EP0688024A2 - Appareil pour augmenter la longueur d'un but SCSI par l'augmentation de la propagation de transmission de seulement deux signaux du but - Google Patents

Appareil pour augmenter la longueur d'un but SCSI par l'augmentation de la propagation de transmission de seulement deux signaux du but Download PDF

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Publication number
EP0688024A2
EP0688024A2 EP95304217A EP95304217A EP0688024A2 EP 0688024 A2 EP0688024 A2 EP 0688024A2 EP 95304217 A EP95304217 A EP 95304217A EP 95304217 A EP95304217 A EP 95304217A EP 0688024 A2 EP0688024 A2 EP 0688024A2
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EP
European Patent Office
Prior art keywords
cable
signals
bus
glitch
wired
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP95304217A
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German (de)
English (en)
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EP0688024A3 (fr
Inventor
William E. Ham
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Digital Equipment Corp
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Digital Equipment Corp
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Filing date
Publication date
Priority claimed from US08/261,571 external-priority patent/US5527996A/en
Priority claimed from US08/262,083 external-priority patent/US5740198A/en
Application filed by Digital Equipment Corp filed Critical Digital Equipment Corp
Publication of EP0688024A2 publication Critical patent/EP0688024A2/fr
Publication of EP0688024A3 publication Critical patent/EP0688024A3/fr
Withdrawn legal-status Critical Current

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B11/00—Communication cables or conductors
    • H01B11/02—Cables with twisted pairs or quads
    • H01B11/12—Arrangements for exhibiting specific transmission characteristics
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B11/00—Communication cables or conductors
    • H01B11/02—Cables with twisted pairs or quads

