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ANOTHER COMPUTER KNOWLEDGE SITE : CROY WORLD PRODUCTIONS
SCSI

ON THIS PAGE ,YOU WILL FIND,SCSI DEFINED,
ALONG WITH CABLES OF,INCLUDING ALT 1 AND 2,
A AND B,L,P,AND Q.ALSO ARE ALL OF THE CABLE
LENGHTS,CONNECTORS AND TRANSFER RATES.AS WELL
AS RAID,INCLUDING RAID O,1,3,AND 5.

SCSI :
SMALL COMPUTER SYSTEM INTERFACE
IS A COLLECTION OF INTERFACE STANDARDS THAT COVERS
A WIDE RANGE OF PERIPHERAL DEVICES,INCLUDING HARD
DRIVES,TAPE DRIVES,CD-ROM'S,AND RAID.
( REDUNDANT ARRAY OF INDEPENDENT DISKS )


Cables:
The single most critical item in the installation of a successful,high-reliability,maximum throughput SCSI system.

The longer the cables and the faster the data
throughput,the more critical cables become.
Internal SCSI cables are usually non-shielded
ribbon cables and external cables are usually
round,shielded cables with shielded connectors.
Cable impedance must be matched to the
requirements of SCSI devices and cable pairs
must be carefully selected for the correct SCSI
signal lines.


Alt 1 Cable -
An A-cable having 50-pin male high-density
micro-D connectors with spring-latch fasteners.
May be nonshielded or shielded.

Alt 2 Cable -
An A-cable having 50-pin male Centronics-type connectors
for round,shielded cabling or two rows of 25 pins on
0.1 inch centers for internal nonshielded ribbon cabling.

A-Cable -
A 50 pin narrow SCSI cable.See Alt 1 and Alt 2
above for the 4 types of A-Cable.

B-Cable -
Identical in construction to the A-cable introduced
in SCSI-2 for use with WIDE SCSI.Never popular because
of requirement for two cables.The B-cable was rendered
obsolete by the introduction of the P-cable in SCSI-3.
Use of WIDE SCSI immediately began to increase.

L-Cable - A 110-pin high-density male micro-D
connector considered for use in 32-bit WIDE SCSI
systems.No longer viable as 32-bit SCSI was
obsoleted in the SPI-3 document of SCSI-3.

P-Cable -
In SCSI-3,the P-cable is defined for 16-bit
WIDE SCSI systems to eliminate the necessity
to use two cables an A-cable and a B-cable)
for 16-bit WIDE SCSI.Uses 34 twisted pair cable
(68 wires)and 68-pin high-density male micro-D
connectors with 2-56 thumbscrew fasteners.

Q-Cable -
Physically and electrically identical to the P-cable.
Both a P-cable and a Q-cable were specified for 32-bit
WIDE SCSI buses.This cable is no long viable as 32-bit
wide SCSI was rendered obsolete in the SPI-3 document
of SCSI-3.

CABLE LENGHTS :
The SCSI specifications provide maximum recommended
cable lengths for various implementations of SCSI.
It is further Recognized that in "engineered"
installations these cable lengths may be exceeded.
Maximum recommended SCSI cable lengths:


-SCSI TYPE - SPEED---MAX CABLE LENGTH

SINGLE ENDED - SLOW (FAST-5) 6 Meters / 19.7 Feet

SINGLE ENDED - FAST (FAST-10) 3 Meters / 9.8 Feet

SINGLE ENDED - ULTRA (FAST-20) 1.5 Meters* / 4.9 Feet
Maximum cable length with eight addresses.
May be up to 3 meters With four devices.

DIFFERENTIAL (HVD) -(ANY SPEED) 25 Meters / 82 Feet

LVD -ULTRA2,ULTRA3 or ULTRA4 12 Meters / 39.4 Feet
This may be increased to 25 meters
(82 Feet in point to point applications.


Connectors:

Standard Connectors:

IDC HEADER :
50 pin insulation displacement connector(IDC)
used with ribbon cables for nonshielded,
internal SCSI cabling.Female type used on cables.
SCSI terminology:
Nonshielded Alternate 2-A Cable.

CENTRONICS TYPE :
This connector is a 50-pin version of the connector
used for parallel printer ports for years.Specified
in SCSI-1 and still an acceptable connector for narrow
SCSI.Male connector used for cables,female for devices.
Uses bail fasteners on the device-mounted female
connector to secure the male connector.Often referred
to as the "SCSI-1" connector.
SCSI terminology:
Shielded Alternative 1, A-cable.

50 -PIN HIGH-DENSITY MICRO-D CONNECTOR -
PIN AND SOCKET CONNECTOR :
Smaller than the centronics-type.Specified to
have spring-latch fasteners.Male connector used
on cables;female on devices.
SCSI terminology:
Shielded(or non-shielded)Alternative 1,A-cable.

68-pin Micro-D high-density-
Originally specified in SCSI-3 to allow the use
of one cable for wide(16-bit)applications.
This cable is called a P-cable.68-pin version
of the 50-pin micro-D high-density connector.
Although the 2-56 thumbscrew fastener is specified
in SCSI-3,some manufacturers have used the spring-latch
fastener.Male connector used on cables;female on devices.
Frequently referred to as the "SCSI-3" connector.
SCSI terminology:
Shielded (or non-shielded)Alternate 3,P-cable.

68-pin VHDCI (Very High Density Cable Interconnect)
Originally specified in SCSI-3.Ribbon contact
construction similar to centronics type connector,
but on smaller 0.8 mm(0.0315")pitch.Much smaller
footprint allows up to four connectors to be mounted
on a PC backplate.Called the VHDCI connector.
SCSI terminology:
Shielded Alternate 4 - P cable.

