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The Small Computer Systems Interface (SCSI) seems to be almost unlimited in its ability to adapt to new applications. Originally developed to enable high speed communication between computers and mass storage devices, the SCSI bus is now used to connect printers, scanners, plotters, servers, sensors, hard drive, tape backup, bar code reader, and many more. Besides extensive connectivity, SCSI and its faster cousins, the 40MB/s Wide Ultra SCSI, 80MB/s Wide Ultra2 SCSI, Wide Ultra3/Ultra160 160MB/s, and Wide Ultra4/Ultra320 320MB/s provide multi tasking for the server market, high bandwidth for multimedia and real time video, and backward connectivity to legacy devices.
For many OEMs and systems integrators, SCSI is the bridge that connects the affordable, well known peripherals of the past with the endless array of new high performance system. If there is an area where standard SCSI can not meet the medium's physical specifications, a wide variety of SCSI expanders are available to solve the problem. For example, if a longer than maximum cable length is required, a SCSI extender can be used to connect the devices. Other types of SCSI expanders, including signal converters, repeaters, and regenerators, can also be used to extend the length of the bus.
Even computers and peripherals with different parallel signal communications, i.e. Single Ended, Differential Ended, and Low Voltage Differential (LVD), can communicate via the same SCSI bus with the help of a SCSI converter. By installing the SCSI converter in a chassis with a power supply, the OEM can simply plug the converter into a power outlet, connect the single ended and differential ended cables, and have a true turnkey system.
There's no doubt SCSI expanders have enabled OEMs and systems integrators to take advantage of new technology and advanced peripherals with out sacrificing their investment in existing equipment. SCSI expanders also help reduce design costs and eliminate the need for additional components. But what happens when form factor restrictions, size limitations, power requirements, or performance demands rule out one off the shelf expander after another? For example, system requirements may call for connectors that are not included in either the ANSI X3.131-1986 or X3.131-199X SCSI specifications or in ANSI working draft X3T10. OEMs who want to incorporate early power supply failure warning signals, or use the ground pins on the SCSI connectors for special pass through signals, will also find that most standard SCSI expanders cannot accommodate these features. Other manufacturers who want to design expanders into their won system boards may have to wait months for a custom ASIC expander chip to be produced from scratch. System integrators with a specification that requires the expander be operated from a power source at a remote termination will also be hard pressed to find an off the shelf SCSI expander that can meet this requirement. Fortunately, a number of SCSI product manufacturers have developed custom SCI expanders that, with minimal redesign, can solve a number of integration problems. The step by step SCSI expander Selection Guide can help the OEM decide whether to build or to buy.
One of the most common dilemmas faced by OEMs and integrators is the performance/size ratio. Higher performance systems often require the installation of more components. Yet the market's demand for miniaturization and smaller footprint systems leaves little room in the RAID chassis, workstation, or clustered network server for a SCSI expander. For example, most standard converter boards measure approximately 5-3/4 x 3-1/4 inches. This form factor is too large to be integrated into many manufacturers' small subsystem chassis, including Hewlett Package's DAT drive. To meet minimum space requirements, Ranch Technology, Rancho Cucamonga , California , designed a miniature Ultra4/Ultra3/Ultra2 SCSI converters and the whole SCSI Expanders family measuring only 1-1/2 x 4 inches.
Other small form factor custom SCSI expanders include an Ultra SCSI 16bit Wide Converter and Extenders, both of which measure only 2 x 4 inches, enabling them to be mounted behind any 3.5 inches drive.
In addition to meeting size constraints, custom SCSI expanders can extend cable length beyond what is possible with off the self expanders. This is particularly important in RAID subsystems, networking and server applications, where systems may be separated by considerable physical distance. Unfortunately, the newer SCSI standards have very short cable lengths. Fast Wide SCSI allows only ten feet of connecting cable, while Ultra SCSI bus limits the length of the cables to a maximum of only five feet when five or more peripherals are on the bus.
Custom SCSI expanders can help OEMs save money by maximizing their investment in existing inventory and less expensive technology. For example, RAID applications may have eight or more drives in on enclosure. As single ended drives cost about $50 less than differential drives and a SCSI converter cost $200, the OEM realizes a cost savings with fie drives or more. In addition, by using only single ended drives and a custom CSI converter, only on type of drive need be stocked for variety of application. Plus, should differential ended drives be phased out the market as some industry observers predict, the OEM will not have to mark significant design changes.
SCSI expanders can also increase an OEM's storage options. NetFrame System, Milpitas , Calif. , builds clustered network servers with hot swappable, differential ended Wide Ultra SCSI backplanes. The company includes either a Plexstor CD-ROM drive or a HP DAT drive for software updates and for performing system backups. But as CD and DAT drives are available only with single ended narrow SCSI, Rancho Technology designed an adapter board with a custom single to differential ended converter that plugs directly into either the CD-ROM or DAT drive. Either storage device can then be plugged directly into the network server's differential ended Wide Ultra SCSI back plane with no additional modifications.
In addition to meeting today's unique form factor, size, power and performance requirements, custom SCSI expanders will also enable OEMs and system integrators to take advantage of Ultra2/Ultra3/Ultra4 SCSI and LVD in a multimode configuration. The whole UltraX SCSI family allows LVD and SE to coexist on the same bus. However, if even one single ended device is on the LVD bus, then every device on this bus must operate no faster than Ultra SCSI speed, which is only about 50% of the LVD's bus performance. Consequently, to ensure maximum performance, an LVD to single ended SCSI converter is required. For example, an OEM wants to put both a SE CD0ROM drive and a number of LVD hard drives on the LVD bus, by adding the CD-ROM to the LVD bus via an LVD to SE converter, the host will communicate with the CD-ROM drive at slower speeds, but communicates with the hard drives at Ultra2 SCSI speeds.
Almost every month, manufacturers of custom SCSI expanders develop new solutions to meet the needs of a unique SCSI application. Once developed, most of these custom expander solutions are available to all customers facing similar SCSI challenges. OEMs and systems integrators with custom requirements will do well to look beyond off the shelf SCSI expanders and investigate the wealth of custom SCSI expanders and ASIC.
John A. Fobel is vice president of Rancho Technology, Inc. Rancho Cucamonga , CA.