Definitions

  • the invention relates to communications busses of the type used to connect a digital computer to peripheral devices, and more particularly methods for increasing bus lengths.
  • SCSI Small Computer System Interface
  • the SCSI standards specify the electrical, mechanical and logical characteristics of the SCSI bus, which is an eight or sixteen bit (or thirty two bit in an extended configuration) parallel input/output (I/O) bus. Up to a total of sixteen devices (including the computers) can be connected to the bus.
  • the peripheral devices can include, for example, disk drives, tape drives, printers, compact disk read-only memories (“CD-ROM's”), and scanners.
  • the SCSI standards specify a distributive bus protocol, which facilitates information transfers between devices connected to the bus.
  • the bus protocol refers to the host computers on the bus as “initiators" and the peripheral devices on the bus as “targets".
  • the initiators are capable of initiating operations on the bus, and the targets are capable of responding to the initiators to perform operations.
  • the SCSI standards also specify an arbitration system, under which control of the bus is awarded to the device on the bus having the highest priority level of those contending for control.
  • the bus protocol includes an addressing scheme for identifying the initiators and targets, and specifies control signals used to control operation of the SCSI bus, and to establish communication links between the initiators and targets for information transfers on the bus.
  • control signals are asserted over specified "lines" of the SCSI bus, and include, among others, the following:
  • the initiators and targets use a bi-directional parallel DATA bus (i.e., DATA lines of the SCSI bus) to transfer data information.
  • DATA lines are also used to transfer SCSI ID codes that uniquely identify the devices on the SCSI bus, and specify their relative priority during arbitration.
  • SCSI busses can be either "differential” or “single-ended".
  • a single-ended configuration in which the voltage on a single conductor determines the assertion or deassertion of the signal, uses a cable limited to six meters in length for each signal line.
  • a differential configuration wherein the voltage difference between two conductors (referenced to ground) determines the assertion or deassertion of a signal, uses cables limited to 25 meters in length for each signal line.
  • the devices "losing" the arbitration will deassert the BUSY line, and thus drop off the bus.
  • a current differential arises, which results in a voltage wavefront traveling the length of the line.
  • the wavefront reaches the other end, it is reflected back.
  • This wavefront is called a BUSY glitch.
  • the wavefront is essentially a voltage pulse or "step".
  • the voltage step can be of sufficient magnitude to cause a false high voltage state ("HIGH") on the BUSY line, i.e., using the negative logic of the SCSI standards, the line will falsely appear to be deasserted at any point along the line until the reflection reaches that point.
  • the false or invalid deassertion of the BUSY signal can "fool" other devices on the bus into “believing" that the bus is free when it is not, thereby adversely affecting bus operation.
  • the SCSI standards contemplate that the devices on the SCSI bus should wait before they again seek control of the SCSI bus for a length of time after first detecting a BUSY glitch equal to that required for the waveform to make a round trip on the bus, which depends on the length of the bus.
  • the specification refers to this length of time as the bus "settle time”.
  • the specification therefore limits the length of the bus cable to guarantee that a wired-or glitch will traverse twice the length of the cable within the "settle time" specified, assuming that signals propagate through the signal carriers in a cable at a typical speed of 0.63ns/cm (1.6ns/ft).
  • the SCSI bus was initially meant for coupling physically small computers with each other and with peripheral devices.
  • the SCSI bus definition has since evolved into a higher performance interconnect, and is now being used in midrange computer applications supporting the interconnection of numerous devices. In such applications it is often desirable to interconnect computers and devices that are at significant distances from one another - potentially, in separate rooms. It is highly disadvantageous, however, to add the cost of expensive bus adapters to these systems in order to achieve the longer bus lengths necessary.
  • the SCSI cable length limitation has thus become increasingly onerous.
  • the invention in its broad from resides in a cable for transmitting communications bus signals, as recited in claims 1 and 11, and a method as recited in claim 9.
  • a cable for transmitting communications bus signals including arbitration control signals subject to a wired-or glitch.
  • the cable includes first conductors for transmitting the signals subject to a wired-or glitch at a first propagation speed, and second conductors for transmitting the other bus signals at a second propagation speed, the first propagation speed being greater than the second propagation speed.
  • a medium surrounds each of the conductors for transmitting the signals subject to a wired-or glitch.
  • the medium has a sufficiently low effective dielectric constant resulting in a propagation speed of the signals subject to a wired-or glitch which is greater than the propagation speed of the other bus signals.
