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 concerned even this may be insufficient.
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 cross-talk 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 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 whilst a suitable controller card can be retrofitted into older PC's.
So is this a major jump in performance? Various evidence suggests a slight improvement but there 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". Another important factor to consider is what happens when same PCI bus is being used by another PCI device, such as a signal capture or generator card. Now the bandwidth has to be shared. So for example if your acquiring at 40 Mbyte/s and capturing to the RAID disk system at the same rate, the total bandwidth required will be 80 Mbyte/s, this being about the maximum under Windows on a continuous basis. The 132 Mbyte/s often quoted is a theoretical best. By the way, there is a way around this using twin PCI bus motherboards, and you are welcome to contact us if you would like more information on this.
Redundant Arrays of Inexpensive Disks; (RAID). The word "redundant" might be a little misleading here, 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. The chart below shows performance comparisons for single drives and the performance increases for a pair of SATA Hard drives running with a RAID controller (see link at the end of this article for more information on this test). Its values should not be taken as absolute as there are many factors to cause variation including the data block size, but it does give some relative idea of the performance advantages that could be gained.
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. Since no redundant information is stored, performance is very good and can be expected to approach double that of a single drive, but the failure of any disk in the array results in data loss.
RAID Level 1. This provides redundancy by writing all data to two (or more) drives in a "mirroring" process. As the data is 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 is lost.
So 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 fastest and therefore the most viable way to capturing 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 on-board connections for internal wiring. Where space is at a premium an external box to hold the drives is a possibility, the connection being via a cable e.g. SCSI. However let us go back to the controller card, the simplest and most cost effective for just two drives.
The card shown here can be configured for RAID 0 or 1 Note the serial ATA cable connections to the rear, these are for direct wiring connection to the drives. Presently its operating system support extends to Windows XP, 2000, RedHat and SuSE Linux. The entry level system consisting of one such controller card plus two 120 Gbyte SATA drives currently costs in the region of £150 to £200 +VAT, it really depends on the storage capacity and the quality of the drives. At these prices it's a good investment not just for PC instrumentation but for everyday office use too.
Finally, for an interesting comparison on performance between PATA, SATA and RAID follow this link: http://www.bjorn3d.com/read.php?cID=647
Dataquest Solutions 22.10.04
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