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S C S I Small Computer Systems Interface

S C S I Small Computer Systems Interface. Contents. Small Computer Systems Interface (SCSI) Overview and History of the SCSI Interface SCSI Standards SCSI Data Transfer Modes and Feature Sets SCSI Protocols and Interface Features Summary of SCSI Protocols and Transfer Modes

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S C S I Small Computer Systems Interface

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  1. S C S ISmall Computer Systems Interface DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  2. Contents • Small Computer Systems Interface (SCSI) • Overview and History of the SCSI Interface • SCSI Standards • SCSI Data Transfer Modes and Feature Sets • SCSI Protocols and Interface Features • Summary of SCSI Protocols and Transfer Modes • SCSI Host Adapters • SCSI Cables and Connectors • SCSI Configuration • IDE/ATA vs. SCSI: Interface Comparison DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  3. 1. History • 1979: Shugart Associates Systems Interface (SASI) • 1,5 Mbytes/second • First SCSI standard published in 1986 • SCSI = system interface • Not tied specifically to hard disk ! • Designed to be high-level, expandable, high performance •  choice for high-end computer users DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  4. 2. SCSI Standards • SCSI – 1 • SCSI – 2 • SCSI - 3 DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  5. 2.1. SCSI-1 Standard • 1979~1986 (approved) • Defines SCSI basics • Cable lenght (6m SE, 12m HVD) • Narrow 8-bit bus • 5 MB/s transfer rate • Only single ended transmission • Passive termination (=resistors) • Obsolete now DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  6. 2.2. SCSI-2 Standard • 1985~1994 (approved) • Fast SCSI: bus speed 10MHz  10MB/s • Wide SCSI: 8  16 bit, 8 16 devices (32-bit) • Improved cables and connectores: +50pin • Active Termination: more reliable • (High-Voltage) Differential Signaling  longer cables • Command Queuing • Additional Command sets for cdroms, scanners, removable media, ... • SCSI-2 is not the same as Ultra2 SCSI !!! DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  7. 2.3. SCSI-3 Standard • 1993, collection of different, but related standards • SCSI-3 Architecture • SCSI-3 Standards • SCSI-3 Parallel Interface (SPI, SPI2-4) DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  8. SCSI-3 Architecture • Three categories: • Commands: These are standards that define specific command sets for either all SCSI devices, or for particular types of SCSI devices. • Protocols: These standards formalize the rules by which various devices communicate and share information, allowing different devices to work together. These standards are sometimes said to describe the transport layer of the interface. • Interconnects: These are standards that define specific interface details, such as electrical signaling methods and transfer modes. They are sometimes called physical layer standards as well. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  9. Overview SCSI-3 Standards DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  10. SCSI-3 Parallel Interface (SPI) • Three Documents to define the standard: • Protocol: The protocol for parallel SCSI was defined in a document entitled SCSI-3 Interlocked Protocol (SIP). • Physical Layer: The physical layer was defined in the SCSI-3 Parallel Interface or SPI document, ANSI standard X3.253-1995. This specification only called for bus speeds of up to 10 MHz, which is so-called "Fast SCSI", first defined in SCSI-2. • Fast-20: This is an addendum to the original SPI document, published as ANSI standard X3.277-1996. It defined faster 20 MHz bus signaling, increasing maximum throughput to as much as 40 MB/s on the SCSI bus. • Retired in 1999 DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  11. SCSI-3 Parallel Interface - 2 (SPI-2) • Most important changes: • Fast-40 Data Transfer: SPI-2 defines another doubling of the maximum speed of the SCSI bus, from 20 MHz to 40 MHz, allowing maximum throughput of 40 MB/s on a narrow (8-bit) channel or 80 MB/s on a wide (16-bit) channel. The document also defines several restrictions associated with these faster signaling speeds, such as the use of differential signaling. • Low Voltage Differential Signaling: A new type of signaling for the SCSI bus, called low voltage differential or LVD signaling, was specified as part of SPI-2. LVD is an attempt to blend the best attributes of conventional single-ended (SE) signaling and the older type of differential signaling that is now called high voltage differential (HVD). LVD (or the older HVD) is required to run the SCSI bus at Fast-40 speeds; • Multimode Operation: Specification is provided for a way to create devices that will automatically work on both LVD and regular single-ended buses; such units are called multimode devices. • SCA-2 Single Connector Attachment Connectors: An improvement to the original SCA connectors, called SCA-2, was defined. • Very High Density Connectors: SPI-2 defined a smaller version of the older high-density 68-pin connectors. This new standard is called Very High Density Cable Interconnect, abbreviated VHDCI. • Marketing names: Ultra2 SCSI or Wide Ultra2 SCSI DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  12. SCSI-3 Parallel Interface - 3 (SPI-3) • Fast-80(DT) Data Transfer: Reflecting the continuing appetite for speed on the SCSI bus, data transfer rates were again doubled, this time to 160 MB/s on a wide bus. This was accomplished not by increasing the speed of the bus from 40 MHz to 80 MHz, but rather through the use of double transition clocking; thus the "DT" sometimes found in the name for this signaling speed. • Cyclic Redundancy Check (CRC): This is a common error checking protocol used to ensure data integrity. It was added as a safety measure since transfer speeds were being increased, leading to the possibility of data corruption. • Domain Validation: This feature improves the robustness of the process by which different SCSI devices determine an optimal data transfer rate; • Quick Arbitration and Selection (QAS): This feature represents a change in the way devices determine which has control of the SCSI bus, providing a small improvement in performance. • Packetization: Another small change to improve performance, packetization reduces the overhead associated with each data transfer; • Marketing name: Ultra3 SCSI when implemented any sub-set of these features ?!?! • Ultra 160 Ultra160/m Ultra160+ DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  13. SCSI-3 Parallel Interface - 3 (SPI-3) • High Voltage Differential: With the widespread adoption of low voltage differential, the older "high voltage" differential became unnecessary. Since it was never very popular, it was removed from the standard. • 32-Bit Bus Width: Introduced in SCSI-2, the 32-bit parallel SCSI option never caught on in the industry and was finally removed from the specification in SPI-3. • SCAM: SPI-3 removed the "SCSI Configured AutoMatically" (SCAM) feature, which was a good idea but never was universally adopted and sometimes led to configuration problems. In doing so, the SCSI world was mercifully rid of one of the worst acronyms in the history of the computer industry. • Narrow High-Speed Transfers: Narrow (8-bit) SCSI hasn't been technically "made obsolete", but 8-bit transfers are not defined for Fast-80 transfers. (Considering that faster transfer modes are used to get more throughput, increasing data transfer speeds while staying on an 8-bit bus never really made much sense.) DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  14. SCSI-3 Parallel Interface - 4 (SPI-4) • Another doublink of maximum throughput • Fast-160(DT) • Double transition clocking • Bus-speed from 40MHz to 80MHz • Theoretical throughput = 320MB/s (16-bit) • LVD required • Marketing: Ultra320 DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  15. 3. SCSI Data Transfer Modes and Feature Sets • "Regular" SCSI (SCSI-1) • Wide SCSI • Fast SCSI • Fast Wide SCSI • Ultra SCSI • Wide Ultra SCSI • Ultra2 SCSI • Wide Ultra2 SCSI • Ultra3 SCSI • Ultra160 (Ultra160/m) SCSI • Ultra160+ SCSI • Ultra320 SCSI • SCSI Transfer Mode and Feature Set Compatibility DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  16. 3.1 “Regular” SCSI (SCSI-1) • Defining Standard:SCSI-1. • Special Features: None. • Bus Width: Narrow (8-bit). • Signaling Method: SE or HVD. • Signaling Speed and Bus Throughput: 5 MHz bus speed; 5 MB/s. • Number of Devices Supported: 8. • Termination: For SE, any type (passive, active or forced perfect termination). For HVD, HVD termination. • Cabling and Maximum Cable Length: "A" cable (50 pins). Maximum of 6m for SE, 25m for HVD. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  17. 3.2 Wide SCSI (Wide SCSI-2) • Defining Standard:SCSI-2. • Special Features: None. • Bus Width: Wide (16-bit). • Signaling Method: SE or HVD. • Signaling Speed and Bus Throughput: 5 MHz bus speed; 10 MB/s. • Number of Devices Supported: 16. • Termination: For SE, any type (passive, active or forced perfect termination). For HVD, HVD termination. • Cabling and Maximum Cable Length: "P" cable (68 pins). (Formerly, "A" cable plus "B" cable.)Maximum of 6m for SE, 25m for HVD. