SCSI "ease of use profiles" help system designers specify mechanical and electrical interfaces.This, article is the second in a two-part series. The first part appeared in the December issue of CTR See click-through rate. . SCSI SCSI in full Small Computer System Interface Once common standard for connecting peripheral devices (disks, modems, printers, etc.) to small and medium-sized computers. SCSI has given way to faster standards, such as Firewire and USB. bus segment terminator issues for single-ended and multimode terminators need close attention, as multimode terminators are capable of LVD See LVDS. LVD - Low Voltage Differential and single ended termination. They can switch to single ended mode when a single-ended device is installed on the bus. A terminator in single-ended mode under high utilization will dissipate 2.5 watts for 27 lines. Adequate heat sinking must be provided for single-ended and multimode terminators. Single-ended interfaces should be active negation drivers that conform to the termination requirements defined in SPI-2. New designs for single-ended devices should use the UltraSCSI speed for transferring data. SCSI Ids SCSI IDs for workstations and PCs are not fixed but there are some general rules that make configurations easier. General rules for SCSI TD settings are: * Boot disk should be ID0 * Tape drive is ID3* * CD-ROM CD-ROM: see compact disc. CD-ROM in full compact disc read-only memory Type of computer storage medium that is read optically (e.g., by a laser). is ID4 * Scanner is ID5 * Optional second controller is ID6 * SCSI adapter (controller) is ID7 * Wide user disk drives * Note: Often multiple CDROM See CD-ROM. , CD-R (CD-Recordable) A writable CD technology using a type of compact disc that can be recorded, but not erased (CD-Rs are "write once" discs). CD-R discs are used to master CD-ROMs, to back up data and to make copies of data for distribution. , CD-RW (CD-ReWritable) The only rewritable CD technology. CD-RW disks look like other CD media, but with close inspection, they have a more polished surface with a very dark blue-gray cast. , DVD-RAM A rewritable DVD disc endorsed by the DVD Forum. Using phase change technology, DVD-RAMs are like removable hard disks, and the media can be rewritten 100,000 times compared to 1,000 times for DVD-RW and DVD+RW. The first DVD-RAM drives with a capacity of 2.6GB (single sided) or 5. , and removable media devices are on the same system. These are all narrow devices and often take IDs 1-4. All devices must have a unique ID. SCSI IDs lot servers and disk arrays are normally the controller (ID7) with a possible second controller (ID6) and the rest of the IDs are for the disk drives. ID0 should always be reserved for the boot device. Other Important Considerations System designs must consider power. Low performance disk drives are in the 15 to 25 watt range and high performance disk drives are in the 25 to 35 watt range. Disk drives need adequate airflow for cooling, otherwise they can overheat o·ver·heat v. o·ver·heat·ed, o·ver·heat·ing, o·ver·heats v.tr. 1. To heat too much. 2. To cause to become excited, agitated, or overstimulated. v.intr. . Airflow is normally front to back, pulling the air from the front of the cabinet around the disk drives and out the back of the cabinet. Holes in the backplanes may be required for ad-equate airflow. It is important to mount the drives in such a way that they do not move with head positioning. High performance disk drives use alot of power to position as many as 22 heads. The drive can twist and snap back when the heads stop, knocking the heads off track and requiring a second seek for head positioning, thereby reducing the performance of the drive. When there are multiple drives in a system, the drive mounting should not transfer energy from one drive to the next as this can knock heads off track on a drive that is reading data while another drive is positioning heads. Consistent labeling is required next to the connectors. Icons have been defined in SPI-2 annex for all the different SCSI interfaces. These icons should be placed as close as possible to the connectors. The Ease of Use Profiles have annexes for each component in the SCSI system. The host bus adapters are the basic interface point because this is the first annex going back through the disk arrays and SCSI devices. Narrow bus devices include termination, but for wide bus devices, the termination is in the cable system. The last annex explains how to work with legacy connections as SCSI technology and products are all intergenerational and therefore interoperable. The Ease of Use Profiles give the engineering community direction and the users a limited set of options for today's high Today's High The intra-day high trading price. Notes: In other words, this is the highest price that a stock traded at during the course of the day. More often than not this is higher than the closing price. See also: Today's Low performance systems. Configuring SCSI systems using the profile that limits options for two basic electrical interfaces dramatically improves products through selective design choices for both single-ended narrow and LVD SCSI wide bus applications. SCSI products have been serving worldwide storage needs for almost twenty years. The next ten years of SCSI technology have been defined by the SCSI Roadmap. SCSI balances performance with cost and case of use for the most effective high-end peripheral attachment bus in the industry. Paul Aloisi is the president of the SCSI Trade Association The SCSI Trade Association, or SCSITA, is an industry trade group which exists to promote the use SCSI technology. It was formed in 1996 [1]. As of 2006, major members include Adaptec, HP, Intel, LSI Logic, Seagate, and IBM [1]. (San Francisco, CA) and the Distinguished Member of the technical staff at Texas Instruments (Dallas, TX). www.scsita.org www.ti.com |
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