By Treacy Sommer
 Figure 1. Internal Wide to Narrow SCSI adapters with termination. |
Designers recognize SCSI as an established standard that offers a high transfer rate and the flexibility to connect peripheral products.
To those on the outside, it would appear that computer peripheral design engineers and product managers are co-conspirators whose sole purpose is to make the lives of those tasked with connecting the chaos difficult. One enduring standard that has not only survived this continual onslaught of change and competing standards but that has actually thrived right along with it is the small computer system interface (SCSI) standard. SCSI is really a group of standards with two steering organizations, the T10 SCSI Committee of the National Committee on Information Technology Standards (NCITS) and the SCSI Trade Association (STA).
The evolving SCSI standard has been at the forefront of data storage and high-speed peripheral data communications since the early days of the personal computer (PC). This article is intended for the design engineers and product managers who are interested in untangling the interconnect, or physical, layer of SCSI bus — where it has been, where it is at and where it is going.
SCSI Evolution
The early days of the SCSI bus, circa 1981, were developed around prevalent connectors that were available in the industry, predominantly DB 25 and Centronics 50-position connectors for external connections and 0.100" centered, dual-row, 50-position insulation displacement connectors for internal connections. The overall transmission speed of the bus was a whopping 5 Mbps and the bus could support a total of eight devices with a total length of 6 m (single-ended [SE]). At this time, Apple Computer was vigorously endorsing the SCSI standard with both internal and external connections in its computers and peripherals, while the "Wintel" world was looking to parallel and serial ports for the external connections, with several competing schemes for the internal drive connections.
 Table 1. Development of the SCSI standard. |
Over the next several years, SCSI saw widespread acceptance throughout the Wintel community of developers as design engineers recognized SCSI as an established standard that offered a high transfer rate, plenty of peripheral product support in the industry and the flexibility to connect peripheral products within a topology that was proven. This acceptance fueled the growth of SCSI though many iterations including SCSI-1, SCSI-2, Fast Wide SCSI, Ultra SCSI, Ultra-2 SCSI, Wide Ultra-2 SCSI and Ultra-160 SCSI. The standard has evolved from a transfer rate of 5 Mbps that supported eight devices on a 6 m bus to a transfer rate of 160 Mbps that supports 16 devices on a 12 m bus!
Keeping Up with Change
To keep up with this rapid pace of change has not been an easy task for the interconnect industry. Many times, a new standard has been released only to receive a tepid acceptance from the computer industry. This has put the product marketing professionals in a quandary as to whether or not they should "tool up" a specific interconnect. But while the ever-evolving SCSI standard may cause marketing managers to have fits, it has always offered design engineers with a unique combination of constant change with well-defined connection standards (see Table 1).
 Figure 2. Internal SCA-80 to Narrow and Fast Wide SCSI adapters. |
While the physical layer of the SCSI bus may seem to have been constantly morphing there has always been a strong commitment in the industry through SCSI's governing institutions, the STA and NCITS, to freely share the electronic configuration of the bus. This freedom of information has enabled connector and cable manufacturers to reliably develop products that can work within a specific SCSI bus standard or transcend it (see Table 2). In fact, every imaginable configuration of adapter, cable or terminator has probably been tooled.
On the surface, it would appear as if the fathers of the SCSI bus envisioned a product legacy that would easily roll from one standard to the next. However, SCSI is a notoriously fickle connection. With between 50 and 68 pin connections and relatively high-bandwidth parallel signals, there is a lot that can shut SCSI down.
 Table 2. Description of SCSI connectors. |
System design engineers that have worked extensively with SCSI devices will say that the largest configuration problem they experience is with a faulty adapter, cable or terminator. Problems range from the typical faulty or intermittent connection to impedance mismatch between the host adapter, the cable and the SCSI peripheral. Many adapters or pass-through terminators may appear to provide the correct physical connection but actually have problems, ranging from the way the ground signals are terminated to a terminator being low-voltage differential (LVD) where active negation termination is required.
