S C S I Small Computer Systems Interface

April 19, 2023 DE NAYER INSTITUUT Hogeschool voor Wetenschap & Kunst

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S C S ISmall Computer Systems

Interface


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Contents Small Computer Systems Interface (SCSI)

1. Overview and History of the SCSI Interface 2. SCSI Standards 3. SCSI Data Transfer Modes and Feature Sets 4. SCSI Protocols and Interface Features 5. Summary of SCSI Protocols and Transfer

Modes 6. SCSI Host Adapters 7. SCSI Cables and Connectors 8. SCSI Configuration 9. IDE/ATA vs. SCSI: Interface Comparison


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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


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2. SCSI Standards

SCSI – 1

SCSI – 2

SCSI - 3


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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


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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 !!!


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1993, collection of different, but related standards

SCSI-3 Architecture SCSI-3 Standards SCSI-3 Parallel Interface (SPI, SPI2-4)


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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.


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Overview SCSI-3 Standards


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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


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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


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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+


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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.)


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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


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3. SCSI Data Transfer Modes and Feature Sets1. "Regular" SCSI (SCSI-1) 2. Wide SCSI 3. Fast SCSI 4. Fast Wide SCSI 5. Ultra SCSI 6. Wide Ultra SCSI 7. Ultra2 SCSI 8. Wide Ultra2 SCSI 9. Ultra3 SCSI 10. Ultra160 (Ultra160/m) SCSI 11. Ultra160+ SCSI 12. Ultra320 SCSI 13. SCSI Transfer Mode and Feature Set Compatibility


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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.


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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.


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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.


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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.


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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.


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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.


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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.


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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.


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.


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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.


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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



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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



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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.)


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.


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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.


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4. SCSI Protocols and Interface Features Most important characteristics of the SCSI

bus: Signaling Bus speed Bus width

Features to improve: Performance reliability


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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


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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:


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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)


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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.


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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).


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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


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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.


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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


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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”.


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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+


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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


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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.


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4.10 SCSI Protocol Map


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5. Summary of Protocols and Transfer Modes


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5. Summary of Protocols and Transfer Modes


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6. SCSI Host Adapters or Host bus adapter (HBA) Host Adapter = SCSI device like any other “SCSI controller” is technically incorrect Motherboard support


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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


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6.2 Protocol Support

Which transfer modes and feature sets are supported ?

Host adapter is backwards compatible with older devices. (When used performance drops)


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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.


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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


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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


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7.1 SCSI External Cable Types Twisted Pair Wiring:

SE: each signal paired with a “signal return” (ground) Differential Signaling: each positive signal paired with is

negative signal. Shielding: metallic shield 3-Layer Structure:

Core layer: most important control signals: REQ en ACK Middle layer: other control signals Outer layer: data signals

Expensive


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7.2 SCSI Internal Cable Types Thickness of the wires Insulation

better cables use teflon Cheaper ones use PVC

Regular flat cables used only for SE up to 20 MHz Not shielded Twist-N-Flat for higher speeds


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7.3 SCSI external Connector Types D-Shell (D-Sub, DB): 50-pin (obsolete)

Centronics:

High-Density (HD): narrow 50-pins, wide 68


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7.3 SCSI external Connector Types Very High Density Cable Interconnect

68-pins Micro-centronics 2 of these can be squeezed on a single expansion

slot insert (Raid-controllers)


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7.3 SCSI internal Connector Types Regular density: 50-pin

High Density: (50-pins) 68-pin

Single Connector Attachment (SCA)


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7.4 Narrow (50-pin) Single-Ended “A” Cable

External Centronics "A" Cables: These are the oldest common type of external SCSI cables, using Centronics-style ("Alternative 2") connectors. They connect older external devices to narrow SCSI buses.

External High Density "A" Cables: These cables are used for newer external devices that have high density ("Alternative 1") connectors instead of Centronics connectors.

Internal Regular Density "A "Cables: Flat, 50-conductor ribbon cables using the older "regular density" ("Alternative 2") connectors. Widely used for connecting to older hard disks and slower devices such as CD-ROMs.

Internal High Density "A" Cables: 50-conductor ribbon cables using the newer high density ("Alternative 1") connectors are used for newer or faster devices (though most of these are now wide devices, and so use wide cabling.)


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7.4 Narrow (50-pin) Single-Ended Most common narrow cable types:


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7.5 Wide (68-pin) Single Ended Cable “P” cable

External High Density Cables: The older style of external wide cables uses the larger high density connector.

External Very High Density Cables: The newer style of external wide cables uses the very high density (VHDCI) connector. It is most common with the newest devices (though typically, VHDCI cables use LVD, not single-ended signaling.)

Internal High Density Cables: Only one general type of internal wide cable is used for single-ended signaling, a 68-wire ribbon cable with high density connectors.


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7.5 Wide (68-pin) Single Ended Cable External Wide cable Internal wide cale


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7.6 LVD cables

Internal 68-wire LVD cable (twisted pair) with LVD/SE terminator.


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7.7 Single Connector Attachment (SCA / SCA-2) Single connector for signaling and power 80 pins SCSI-ID is set by host adapter Make hot plugging of drives easier. SE and LVD version


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7.8 Adapters (convertors) Mechanical Connector Adapters: The most common types of adapters are

used to allow devices with different types of connectors to be used on the same cable. These are simple, purely mechanical adapters that don't contain any logic or live circuitry, and are relatively inexpensive. For example, you might have an existing external SCSI cable using high density connectors, and want to add to the SCSI chain a device that has a Centronics connector. There are dozens of different connector adapters, reflecting the myriad of combinations of connector types.

SCA Adapters: These are adapters that let you use SCA drives on regular systems that don't have SCA backplanes.