  • the medium may have such a characteristic impedance as to ensure that voltage reflections resulting from a signal deassertion on the conductor do not exceed a minimum threshold signal assertion voltage.
  • the BSY glitch is thereby undetectable by the SCSI bus receivers.
  • the bus "settle time" specified can thus be ignored, resulting in a doubling of the maximum length of the SCSI cable.
  • the SCSI bus BSY signal which is subject to a wired-or glitch is propagated faster than the typical 0.052ns/cm(1.6ns/ft) propagation speed of a signal along a typical conductor.
  • the bus length can thereby be increased while maintaining the SCSI bus "settle time" requirement.
  • an apparatus for transmitting communications bus signals between devices connected to the bus including arbitration control signals subject to a wired-or glitch.
  • the apparatus includes first conductive paths for transmitting the signals subject to a wired-or glitch, and second conductive paths for transmitting the other bus signals, the first conductive paths being shorter than the second conductive paths.
  • the SCSI bus BSY signal which is subject to a wired-or glitch is routed between devices via a conductive path which adheres to the maximum bus length specification, allowing the other SCSI bus signals to exceed the maximum bus length specification.
  • the first conductive paths comprises high propagation speed conductors, resulting in a further increase in length of the conductive paths transmitting the SCSI bus signals.
  • novel concepts are applied to provide increased length SCSI cables for use in any SCSI system, allowing greater bus length without the use of expensive bus adapters. These low cost and highly efficient solutions provide a much needed increase in maximum SCSI bus cable length. Furthermore, the novel concepts can be applied to increase the length of any communications bus which is limited in length due to wired-or glitch problems on control signals.
  • a computer system including a computer 10 and I/O devices 12, which are for example disk drives, all interconnected by a communications bus 14 which is for purposes of description a SCSI bus, though the invention is not so limited.
  • the SCSI bus 14 shown is of a differential configuration, wherein the voltage difference between two conductors (referenced to ground) determines the assertion or deassertion of a signal. It is understood that the SCSI bus 14 can also be of a unitary or single ended configuration, in which the voltage on a single conductor relative to ground determines the assertion or deassertion of the signal.
  • the signals of the SCSI bus 14 are transmitted by a SCSI bus cable 30.
  • the SCSI bus cable 30 is connected to the computer 10 by a standard electrical SCSI connector 31 located at one end of the cable 30.
  • the SCSI connector 31 interfaces with a mating SCSI connector 31a on the computer 10.
  • the SCSI connector 31 and mating SCSI connector 31a can be implemented as any number of industry standard SCSI connectors well known in the art.
  • the SCSI bus cable 30 is connected in a similar manner to I/O devices 12.
  • Conductors 32 are coupled to the connectors 31 located at each end of the cable 30 in any of the manners well known in the art.
  • FIG 2 there is shown a cutaway view of the SCSI bus cable 30 of Figure 1.
  • Multiple conductors 32 are bunched together and wrapped by a shield 34 disposed within a jacket 36.
  • Each conductor 32 is shown to include multiple strands of conducting wire 37; however, it is understood that a single solid conductor could also be used. For reasons of convenience, not all of the 25 conductors 32 necessary to propagate all the SCSI signals are shown.
  • a pair of conductors 38 is used to propagate each signal.
  • the voltage difference between the two conductors associated with a bus signal determines the assertion or deassertion of the signal.
  • a signal-carrying conductor 32 is paired with a ground conductor 32 for each bus signal. In either case, the two conductors are twisted about each other for noise reduction reasons as is known in the art.
  • the bus signal BSY when asserted indicates that the bus is currently controlled by another device; thus, the device wishing to gain control must wait until the BSY signal is deasserted, indicating that the bus is free. All devices connected to the SCSI bus are capable of driving the BSY signal; that is, the BSY signal is wired-or between the devices. The BSY signal is therefore subject to the previously described wired-or glitch problem.
  • the bus "settle time” parameter is based on some multiple of the round-trip propagation time of a reflection on the BSY line - which is in turn based on the typical propagation speed of the signal and on the length of the cable. That is, the round trip propagation time is equal to the length of the cable divided by the speed at which the BSY reflection propagates down its particular conductor. It thus becomes apparent that cable length can be increased over what is specified by a number of methods affecting the propagation of BSY reflections.
  • one way of increasing SCSI cable length is to provide a cable having conductors capable of transmitting the signals subject to the wired-or glitch at a greater propagation speed than that provided through the conductors of a typical cable.