80-pin SCA (Single Connector Adapter) -
Originally specified in the SCSI-3 specifications
for device connection on hot-swap backplanes.Device
ID is assigned by the backplane and power connections
are included on the connector,so using peripherals with
an SCA connector in a cabled system requires the use
of an SCA to 68 pin (or 50 pin)adapter with provisions
for peripheral power and assignment of ID.
SCSI terminology:
Non-shielded Alternate 4 - P cable.

Non-Standard Connectors:

DB-25 - Not specified in the SCSI specifications,
this 25-pin connector can support SLOW 8-bit(narrow)
SCSI only and then only with very short cables.
Apple Macintosh version became a de facto standard
totally incompatible with the pinout of the old -
Future Domain 25-pin SCSI connector).Male connector
on cable;female on devices.

30-pin HDI -
A non-standard connector created by Apple for
reduced mounting space on their PowerBook notebooks.
Not suitable for multiple SCSI devices or long cables
because there are only 30 pins.

DB-50, D-sub connector was used on some older
Sun and DG computers for Narrow SCSI applications.
50 pins arranged in three rows.

60-pin high-density -A non-standard connector used
by IBM.Early in the process of writing the
SCSI-2 specification,the ANSI X3T9.Two committee
specified a 60-pin connector for 8-bit SCSI that
was later abandoned,IBM,however,retained this connector
for their PS/2 systems.It is a 60-pin high-density
micro-D male connector with spring-latch fasteners.
The first 50-pin assignments are identical to the
SCSI-2 high-density pinout and pins 51 to 60
are designated as "reserved".

MORE SCSI CONNECTOR INFO


CONNECTOR :

END P = MALE
S = FEMALE

RC - 50 P/S -------FULL-PITCH CENTRONICS 50-PIN

DB - 25 P/S--------FULL-PITCH D-SUB 25-PIN

DB - 50 P/S -------FULL-PITCH D-SUB 50-PIN

RCII -50 P/S------CENTRONIC HALF-PITCH
---------------------- MICRO CONNECTOR 50 PIN

DBII - 50 P/S----D-SUB HALF-PITCH MICRO
-------------------------- CONNECTOR 50 - PIN

RC3 -68 P/S-------CENTRONIC HALF-PITCH
---------------------- MICRO CONNECTOR 68 -PIN

DB3 - 68 P/S ------D - SUB HALF-PITCH MICRO
------------------------CONNECTOR 68 - PIN

TRANSFER RATES


SCSI - 1 ( ASYNCHRONOUS )--2 Mbps

SCSI - 1 ( SYNCHRONOUS )---- 5 Mbps

SCSI - 2 FAST-----------------------10 Mbps

SCSI - 2 16 - BIT- WIDE----------10 Mbps

SCSI-2 FAST AND 16-BIT-WIDE--20 Mbps

SCSI -2- 32 BIT-WIDE--------------- 20 Mbps

SCSI -2-FAST AND 32-BIT-WIDE--40 Mbps

WHEN SCSI-2 AND SCSI-3 DEVICES FIRST CONNECT ASYNCHRONOUSLY,THEY HANDSHAKE IN 8-BIT NARROW MODE TO ENSURE THAT THE DATA WILL TRANSFER PROPERLY.THE INITIATOR AND TARGET THEN NEGOTIATE THE MAXIMUM MUTAL CAPABILITIES.

SPEED-----FREQUENCY-----NARROW SPEED----WIDE SPEED

SCSI---------5MHz-----1,5 to 5 MB/s---/[no wide device]

Fast SCSI----10MHz--------10 MB/s---------------20 MB/s

Ultra SCSI---20MHz--------20 MB/s---------------40 MB/s

Ultra2 SCSI---40MHz-------40 MB/s----------------80 MB/s

Ultra 160/m--80MHz--/ [no narrow device]---- 160 MB/s

RAID :
REDUNDANT ARRAY OF INDEPENDENT DISK
IS A STORAGE TECHNOLOGY THAT USES TWO OR MORE HARD
DRIVES IN COMBINATION FOR HIGH AVAILABILITY,
FAULT TOLERANCE,AND PERFORMANCE.

RAID 0 - DATA STRIPING :
INTERLEAVES DATA ACROSS MULTIPLE DRIVES.
DOESN'T INCLUDE MIRRORING,REDUNDANCY,
OR ANY OTHER PROTECTION AGAINST DEVICE
FAILURE.IT IS NOT FAULT TOLERANT

RAID 1 - DATA MIRRORING :
PROVIDES FAULT TOLERANCE BY COMPLETELY DUPLICATING
DATA ON TWO INDEPENDENT DRIVES.IN CASE ONE OF THE
MIRRORED DRIVES FAIL.

RAID 3 - PARALLEL TRANSFER WITH PARITY :
PROVIDES FAULT TOLERANCE BY TRANSFERING DATA TO AND
FROM THREE OR MORE HARD DRIVES WITH DATA STRIPED ACROSS
THE DRIVES AND THE PARITY BITS,WHICH ARE USED TO RECON-
STRUCT THE DATA IN THE EVENT OF A DRIVE FAILURE AND STORED ON A SEPERATE AND DEDICATED DRIVE.

RAID 5 - DATA STRIPING WITH PARITY :
PROVIDES FAULT TOLERANCE BY EMPLOYING ESSENTIALLY THE
SAME APPLICATION AS RAID 3.HOWEVER,RAID 5 STORES THE
PARITY BIT FROM TWO DRIVES ON A THIRD DRIVE TO PROVIDE
FOR DATA STRIP ERROR CORRECTION.THIS IS THE MOST POPULAR RAID TECHNOLOGY USED TODAY.

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