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  18. 3.3 Fast SCSI (Fast SCSI-2) • Defining Standard:SCSI-2. • Special Features: None. • Bus Width: Narrow (8-bit). • Signaling Method: SE or HVD. • Signaling Speed and Bus Throughput: 10 MHz bus speed; 10 MB/s. • Number of Devices Supported: 8. • Termination: For SE, either active or forced perfect termination. For HVD, HVD termination. • Cabling and Maximum Cable Length: "A" cable (50 pins). Maximum of 3m for SE, 25m for HVD. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  19. 3.4 Fast Wide SCSI (Fast Wide SCSI-2) • Defining Standard:SCSI-2. • Special Features: None. • Bus Width: Wide (16-bit). • Signaling Method: SE or HVD. • Signaling Speed and Bus Throughput: 10 MHz bus speed; 20 MB/s. • Number of Devices Supported: 16. • Termination: For SE, either active or forced perfect termination. For HVD, HVD termination. • Cabling and Maximum Cable Length: "P" cable (68 pins). (Formerly, "A" cable plus "B" cable.)Maximum of 6m for SE, 25m for HVD. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  20. 3.5 Ultra SCSI (Fast-20 SCSI) • Defining Standard:SCSI-3 / SPI (including Fast-20 addendum). • Special Features: None. • Bus Width: Narrow (8-bit). • Signaling Method: SE or HVD. • Signaling Speed and Bus Throughput: 20 MHz bus speed; 20 MB/s. • Number of Devices Supported: 8 if HVD signaling is used or SE signaling is used with a maximum cable length of 1.5m; 4 if SE signaling is used with a cable length of over 1.5m. • Termination: For SE, either active or forced perfect termination. For HVD, HVD termination. • Cabling and Maximum Cable Length: "A" cable (50 pins). Maximum of 3m for SE if no more than 4 devices are used, otherwise 1.5m; 25m for HVD. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  21. 3.6 Wide Ultra SCSI (Fast-20 Wide SCSI or Ultra Wide SCSI) • Defining Standard:SCSI-3 / SPI (including Fast-20 addendum). • Special Features: None. • Bus Width: Wide (16-bit). • Signaling Method: SE or HVD. • Signaling Speed and Bus Throughput: 20 MHz bus speed; 40 MB/s. • Number of Devices Supported: 16 if HVD signaling is used; 8 if SE signaling is used with a maximum cable length of 1.5m; 4 if SE signaling is used with a cable length of over 1.5m. • Termination: For SE, either active or forced perfect termination. For HVD, HVD termination. • Cabling and Maximum Cable Length: "P" cable (68 pins). Maximum of 3m for SE if no more than 4 devices are used, otherwise 1.5m; 25m for HVD. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  22. 3.7 Ultra2 SCSI (never been popular) • Defining Standard:SCSI-3 / SPI-2. • Special Features: LVD signaling; multimode (LVD/SE) optional. • Bus Width: Narrow (8-bit). • Signaling Method: LVD or HVD. (HVD is officially supported for Ultra2 SCSI, though it is not generally used; LVD offers significant advantages over HVD and has become the standard for modern high-speed SCSI buses.) Note that multimode drives may optionally run in SE mode, but throughput will drop to Fast-20 (Ultra) levels if this is done. • Signaling Speed and Bus Throughput: 40 MHz bus speed; 40 MB/s. • Number of Devices Supported: 8 for HVD or LVD cables up to 12m in length; 2 for LVD cables over 12m. • Termination: For LVD, LVD termination; for HVD, HVD termination. • Cabling and Maximum Cable Length: "A" cable (50 pins). Maximum of 25m for LVD if no more than 2 devices are used, otherwise 12m; 25m for HVD. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  23. 3.8 Wide Ultra2 SCSI (Ultra 2 SCSI) • Defining Standard:SCSI-3 / SPI-2. • Special Features: LVD signaling; multimode (LVD/SE) optional. • Bus Width: Wide (16-bit). • Signaling Method: LVD or HVD. (HVD is officially supported for Wide Ultra2 SCSI, though it is not generally used; LVD offers significant advantages over HVD and has become the standard for modern high-speed SCSI buses.) Note that multimode drives may optionally run in SE mode, but throughput will drop to Fast-20 (Ultra) levels if this is done. • Signaling Speed and Bus Throughput: 40 MHz bus speed; 80 MB/s. • Number of Devices Supported: 16 for HVD or LVD cables up to 12m in length; 2 for LVD cables over 12m. • Termination: For LVD, LVD termination; for HVD, HVD termination. • Cabling and Maximum Cable Length: "P" cable (68 pins). Maximum of 25m for LVD if no more than 2 devices are used, otherwise 12m; 25m for HVD. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  24. 3.9 Ultra 3 SCSI • Defining Standard:SCSI-3 / SPI-3. • Special Features: Ultra3 SCSI devices include support for at least one of the following five features: • Fast-80(DT) data transfer • Cyclic Redundancy Check (CRC) • Domain validation • Quick Arbitration and Selection (QAS) • Packetization • Bus Width: Wide (16-bit). Narrow mode is not supported. • Signaling Method: LVD only, if Fast-80 is being used. (Multimode drives may optionally run in SE mode, but throughput will drop to Fast-20 (Ultra) levels.) • Signaling Speed and Bus Throughput: Depends on implementation; assuming Fast-80, 40 MHz bus speed; 160 MB/s. • Number of Devices Supported: 16 for cables up to 12m in length; 2 for cables over 12m. • Termination: LVD termination. • Cabling and Maximum Cable Length: "P" cable (68 pins). Maximum of 25m if no more than 2 devices are used, otherwise 12m. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  25. 3.10 Ultra 160 (Ultra160/m) SCSI • Defining Standard:SCSI-3 / SPI-3. • Special Features: Ultra3 SCSI devices include support for the following three SPI-3 features: • Fast-80(DT) data transfer • Cyclic Redundancy Check (CRC) • Domain validation • The following features are considered optional for Ultra160: • Quick Arbitration and Selection (QAS) • Packetization • Bus Width: Wide (16-bit). Narrow mode is not supported. • Signaling Method: LVD only. (Multimode drives may optionally run in SE mode, but throughput will drop to Fast-20 (Ultra) levels.) • Signaling Speed and Bus Throughput: 40 MHz bus speed; 160 MB/s. • Number of Devices Supported: 16 for cables up to 12m in length; 2 for cables over 12m. • Termination: LVD termination. • Cabling and Maximum Cable Length: "P" cable (68 pins). Maximum of 25m if no more than 2 devices are used, otherwise 12m. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  26. 3.11 Ultra160+ SCSI • Defining Standard:SCSI-3 / SPI-3. • Special Features: Ultra3 SCSI devices include support for all of the following five SPI-3 features: • Fast-80(DT) data transfer • Cyclic Redundancy Check (CRC) • Domain validation • Quick Arbitration and Selection (QAS) • Packetization • Bus Width: Wide (16-bit). Narrow mode is not supported. • Signaling Method: LVD only. (Multimode drives may optionally run in SE mode, but throughput will drop to Fast-20 (Ultra) levels.) • Signaling Speed and Bus Throughput: 40 MHz bus speed; 160 MB/s. • Number of Devices Supported: 16 for cables up to 12m in length; 2 for cables over 12m. • Termination: LVD termination. • Cabling and Maximum Cable Length: "P" cable (68 pins). Maximum of 25m if no more than 2 devices are used, otherwise 12m. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  27. 3.12 Ultra 320 SCSI • Defining Standard:SCSI-3 / SPI-4 (in development). • Special Features: Fast-160(DT) data transfer; others not yet confirmed at this time. • Bus Width: Wide (16-bit) only. • Signaling Method: LVD only. (Multimode drives may optionally run in SE mode, but throughput will drop to Fast-20 (Ultra) levels.) • Signaling Speed and Bus Throughput: 80 MHz bus speed; 320 MB/s. • Number of Devices Supported: 16 for cables up to 12m in length; 2 for cables over 12m. • Termination: LVD termination. • Cabling and Maximum Cable Length: "P" cable (68 pins). Maximum of 25m if no more than 2 devices are used, otherwise 12m. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  28. 3.12 SCSI Protocol Compatibility • There are no hard and fast rules regarding the compatibility of different SCSI transfer modes and feature sets, especially if they are very different in terms of key attributes. Here are some issues that you should keep in mind as you consider device compatibility: • Age:  The greater the difference in age between two devices, the greater the difficulties associated with getting them to work together. The extreme example I gave above of trying to get an Ultra160 drive to work with a SCSI-1 host adapter (or vice-versa) would probably not be much fun. However, mixing Ultra160 and Ultra2 devices is fairly straightforward. • Drive and Host Speed Negotiation: You can use faster drives on slower host adapters or vice versa, but communication will only occur as fast as the slowest device can handle. For example, you can connect a Wide Ultra SCSI drive to an Ultra160 host adapter, but the drive will only run at a maximum of 40 MB/s throughput, not 160 MB/s. • Signaling: Mixing different types of signaling on the same bus can lead to problems ranging from slowdowns to disaster. The older (high voltage) differential signaling is not electrically compatible with either single-ended or LVD devices, and should never be mixed with those types, or you risk disaster such as smoked hardware. Multimode LVD devices can be mixed with SE devices, but they won't function at Ultra2 or higher speeds if you do so. • Bus Width: You can mix wide and narrow devices on the same SCSI bus, but there are specific requirements in doing this, to ensure that the bus functions properly. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  29. 4. SCSI Protocols and Interface Features • Most important characteristics of the SCSI bus: • Signaling • Bus speed • Bus width • Features to improve: • Performance • reliability DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  30. 4.1 SE and HVD Signaling • SE signaling: Positive voltage is “one”, zero voltage (ground) is “zero”. • SE: longer cables, higher speeds: problem with signal integrity • Differential signaling: two wires for each signal (+DB(0) and –DB(0)): • Zero: zero voltage on both wires • One: one wire positive voltage, other negative DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  31. 4.1 HVD Signaling • Only on servers • High cost: • Complex circuits • Uses more power • Solution: Low Voltage Differential or LVD • Never mix SE or LVD devices with HVD devices on the same bus (the cables do match !) SMOKE is possible !!! • Always check the SCSI symbols: DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  32. 4.2 LVD Signaling • SE: 40MHz  max cable length 0,75m • HVD: high cost, electrically incompatible • The best of SE and HVD  LVD: • Two wires for each signal line • Lower voltage to create the complementary signal pairs • Electrically compatible (no smoke !) • Some LVD devices can function on SE bus (=multimode LVD device, LVD/SE or LVD/ME). • LVD required for all SCSI modes faster than Ultra (cable length 12 meters) DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  33. 4.2 LVD Signaling • LVD operation requires the following: • All devices on the chain must be LVD-capable; if even one device is only SE, all devices "drop down" and run as single-ended. • All devices must not be set to run in SE mode; some multimode devices have a jumper to "force" SE operation, which will cause the entire SCSI chain to not work in LVD. • LVD (or multimode LVD/SE) terminators must be used. • Bus speeds over 20 MHz are not supported under single-ended operation. This means that a multimode LVD/MSE Ultra160 device will run at only a maximum of 40 MB/s if it is connected to a SCSI chain with single-ended devices. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  34. 4.2 LVD Signaling • Warning: As soon as multimode LVD devices begin running as single-ended, all the rules and restrictions of single-ended operation apply, including cable length. For example, suppose you have a 4m cable connecting an LVD Ultra160 host adapter to a multimode LVD Ultra160 device; this is perfectly fine. Now, let's say you decide to add to this cable a Wide Ultra single-ended device. As soon as this happens, the other devices will drop down to single-ended operation, and probably will try to run at Ultra speeds (Fast-20). Communication problems will then result due to the fact that a 4m cable is not supported at Ultra speeds in single-ended operation. • Warning: Low voltage differential devices are not electrically compatible with high voltage differential hardware; do not mix them on the same SCSI cable or damage to the LVD devices may occur. • For Ultra3, Ultra160, Ultra160+ and faster speeds, LVD is the only option (HVD was made obsolete for these). DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  35. 4.3 SCSI Bus Width • Two commonly used SCSI bus widths: • Narrow (=regular) • 8 bits data pathway • 8 devices • 50 conductor “A” cable • Becoming obsolete • Wide • 16 bits data pathway • 16 devices • Two cables “A” cable + 68-pin “B” cable • Now= single 68-pin “P” cable • Modes faster than Ultra 2 only use wide • (32-bit) • 32 bits data pathway • Two 68-pin cables • Never very popular DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  36. 4.4 SCSI Bus Speed • Three parameters • Clock speed: frequency of the clock (5,10,20,40,80 MHz) • Transfer rate: times per second data is transferred • Throughput: theoretical maximum amount of data that can be moved across the SCSI bus. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  37. 