SCSI Cables
 Figure 3. Ultra-160 internal terminators on double-laminated, twisted-pair TPE cable. |
Today's market for external SCSI cables offers Fast Wide SCSI, Ultra SCSI and Wide Ultra-2 SCSI with internal or external adapters to connect them all. Key elements to a successful cable design are correct cable impedance, and good quality connectors that are reliably terminated by solder or insulation displacement connection. External, 0.050" pitch, 50- or 68-position connectors are typically overmolded while very high-density connector interface (VHDCI) 0.8 mm connections may have a polyvinyl chloride (PVC) boot that slips over a tin shell. All require 360° shielding and shell-to-shell grounding. Cable impedance can range from the low 90s for SE cables to the 130s for high-voltage differential (HVD) or LVD cables.
Internal cables keep pace with externals where variety and configuration are concerned. Currently, jacketing materials include PVC, PTFE, thermoplastic ethylene (TPE) and thermoplastic olefin (TPO). Cable construction includes solid or stranded, flat or double-laminated, twisted pair, 32 to 28 AWG. Impedance can range from low 90s for SE cables to the low 130s for LVD applications. Internal cables also come in another unique configuration, internal-to-external bracket adapter cables. These cables are typically used to transition from an internal SCSI bus to either a bracket for the back of an internal SCSI storage array or the back of a standard PC case.
Adapters
Adapters for transitioning the SCSI bus are probably the most free-form products to be developed around the various evolving SCSI standards (see Figure 1). These adapters can be straight 180° and right-angle 90°, different colors, light-emitting diode (LED) indicators, and have embossed logos or on-board termination. If it can be conceived, it has probably already been tooled! The most popular designs in the current market continue to be various configurations of Fast Wide to Narrow SCSI adapters. Recently, adapters that transition from SCA-80 to Fast Wide or Narrow SCSI or both have become popular (see Figure 2).
Terminators
The end of the line can be the most important stop for the SCSI bus. The terminator will ultimately fail or enhance a SCSI bus. Over the years, much improvement in the reliability of terminators has occurred because of more sophisticated and reliable manufacturing techniques for both the processes and the components. LVD and LVD multimode for mixed SCSI buses offer design engineers the flexibility of accommodating the unknown — potential SCSI bus configurations that may or may not be populated with 100 percent LVD devices. Active terminators for the Fast Wide SCSI bus also maintain high levels of popularity.
Conclusion
Ultra-160 is the latest iteration of SCSI standard with peripherals that are available (see Figure 3). This standard requires only Ultra-160-qualified components including cables and terminators, but offers up to 16 devices at the blinding-fast speed of 160 Mbps and the LVD cable can be extended up to 12 m in length. Around the corner, the STA has defined the next generation of SCSI — Ultra-320. Yes, 320 Mbps!
While competing interconnect technologies such as Ultra ATA (Advanced Technology Attachment), Fibre Channel and IEEE 1394 Firewire try to make inroads on various market segments of the SCSI marketplace, SCSI remains the one constant. Just when it seems the market is finally ready to migrate to something new, the unsinkable SCSI standard just keeps sailing on!
TREACY SOMMER is vice president, Oasis Computer Products Inc. (OCP), 7130 Engineer Road, San Diego, CA 92111; (800) 903-7816 ext. 102; Fax: (858) 279-0079; E-mail: treacy@ocp.com; Web site: www.ocp.com.
SPEC SHEET
End Applications: Data storage, high-speed peripheral data communications
Related Products: SCSI connectors, adapters, cable, terminators
Main Point: Over the past several years, SCSI standards have had widespread acceptance as design engineers recognize SCSI as an established standard. While competing interconnect technologies try to make inroads in the various market segments, SCSI remains the one constant. Just when it seems the market is finally ready to migrate to something new, the unsinkable SCSI standard keeps sailing on!
