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SCSI stands for Small Computer Systems Interface. It´s a standard for connecting peripherals to your computer via a standard hardware interface, which uses standard SCSI commands

The SCSI standard can be divided into SCSI1, SCSI2, SCSI wide and so on. SCSI advantages includes flexible device attachment (up to 7-15 devices per SCSI bus), support for almost any peripheral type (disk, tape, CDROM, scanner etc), greater speed. All SCSI commands can overlap with commands on other devices. SCSI is widely used in servers, but due to it cost less used in end-user computers (ATA devices are cheaper and offer comparable perfomance).

TermsNameMHzBus
Width
MB/sMBit/s
SCSI-1 (previously noted as SCSI-1 X3.131-1986 )SCSI-158540
Fast SCSI (SCSI-2 X3.131-1994)SCSI-21081080
Fast-Wide SCSISCSI-2 /
SCSI-3
101620160
Ultra SCSI (deprecated terms Fast 20,
SCSI-3, Double-speed SCSI )
SCSI-320820160
Ultra-Wide SCSISCSI-3201640320
Ultra2 SCSI (Fast 40, SCSI-3)SCSI-440840320
Ultra2-Wide SCSISCSI-4401680640
Ultra3 SCSI (Fast 80, SCSI-4)Ultra160/m40*2880640
Ultra3-Wide SCSIUltra160/m40*2161601280
Ultra4 SCSIUltra32080*281601280
Ultra4-Wide SCSIUltra32080*2163202560

Ultra3 features the same base frequency as Ultra2 (40MHz) but transmits 2 bytes per data clock thus doubling the total throughput. This is the same technique used in SDRAM, DDR and RAMBUS Memory where one data word (8 or 16 Bits) is transmitted at the raising edge and one at the falling edge. Ultra4 doubles the base frequency of Ultra2 (80MHz) and like U3 transmits 2 bytes per data clock thus doubling the total throughput vs U3. Ultra4 also features AAF (Adjustable Active Filter), QAS (Quick Arbitration and Selection). This enables these fast speeds at these long cable lengths and decreases the overall latencies due to the protocol overhead.

Fast-Wide SCSI is actually SCSI-2, however many cable manufacturers refer to the 68-pin plugs as SCSI-3.
Most SCSI 1-4, 160/m Ultra320 Devices can be mixed on the same bus, however, the slowest device/controller determines the bus speed during negatiating and in some cases also during data transfers!
You can not mix single ended and High Voltage (HV) differential! But you can mix most LVD (Low Voltage Differential) and single ended devices. Fast-Wide SCSI-2 is also sometimes called SCSI-3 (in particular when referring to the 68 HP connector).

Name Common Name Application Bus Width SCSI Standards Comment
D-Sub 25 pinAppleExternal8 narrowSCSI 1+2Primarily for Microtest Discport, ZIP drives and Apple Computers
50 pin CentronicsSCSI-1External8 narrowSCSI 1+2+3Most common for Scanners, Tapes and external CD-ROM"s and other slower periphereals
50 pin (HP)SCSI-2External8 narrowSCSI 2+3External Raid-Disks and high performance Tape Libraries
68 pin (HP)SCSI-3External16 wideSCSI 2+3External Raid-Disks and high performance Tape Libraries
50 pin ribbonnormalInternal8 narrowSCSI 1+2+3Standard internal narrow SCSI Disks, CD-ROM"s, Tapes and optical disks
68 pin ribbonwideInternal16 wideSCSI 2+3Wide SCSI Disks and other high performance drives

Interfaces pinoutss:

The words "narrow" stand for 8 bits and "wide" is 16 bit. The majority of the millions of SCSI devices in use today use single-ended transceivers (Asymmetrical, ground referenced). Differential (symmetrical +-) devices, which provide for greater bus lengths, are typically used in applications which can properly bear the added expense. A transceiver technology, Low-Voltage Differential (LVD), will combine the best features of single-ended and high-powered differential transceivers. LVD will also enable higher speeds. To ease the migration to LVD, most new SCSI devices will support Universal transceivers which include both single-ended and LVD transceivers.

