated to cause data integrity problems. This allows for serial transmission methods to run at much higher speeds than the equivalent parallel methods. Companies such as Dell are now starting to fit these drives as standard into many of their computers, and a suitable controller card can be retrofitted into older PCs. So is this a major jump in performance? Evidence suggests a slight improvement but there are other factors such as a hard drives cache size, spindle speed and access times to take into account. Having a lightening fast connection bus connection is not the "be and end all" however RAID systems do show much promise.RAID stands for redundant arrays of inexpensive disks, but the word "redundant" might be a little misleading. In fact RAID usefully combines multiple small, inexpensive disk drives into an array of disk drives that yields performance and data security benefits which can exceed that of a single large (more expensive) drive. This array of drives appears to the computer as a single logical storage unit or drive, but it must be noted that there is no gain in storage size in this arrangement, for example using two 80Gbyte drives will yield 80Gbyte. The key to increased performance under RAID is parallelism, where simultaneous access to multiple disks allows data to be written to or read from a RAID array faster than would be possible with a single drive. RAID is commonly available in configurations RAID 0, 1, 2, 3, 4, 5 or 10 (with more are being added over time), but how to choose? Here we will look closer at systems 0 and 1, both of which will work with just two drives, and represent the entry level system most applicable for a PC based instrumentation system. RAID Level 0. At this level, data is split across drives by a process called "striping", resulting in higher data throughput. As no redundant information is stored, performance is very good and can be expected to at least double that of a single drive, but the failure of any disk in the array results in data loss. RAID Level 1 provides redundancy by writing all data to two (or more) drives in a "mirroring" process. As the data are identical on each drive, having a redundant drive has the advantage of always having a copy of the data safe. The performance of a level 1 array tends to be faster on reads and slower on writes compared to a single drive or Raid 0, but if either drive fails, no data are lost. The choice really comes down to which is the most important to your application performance or data security. Most users of PC instrumentation will go for RAID 0 as the best and most viable way to capture gigabytes of data at the highest speed, with quality hard drives minimising the possibility of data loss through drive failure. There is a way of combining RAID 1 and 0 to get the best of both worlds, but four drives are required. The common way to undertake connection of two drives into a RAID system is by use of a controller card. If there is room internally for two drives in the PC a PCI controller card may be used, with onboard connections for internal wiring. Where space is at a premium an external box to hold the drives is a possibility, the connection being via SCSI or fibre channel, the latter being only of real advantage if you have more than five drives, though it can communicate to 10km if converted to optical cable. However the controller card should be considered as the simplest and most effective method for just two drives and can be configured for RAID 0 or 1. Such cards usually support the main operating systems Windows XP, 2000, Red Hat and SuSE Linux. The entry level system consisting of one such controller card plus two 120Gbyte SATA drives currently costs in the region of GBP 150 to GBP 200 plus VAT, it really depends on the storage capacity and the quality of the drives. At these prices it's a good investment not only for ultra high speed PC instrumentation, but for everyday office use too. If it is a ultra high speed signal capture system you are after, Dataquest Solutions can provide further advice on choosing the best system to meet your requirements.

[27 September 2004]: Capturing external signals at extreme speeds directly and continuously onto a computer inherently has its problems, most notably the hard drive, where storage rates can rarely exceed 20Mbyte/s.
Many ultrafast analogue and digital signal cards can only operate at their highest speeds by using onboard memory, which is inherently limited in size. Spectrum, one of the leading designers of ultra high-speed capture cards, has met the challenge with SPViewIT; a new generation of ultra-high-speed signal capture software. Using its support of Serial ATA hard disks with RAID systems, data transfer rates of to 80Mbyte/s are possible. This outstanding jump in performance is combined with a user interface that requires no code programming, provides the flexibility of real time displays, on screen disk space monitoring and advanced mathematical and filter functions. A particularly useful tool is the event marker, which is instantly activated by a key press during runtime, which may be combined with zoom facility for a closer look. All events are logged for later retrieval as part of a post process for investigation and documenting, indeed it is documenting of test measurements has become particularly important task - often to meet ISO9000 compliance. SPViewIT can automatically generate protocol using ActiveX "hot-scripts" to automate and reduce the burden of this task, exporting the data into professional formatted reports, with direct linking to many third-party software packages, including Excel, Matlab, FlexPro and DIAdem. It is also possible to save in the ASCII format for almost universal access. Dataquest Solutions is the official distributor in the UK and Ireland for Spectrum. SPViewIT is currently priced at just GBP 695 plus VAT for the standard package, which includes the full complement of display and processing modules.

[20 July 2004]: Over time an increasing number of digital logic standards have evolved.
Where originally TTL and CMOS were the main standards, the newer low-voltage versions LVTTL and LVCMOS are now in common use. Another factor in the evolution of digital communication is the need for high-bandwidth low-noise communication, and this has led to the introduction of ECL, PECL and most recently LVDS, whose differential signal connection provides greater noise rejection. For the test engineer and system designer, finding PC-based hardware specifically designed to generate high-speed digital patterns covering these all these standards can be a problem, particularly when there is a requirement to produce unique custom patterns at high speeds from large stored data sets of megabyte or even gigabyte scale. Spectrum, one of Europe's leading manufacturers of innovative PC instrumentation, has met this requirement with a series of pattern generation boards available in PCI (Mi72xx series), 6U CompactPCI (Mc72xx) and PXI (Mx72xx) formats. These boards feature programmable outputs whose logic low and high levels may be set to an accuracy of +/-10mV anywhere between -2 and +10V, thus covering all the recommended levels for the aforementioned logic families. Patterns generated can be up to 32bit wide, but this may be subdivided into 4 bit "nibbles", each individually programmed to produce a versatile mix of logic levels including the electrically differential outputs for ECL, PECL and LVDS. This feature when combined with the ability to separately disable the (tristate) outputs, provides for easy, flexible, circuit interface options, all in one piece of hardware. Where the requirement is for even greater numbers of bit patterns, or simultaneous capture or generation of analogue data, this series can be synchronised with others from the manufacturer's ultra-high-speed range to make an extensive analogue/digital system. With signal bit updates from 1Ksample/s to 40Msample/s, the speed range is designed to meet the demanding bit rate requirements in industries such as semiconductor circuit design and testing, communications and digital control. However, generating such long streams of data does require deep memory. These boards offer up to 512Mbyte on the PCI and cPCI versions and 128Mbyte on the physically more compact PXI format. Alternatively FIFO mode can be used, with data streamed directly from PC RAM or hard drive. In addition to 16 and 32bit-wide patterns, a memory-saving 8 bit-wide mode can also be programmed to further extend the data streaming period. Spectrum's official UK distributor, DataQuest Solutions, is on-hand to provide expertise and a wide selection of Windows and Linux code drivers. Third-party packages are also well catered for, with driver and examples options for Matlab, LabView, LabWindows CVI, DasyLab and Agilent VEE. All the boards from the Spectrum range come with free lifetime technical support and a 2-year warranty.


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