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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