4.5 Bus Parity and Cyclic Redundancy Checking (CRC) • Bus Parity • Universally supported • CRC • Bus Parity unnecessary (except for compatibility) • Optional for Ultra-3 • Required for Ultra 160 or Ultra 160+ and up DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  38. 4.6 Command Queuing and Reordering • IDE/ATA: allows only a single command at a time to any device • Command Queuing: device accepts as many as 64 or even 256 concurrent commands. • Command Reordering: the device can execute the outstanding commands “out-of-order”. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  39. 4.7 Negotiation and Domain Validation • Negotiation • Host adapter interrogates all devices on the bus to find out what speeds they support. • Theory OK, Practice ??? • Domain Validation: • Host adapter tests (read/write) each device according to their possibilities and matches a working speed (possible fallback to slower speeds). • Required for Ultra160 and Ultra160+ DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  40. 4.8 Quick Arbitration and Selection (QAS) • Regular SCSI configurations: • Device “bids” for the bus if it wants to use it. • Process to decide who gets to use the bus is called “arbitration”. • Arbitration is based on priority levels. • No data can be transmitted on the bus during arbitration.  overhead + slow • SPI-3 standard: “optional “feature QAS • Number of times arbitration must occur is reduced. • A device waiting for the bus can grab it more quickly after the last device sends a “done” signal without having to start an arbitration process. • Provision is made so that one device cannot dominate the bus. • Persent in Ultra160+ and up DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  41. 4.9 Packetization • To improve performance by reducing overhead: “optional” feature “packetization” or packetized SCSI: • Phases to setup a command request and data transfer are combined. • Traditional SCSI: commands, data, status, are all sent seperately over the bus • Packetization: all grouped into packets and sent as a single entity to reduce the waste of bus cycles • Present in Ultra 160+ and up. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  42. 4.10 SCSI Protocol Map DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  43. 5. Summary of Protocols and Transfer Modes DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  44. 5. Summary of Protocols and Transfer Modes DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  45. 6. SCSI Host Adapters or Host bus adapter (HBA) • Host Adapter = SCSI device like any other • “SCSI controller” is technically incorrect • Motherboard support DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  46. 6.1 Adapter Types and PC Bus Connections • PC I/O bus: ISA EISA VLB, MCA, PCI • Fast SCSI through ISA bus (8MB/s) ?!?! • PCI had more than enough capacity to handle any SCSI bus, until recently. • 32bit, 33MHz PCI  100MB/s <> Ultra160 ?? • 64bit PCI  200MB/s • PCI-X  1GB/s • PCI: supports bus mastering DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  47. 6.2 Protocol Support • Which transfer modes and feature sets are supported ? • Host adapter is backwards compatible with older devices. (When used performance drops) DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  48. 6.3 Signaling Type Support • SE, HVD, LVD • Host adapter must be electrically compatible with the devices that are to be used. • Do not attempt to connect HVD drives on the same SCSI-chain with non-HVD-drives • LVD and SE: electrically compatible assuming the LVD-devices are multimode capable (LVD/SE or LVD/MSE). • One SE device on a LVD chain  all SE and max. cable length reduced from 12m to 3m or 1,5m ! • Solved by adding an additional SE segment on the host adapter electrically separated. DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  49. 6.4 Extra features • Read Cache • Multiple Segments on a single channel • Electrically separated • Part of the same SCSI bus • Internal / external segment • Each segment can have maximum allowed cable length. • Multiple channels • Completely independent (logically and electrically) • Run in parallel • Twice as much devices and throughput • More expensive. • SCSI RAID Controllers DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

  50. 7. SCSI Cables and Connectors • Cable type: internal / external • Connector Type • Cable Length • Number of connectors • Connector Spacing • Termination • General Quality • Cables labeled as SCSI-1, SCSI-2 or SCSI-3 are not correctly labeled DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

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