Wide/Narrow Adapters: Adapters that let you put a narrow SCSI device on a wide cable or vice-versa. There are complications involved in doing this; see here for more.

Internal/External Adapters: Adapters that let you use an internal cable outside the PC.

Signaling Method Adapters: Adapters that let you use differential drives on a single-ended SCSI chain or vice-versa. These are electrically active adapters and are generally expensive.

Interface Adapters: Adapters that let you use SCSI devices on other interfaces; the most common is an adapter to let you use certain types of SCSI drives on a PC's parallel port. Again, these can be a bit expensive. Also, you will be limited to (at best) the maximum speed of whichever interface is slowest.


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7.8 Adapters (convertors)


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7.9 SCSI Bus termination Passive Termination: This is the oldest, simplest and least reliable type of

termination. It uses simple resistors to terminate the bus, similar to the way terminators are used on coaxial Ethernet networks. Passive termination is fine for short, low-speed single-ended SCSI-1 buses but is not suitable for any modern SCSI speeds; it is rarely used today.

Active Termination: Adding voltage regulators to the resistors used in passive termination allows for more reliable and consistent termination of the bus. Active termination is the minimum required for any of the faster-speed single-ended SCSI buses.

Forced Perfect Termination (FPT): This is a more advanced form of active termination, where diode clamps are added to the circuitry to force the termination to the correct voltage. This virtually eliminates any signal reflections or other problems and provides for the best form of termination of a single-ended SCSI bus.

High Voltage Differential (HVD): Buses using high voltage differential signaling require the use of special HVD terminators.

Low Voltage Differential (LVD): Newer buses using low voltage differential signaling also require their own special type of terminators. In addition, there are special LVD/SE terminators designed for use with multimode LVD devices that can function in either LVD or SE modes; when the bus is running single-ended these behave like active terminators.


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7.9 SCSI Bus termination

Termination to prevent refection Exactly two terminators per bus or bus

segment. Many devices contain terminators that can be

activated. Differential and SCA devices DO NOT have

this capability ! Host adapters can terminate the bus.


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7.10 Summary of cables and connectors


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8 SCSI Configuration SCSI Bus Topology Number of Devices SCSI Device IDs Mixing Narrow and Wide Devices


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8.1 SCSI Topology

Term - DevA – DevB – Term Term - DevA – DevB – DevC –DevD – Term T – DA – unconn – DB – DC – unconn –T T – DA – DB – DC – T unconn – unconn T – DA – unconn – DB – unconn - DC –T


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8.2 Number of Devices Defined by:

Bus Width: SCSI was originally defined as a narrow, 8-bit bus, with support for 8 different devices. When wide 16-bit SCSI was created, this was expanded to support for 16 devices. Since the host adapter is itself a SCSI device, this means a theoretical maximum of 7 other devices for narrow SCSI, or 15 for wide.

Transfer Mode and Signaling Method: Certain transfer modes and signaling methods limit the theoretical maximum number of devices, due to electrical signal integrity considerations. This is particularly true of Wide Ultra SCSI, where single-ended implementations are limited to 8 devices despite being 16 bits wide, because single-ended signaling can't handle 16 devices on a cable at that speed.

Cable Length: Many SCSI transfer modes involve a "tradeoff" between cable length and the number of devices that can be supported. For Ultra SCSI in particular, reducing the number of devices on the chain allows the use of longer cables and vice-versa. Newer implementations that use LVD signaling are normally limited to 12m in length, but this can be extended to 25m if only two devices are used on the cable.

Practical Considerations: The limitations above are all based on theoretical considerations. There are also practical issues involved in running many devices on a SCSI bus


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8.3 SCSI Device IDs Narrow: 0 – 7 Wide: 0 – 15 Priority (high to low) for arbitration:

7 6 5 4 3 2 1 0, 15 14 13 12 11 10 9 8 Slower devices best set to higher priority IDs

in busy systems so they get a chance to get on the bus.

CD/DVD recorder -> high priority IDs Automatic assignment of the IDs.


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8.4 Mixing narrow with wide devices Adapters: You will have to use either a wide or a narrow cable, depending on whether the

host adapter you are using is wide or narrow. Then, whichever drives are the opposite width will need an adapter so they fit onto the cable. For example, if you have a wide cable connected to a wide host adapter and several wide devices, to add a narrow device to this chain you will need an adapter for the narrow device to let it plug into the wide cable.

Performance: If you put a wide device on a narrow SCSI channel, you will cut its potential maximum performance since it will only be able to send data 8 bits at a time. This is generally only a big issue for hard disk drives. The converse is not true, of course; putting narrow devices on a wide channel will not double their performance. Nice try though. ;^)

Device Addressing: Narrow devices cannot see or access device IDs over 7. If you are going to use narrow devices on a wide host adapter, set the host adapter's device ID to something between 0 and 7 or the device won't work. (It's a good idea to just leave host adapters at device 7 period.)

Termination: This is really the biggest issue with mixing devices. The problem is this: what happens to the extra 8 data bits present on the wide SCSI bus when they are connected to narrow devices? If you just connect the narrow data signals and leave the wide signals "hanging", then that part of the bus will be unterminated! Similarly, many wide devices, when connected to narrow buses, won't work properly if the high bits are not properly dealt with. Therefore, the wide data signals must be terminated when bus widths are mixed. This is often done by the adapters used for mixing devices, as long as you purchase the right hardware. Connectors that automatically terminate the extra "wide" signals are said to include high byte termination.


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9. IDE/ATA vs. SCSI


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Sources

http://www.storagereview.com/guide2000/ref/hdd/if/scsi/index.html

S C S I Small Computer Systems Interface

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