  • a cable is provided including first conductors 40 for transmitting the BSY signal which is subject to the wired-or glitch at a first propagation speed, and including second conductors 42 for transmitting the other bus signals at a second typical propagation speed, where the first propagation speed is greater than the second propagation speed.
  • One way of so doing is to construct a cable 30 wherein the second conductors 42 are of the typical variety propagating signals at approximately 0.052ns/cm(1.6 ns/ft), but wherein the first conductors 40 are specially constructed so as to propagate the BSY signal faster that 0.052ns/cm(1.6ns/ft).
  • One way of increasing the propagation speed of the wired-or BSY signal is by surrounding the BSY signal conductors 40 with a dielectric medium 44 having a relatively low dielectric constant.
  • the dielectric constant ⁇ r of a material is a dimensionless quantity which when multiplied by the permittivity of free space, designated ⁇ o and measuring 8.854 x 10 ⁇ 12 F/m, gives the absolute permittivity of the material measured in units of capacitance per unit length. It is known that the propagation speed of a signal through a conductor increases in proportion with the square root of the decrease in the permittivity of the material surrounding the conductor. Thus, a conductor spaced from the surrounding conductors of a cable by a medium having a low dielectric constant will propagate a signal faster than typical conductors within a cable, which are more closely surrounded by other conductors.
  • the medium 44 could be constructed entirely of some other material having a relatively low dielectric constant.
  • Feasible materials include Teflon®, having a dielectric constant of approximately 2.1, and polyethylene, which has a dielectric constant of approximately 2.3.
  • a signal traveling through a conductor 40 surrounded by a medium 44 with a dielectric constant of approximately 2 will travel at approximately 0.7 times the speed at which it would travel through the same conductor 40 were the medium 44 air.
  • the dielectric medium 44 could be constructed of an air-filled foam material such as foam Teflon®.
  • the medium 44 would then have an "effective" dielectric constant ⁇ f dependent upon the volume distribution of materials making up the medium 44.
  • the medium 44 includes a cellular distribution of Teflon® 46 having a dielectric constant ⁇ 2 , the cells 47 being filled with air 48 having a dielectric constant ⁇ 1.
  • ⁇ f ⁇ 1(V1/(V1+V2)) + ⁇ 2(V2/(V1+V2)), where V1 is the volume of air 48 surrounding the conductor 40 while V2 is the volume of Teflon® 46 surrounding the conductor 40.
  • V1 is the volume of air 48 surrounding the conductor 40
  • V2 is the volume of Teflon® 46 surrounding the conductor 40.
  • larger celled foams will provide the larger propagation speed increases.
  • FIG. 5 A third way of providing a dielectric medium 44 having a sufficiently low effective dielectric constant is shown in Figure 5.
  • the BSY signal conductors 40 are disposed within a tubular insulating core 50.
  • An insulating filament 52 is helically wrapped about the length of the conductor 40 to provide a space between the conductor 40 and the interior wall 54 of the core 50. There is thus provided an air gap or space 56 between the conductor 40 and the interior wall 54 of the core 50.
  • an effective dielectric constant ⁇ f surrounds the conductor 40, the effective dielectric constant ⁇ f again being dependent upon the volume distribution of materials making up the medium 44.
  • a 26 AWG (7/34) silver plated copper conductor when helically wrapped with an FEP filament of a 0.254cm (.010 inch) diameter and disposed within an FEP tubular core of a 1.09cm (.043 inch) diameter, will provide a propagation delay of 0.04477 ⁇ .00118ns/cm (1.14 ⁇ .03 ns/ft); thus providing about a 30% increase in speed over the conventional conductor propagation delay of 0.052ns/cm (1.6 ns/ft).
  • Fig. 5 Although the space 56 is maintained In Fig. 5 by use of the filament 52 helically wrapped about the conductor 40, many ways of maintaining the space 56 can be implemented within the principles of the invention. For example, strips of insulating material might be circularly wrapped about the conductor 40 at intervals along the length of the conductor 40.
  • any of the previously described ways of surrounding a conductor 40 with a dielectric medium 44 to increase the propagation speed of a signal through the conductor 40 can be applied to each of the conductors 40 of the BSY signal conductor pair 60 to increase the propagation speed of the BSY signal. Any wired-or glitch will thereby propagate faster, thus allowing a proportional increase in the length of the cable 30 without violating the bus settle time specification.
  • the SEL signal is also wired-or between the devices 12 connected to the bus 14; therefore, higher propagation speed conductors 40 should comprise the SEL signal conductor pair 60 as well in order to increase the length of the bus.
  • the other SCSI bus signals can be transmitted by way of typical conductor pairs 38. Alternatively, all the SCSI bus signals can be transmitted via the high propagation speed conductors 40.
  • the other SCSI bus signals, such as the DATA signals cannot, however, be propagated via a mixture of high propagation speed conductors 40 and typical propagation speed conductors 42 without violating the SCSI signal skew specification for those signals.
  • SCSI bus lengths are thereby provided by a SCSI cable 30 that interfaces to devices such as the computer 10 or I/O devices 12 via industry standard SCSI connectors 31 in the same standard manner as any presently available SCSI cable.
