Application Note 904 An Introduction To The Differential SCSI Interface 0904

User Manual: AN-0904

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An Introduction to theDifferential SCSI InterfaceOVERVIEWThe scope of this application note is to provide an introduc-tion to the SCSI Parallel Interface and insight into the differ-ential option specified by the SCSI standards. This applica-tion covers the following topics:•The SCSI Interface•Why Differential SCSI?•The SCSI Bus•SCSI Bus States•SCSI Options: Fast and Wide•The SCSI Termination•SCSI Controller Requirements•Summary of SCSI Standards•References/StandardsTHE SCSI INTERFACEThe Small Computer System Interface is an ANSI (AmericanNational Standards Institute) interface standard defining apeer to peer generic input/output bus (I/O bus). The intentionof the SCSI standard is to provide a fast, multipoint parallelbus that is easily upgradeable and keeps pace with advanc-ing technologies.The SCSI interface is commonly the interconnect of choicefor high performance hard disk drives. Being a generic inter-face, the SCSI bus is not limited to only one type of periph-eral. It is also commonly used to interconnect optical drives,tape drives, disk arrays, scanners, printers, and other targetsto a wide range of terminals, computers, and other hosts. Itis important to also remember that a SCSI bus is not a pointto point bus, but rather a multipoint bus, allowing up to eightdifferent devices to be connected to the same daisy chainedcable (SCSI-1 and 2 allows up to eight devices while the pro-posed SCSI-3 standard will allow up to 32 devices). A typicalSCSI bus configuration is shown inFigure 1.WHY DIFFERENTIAL SCSI?In comparison to single-ended SCSI, differential SCSI costsmore and has additional power and PC board space require-ments. However, the gained benefits are well worth the addi-tional IC cost, PCB space, and required power in many appli-cations. Differential SCSI provides the following benefitsover single-ended SCSI:•Reliable High Transfer Rates — easily capable of operat-ing at 10MT/s (Fast SCSI) without special attention to termi-nations. Even higher data rates are currently being standard-ized (FAST-20 @20MT/s).The companion Application Note (AN-905) focuses on thefeatures of National’s new RS-485 hex transceiver. TheDS36BC956 specifically designed for use in differential SCSIapplications is also optimal for use in other high speed, par-allel, multipoint applications.•High Noise Rejection — the differential transmissionscheme provides excellent common mode rejection over awide bus voltage range.•Long Cable Lengths — cables can be as long as 25meters in length compared to 3 meters or less forsingle-ended interfaces.•Superior AC Performance — high performance transceiv-ers with tightly specified and guaranteed AC performance.•Fault Tolerance — current limiting and thermal shutdownprotection integrated into the differential driver design.Signal quality and long cable runs are the two major en-hancements differential SCSI offers over single-ended SCSI.As stated above, differential SCSI allows for cable runs up to25 meters in length compared to only 3 meters ofsingle-ended SCSI. Differential SCSI is optimal for connect-ing together terminals with storage arrays located in a sepa-AN011897-1FIGURE 1. Typical SCSI Bus Configuration-Multiple Hosts/Multiple TargetsNational SemiconductorApplication Note 904John GoldieAugust 1993An Introduction to the Differential SCSI Interface AN-904© 1998 National Semiconductor Corporation AN011897 www.national.com
rate cooled computer room. The differential transmissionscheme offers superior noise rejection and signal qualitycompared to a TTL single-ended bus.Differential buses are also immune to minor terminationproblems that commonly plague the single-ended SCSI bus.These problems can, and commonly do have major impacton single-ended system performance. By expanding thecable length beyond 3 meters, by mixing different cabletypes (impedance), by using different types of termination, orby using the standard passive termination, system through-put may be reduced as great as 50%. Since it has been de-termined that the original single-ended termination recom-mended in the SCSI-1 standard does not provide adequatesignal termination performance for Fast SCSI, the SCSI-2and proposed SCSI-3 standards recommend the use of al-ternate terminations. There are three popular alternatives tothe passive resistive terminators. These are the Boulay ter-mination (voltage regulated), Current Regulated Termina-tions, and the FPT (forced perfect termination). Each has itsown merits and limitations, and in fact the FPT offers goodperformance but is not sanctioned by the standard. Troublecan arise in single-ended SCSI applications when differenttypes of termination are used on the bus. In addition, someSCSI controllers now provide totem pole outputs on the highspeed lines (REQ and ACK) to improve the signal quality onthose lines on the de-assert edge (active negation in industryjargon). These active negation drivers can become in con-tention with the alternative termination techniques and causethermal problems and data corruption. Single-ended SCSItermination have caused much grief, and discussion in theSCSI standard committee.In contrast Differential SCSI has not encountered the prob-lems that drove the single-ended interface to develop somany alternative terminations. Differential SCSI uses a stan-dard passive resistor termination (described in detail later inthis application note). This terminator remains unchangedfrom the original SCSI-1 standard to the proposed SCSI-3physical layer.National’s DS36954 Quad Differential Bus Transceiver is de-signed for Differential SCSI applications up to 10 MT/s.THE SCSI BUSThe SCSI bus is composed of a minimum of 18 signal lines.An option is provided to add extra bytes to boost systemthroughput (Mega Bytes per second (MB/s)) if required bythe application. The SCSI 1 and 2 standards define twotypes of electrical characteristics; single-ended and differen-tial.Single-ended drivers (typically 48 mA open drain drivers)and receivers are commonly integrated onto the SCSI con-troller chips. For the differential option, external RS-485transceivers are required. Integrating the differential trans-ceivers onto the SCSI controller is not feasible due to the ad-ditional pins required for differential operation, and the addi-tional power dissipation. Additionally the semiconductorprocesses commonly used for the controllers are not com-patible with the special high speed/high voltage breakdownprocesses used for RS-485 transceivers.The single-ended and differential modes are exclusive, andcan not inter-operate. Of the 18 lines, 9 are data path (dataplus parity) and the others are control. The lines are:•Data Path— DB(7–0,P) — Data Bus•Control— REQ — Request— ACK — Acknowledge— BSY — Busy— SEL — Select— C/D — Control/Data— I/O — Input/Output— MSG — Message— ATN — Attention— RST — ResetThe SCSI Standard has two types of devices, which are “Ini-tiators” (typically a host computer); and “Targets” (typicallydrives). Of the 18 lines, 9 are bi-directional, 7 areuni-directional direction, and 2 are wire-ORed. The data bus(DB0–DB7 and DBP) are the bi-directional lines. Three con-trol lines are lnitiator to Target only lines; these are the ACK,ATN, and SEL*lines. Four lines are Target to lnitiator onlylines; these are the C/D, I/O, REQ, and MSG lines. A pictorialrepresentation of the signal lines is shown inFigure 2.(*SEL can also be a wire-ORed line, but is more commonlyimplemented as a initiator to target line).www.national.com 2

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