SCSI terms:

Host adapter:
The card that connect your computer to the SCSI-bus. Usually called SCSI-controller by marketing droids.

Terminators (passive).
A group of resistors on the physical ends of a single ended SCSI-bus (and only on these ends) that dampens reflected signals from the ends of the bus.
Each terminated signal is connected by:
* 220 Ohm to +5 volt (TERMPWR)
* 330 Ohm to ground.
The 18 signals that are terminated are: I/O, Req, C/D, Sel, Msg, Rst, Ack, Bsy, Atn, DB(p), DB(7) ... DB(0).

Terminators (active).
Rather than passive terminators that use TERMPWR which may not be exactly +5v, active terminators use a voltage regulator.

Single ended:
"Normal" electrical signals. Uses open collector to the SCSI bus, [usually] survives wrong cable insertion. DIFFSENSE signal is used to detect
connection of wrong type devices. The max. length for SCSI-1 is a 6 meter cable with stubs of max 10cm allowed to connect a device to the main-cable. Most devices are single ended.

Differential:
Uses two wires to drive one signal. Max. cable length of 25 meters. Electrically incompatible with single ended devices! SCSI-1 and upwards. "Normal" SCSI is also called "Single-ended" SCSI. For each signal that needs to be sent across the bus, there exists a wire to carry it. With differential SCSI, for each signal that needs to be sent acrossthe bus, there exists a pair of wires to carry it. The first in this pair carries the same type of signal the single-ended SCSI carries.The second in this pair, however, carries its logical inversion. The receiver takes the difference of the pair (thus the name differential), which makes it less susceptible to noise and allows for greater cable length.

Apple kludge:

The single ended 50 pins cable has been reduced to 25 pins by tying most grounds together. DB25 connector (like a parallel port). Often used as the external SCSI connector.

Asynchronyous SCSI:
A way of sending data over the SCSI-bus. The initiator sends a command or data over the bus and then waits until it receives a reply (e.g. an ACKnowledge). All commands are send asynchronously over the 8 bit part of the SCSI-bus.

Synchronous SCSI.
Rather then waiting for an ACK, devices that both support synchronous SCSI can send multiple bytes over the bus. This improves throughput, especially if you use long cables. (The time that a signal travels from one end of the cable to the other end of the cable IS relevant.)

RAID:
A Redundant Array of Independant Disks is a set of drives connected to a special dual ported SCSI adapter that allows certain types of access
optimization. A RAID 0 array stripes the data accross multiple drives to decrease data latency. A RAID 1 array mirrors the data on multiple drives
for increased data integrity. A RAID 5 array contains extra drives that are used to apply ECC data correction and provide high reliability.

Wide SCSI:
Uses an extra cable (or 68 pin P cable) to send the data 16 or 32 bits wide. This allows for double or quadruple speed over the SCSI-bus.
Note that no *single* drive reaches these speeds, but groups of several drives can.

Cables are:

DescriptionLengthDescriptionLengthInternal Cables and Adapter
DB25 - SCSI10.9mSCSI1 - SCSI33.6m50pin ribbon2 pos no Termination
SCSI1 - SCSI10.9mSCSI2 - SCSI30.9m50pin ribbon4 pos w/ Termination
SCSI1 - SCSI11.8mSCSI2 - SCSI31.8m50pin ribbon4 pos w/ Termination
SCSI1 - SCSI20.9mSCSI3 - SCSI30.9m50pin ribbon7 pos w/ Termination
SCSI1 - SCSI21.8mSCSI3 - SCSI31.8m68pin ribbon4 pos w/ Termination
SCSI2 - SCSI20.9mVHDCI - SCSI11.8m68pin ribbon4 pos w/ Termination
SCSI2 - SCSI21.8mVHDCI - SCSI13.6m68pin ribbon7 pos w/ Termination
SCSI1 - SCSI30.9mVHDCI - SCSI31.8m50pin - 68pinNarrow 2 Wide Adapter
SCSI1 - SCSI31.8mVHDCI - SCSI33.6m  

(SCSI3 Refers to Wide 68pin Connectors rather than Ultra SCSI)

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