  • a second way of increasing the length of the SCSI bus cable 30 arises from the realization that adherence to the SCSI cable length specification is in fact necessary only for those signals subject to the wired-or glitch problem; i.e. the BSY signal.
  • the conductive paths for transmitting the other SCSI signals can exceed the cable length specification.
  • first conductive paths 62 can be provided for transmitting the signals subject to a wired-or glitch
  • second conductive paths 64 can be provided for transmitting the other bus signals, the first conductive paths 62 being shorter than the second conductive paths 64.
  • conductive paths 62 and 64 for routing the signals of a SCSI bus to device connectors 66 on a backplane 68; for example, a RAID (redundant array of inexpensive disks) system backplane.
  • a single-ended SCSI bus 14 routed between device connectors 66 on a backplane 68.
  • the BSY and SEL signals subject to a wired-or glitch are routed via external wires 70 between the device connectors.
  • the other SCSI bus signals are routed between the connectors 66 via PCB etch 72, and may be many inches to even a foot or more long. Lengths of ribbon cable 74 may extend from each connector 66, further increasing the length of the bus 14.
  • the length of the conductive paths 62 carrying the BSY and SEL signals subject to the wired-or glitch include the lengths of the conductor pairs 76 within the ribbon cables 74 plus the lengths of the wires 70 connecting the signals between the device connectors 66. As long as the total length of the conductive paths 62 carrying the BSY and SEL signals subject to the wired-or glitch remain within the SCSI cable length specification, the conductive paths 64 carrying the other bus signals, including etch 72 and the conductor pairs 78 within the ribbon cables 74, may exceed the 25 meter cable length limit.
  • a further bus length advantage can be obtained by combining the previously described ways of increasing cable length. If the wires 70 connecting the BSY and SEL signals between the connectors 66 in Fig. 6 are high propagation speed conductors 40 as shown in Figs. 3, 4, or 5, the lengths of the conductive paths 62 transmitting the BSY and SEL signals may now also exceed the 25 meter SCSI cable length specification for the reasons previously described. The increased length will depend upon the proportionate length of the wires 70 to the conductive path 62 and the proportionate increase in propagation speed through the wires 70. Alternately, both the wires 70 and the conductors 76 in the ribbon cables 74 can be implemented as high propagation speed conductors 40, providing an even greater increase in bus length.
  • a third way of increasing SCSI bus cable length applies a different principle to the wired-or glitch problem.
  • the SCSI bus cable length is limited in the SCSI bus specification to allow a wired-or glitch on the BSY signal to settle to a voltage below the signal assertion voltage threshold of devices 12 on the bus so as to avoid bus contention due to "false" signal assertions.
  • the bus settle time parameter which currently limits the SCSI bus cable 30, is no longer the time in which a round trip propagation of a signal must occur, but is the time in which the signal must travel one length of the cable. The maximum cable 30 length is thereby effectively doubled.
  • a cable 30 for transmitting SCSI bus signals which includes amongst various conductors 32 particular conductors 80 having a low characteristic impedance for transmitting the signals subject to a wired-or glitch.
  • the characteristic impedance of the conductors 80 is sufficiently low to ensure that voltage reflections resulting from a signal deassertion on the conductors 80 do not exceed a minimum threshold signal assertion voltage.
  • the signal voltage V decreases proportionately.
  • the maximum magnitude of a BSY glitch can be controlled by adjusting the characteristic impedance of the BSY signal conductor 80.
  • One way of providing a cable 30 including conductors 80 for transmitting the BSY signal having a relatively low characteristic impedance is by surrounding the BSY signal conductors 80 with a dielectric medium 82 having a relatively high dielectric constant. It is known that as the dielectric constant of material surrounding a conductor increases, the characteristic impedance of the conductor decreases.
  • the medium 82 can be for example a metal-filled dielectric such as polyolefin having a relative dielectric constant of approximately 5 to 10.
  • the propagation speed of the BSY signal will decrease as the dielectric constant of the surrounding medium 82 increases; however, as long as the maximum voltage level of the BSY glitch remains below .8 volts, the initial signal wavefront can take up to the specified bus settle time to travel the length of the cable 30 (half the round-trip time). The allowable cable length is thus at least doubled.
  • the SEL conductors must also be transmitted via conductors 80 in the cable 30 having a relatively low characteristic impedance. According to this method of increasing SCSI bus cable length, only the BSY and SEL arbitration control signals should be transmitted via the low characteristic impedance conductors. Transmitting high speed bus signals such as the DATA signals via the low characteristic impedance conductors may adversely affect voltage levels and signal quality on these lines.

