S C S I Small Computer Systems Interface

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S C S ISmall Computer Systems Interface
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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

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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

• 19791986 (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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2.2. SCSI-2 Standard

• 19851994 (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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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)

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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 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

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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.
• 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.

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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 of 25m if no more than 2 devices
  are used, otherwise 12m.

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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 of 25m if no more than 2 devices
  are used, otherwise 12m.

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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.)
• 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.

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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
43
5. Summary of Protocols and Transfer Modes
44
5. Summary of Protocols and Transfer Modes
45
6. SCSI Host Adapters or Host bus adapter (HBA)

• Host Adapter SCSI device like any other
• SCSI controller is technically incorrect
• Motherboard support

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 ltgt Ultra160 ??
• 64bit PCI ? 200MB/s
• PCI-X ? 1GB/s
• PCI supports bus mastering

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)

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.

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

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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

52
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

53
7.3 SCSI external Connector Types

• D-Shell (D-Sub, DB) 50-pin (obsolete)
• Centronics
• High-Density (HD) narrow 50-pins, wide 68

54
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)

55
7.3 SCSI internal Connector Types

• Regular density 50-pin
• High Density (50-pins) 68-pin
• Single Connector Attachment (SCA)

56
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.)

57
7.4 Narrow (50-pin) Single-Ended

• Most common narrow cable types

58
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.

59
7.5 Wide (68-pin) Single Ended Cable

• External Wide cable

• Internal wide cale

60
7.6 LVD cables

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

61
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

62
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.

63
7.8 Adapters (convertors)
64
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.

65
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.

66
7.10 Summary of cables and connectors
67
8 SCSI Configuration

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

68
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

69
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

70
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 -gt high priority IDs
• Automatic assignment of the IDs.

71
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.

72
9. IDE/ATA vs. SCSI
73
Sources

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