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  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
EP95304217A 1994-06-17 1995-06-16 Appareil pour augmenter la longueur d'un but SCSI par l'augmentation de la propagation de transmission de seulement deux signaux du but Withdrawn EP0688024A3 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US08/261,571 US5527996A (en) 1994-06-17 1994-06-17 Apparatus for increasing SCSI bus length by increasing the signal propogation velocity of only two bus signals
US08/262,083 US5740198A (en) 1994-06-17 1994-06-17 Apparatus for increasing SCSI bus length through special transmission of only two bus signals
US262083 1994-06-17
US261571 1994-06-17

Publications (2)

Publication Number Publication Date
EP0688024A2 true EP0688024A2 (fr) 1995-12-20
EP0688024A3 EP0688024A3 (fr) 1996-04-17

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EP95304217A Withdrawn EP0688024A3 (fr) 1994-06-17 1995-06-16 Appareil pour augmenter la longueur d'un but SCSI par l'augmentation de la propagation de transmission de seulement deux signaux du but

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000063925A1 (fr) 1999-04-15 2000-10-26 Adaptec, Inc. Cable scsi ultra mince et flexible et son procede de fabrication
WO2006117698A1 (fr) * 2005-04-29 2006-11-09 Nexans Cable a paires torsadees non blindees ameliore et son procede de fabrication
GR1006413B (el) * 2008-05-19 2009-05-29 Κωνσταντινος Παναγιωτη Βαρουδης ΟΜΟΑΞΟΝΙΚΟ ΨΗΦΙΑΚΟ ΚΑΛΩΔΙΟ ΜΕΤΑΦΟΡΑΣ ΗΧΗΤΙΚΟΥ 'Η (ΚΑΙ) ΟΠΤΙΚΟΥ ΣΗΜΑΤΟΣ 75 ohm

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CH324053A (de) * 1954-09-16 1957-08-31 R & E Huber Schweizerische Kab Fernmeldekabel mit Verseilelementen mit Kunststoffisolation
US3273080A (en) * 1963-08-06 1966-09-13 Hackethal Draht & Kabelwerk Ag High-frequency transmission line having plural coaxial conductors of different effective length between source and sink
FR1391953A (fr) * 1964-01-30 1965-03-12 Lcc Steafix Lignes à retard à constantes réparties
US4376920A (en) * 1981-04-01 1983-03-15 Smith Kenneth L Shielded radio frequency transmission cable
JPS62117210A (ja) * 1985-11-15 1987-05-28 株式会社潤工社 伝送線路
US5113098A (en) * 1991-03-29 1992-05-12 Advanced Micro Devices, Inc. Glitch remover circuit for transmission links

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000063925A1 (fr) 1999-04-15 2000-10-26 Adaptec, Inc. Cable scsi ultra mince et flexible et son procede de fabrication
EP1169716B1 (fr) * 1999-04-15 2011-08-03 Adaptec, Inc. Câble scsi ultra mince et flexible et son procédé de fabrication
WO2006117698A1 (fr) * 2005-04-29 2006-11-09 Nexans Cable a paires torsadees non blindees ameliore et son procede de fabrication
US7390971B2 (en) 2005-04-29 2008-06-24 Nexans Unsheilded twisted pair cable and method for manufacturing the same
GR1006413B (el) * 2008-05-19 2009-05-29 Κωνσταντινος Παναγιωτη Βαρουδης ΟΜΟΑΞΟΝΙΚΟ ΨΗΦΙΑΚΟ ΚΑΛΩΔΙΟ ΜΕΤΑΦΟΡΑΣ ΗΧΗΤΙΚΟΥ 'Η (ΚΑΙ) ΟΠΤΙΚΟΥ ΣΗΜΑΤΟΣ 75 ohm

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