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SCSI Means More

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PCQ Bureau
New Update

SCSI-1. Born in 1986, the first SCSI standard was designed to operate at a 5 MHz clock speed and provide a throughput of 5 MB/s using an 8-bit bus. It was adapted from SASI (Shugart Associates Systems Interface), an initiative of the (then) popular drive manufacturer Shugart, aimed at moving away from the head:cylinder:sector addressing structure towards logical block addressing. Released in 1979, SASI was later adapted by ANSI, which made some significant improvements in it and released it as the IEEE X3.131-1986 seven years later. This document formed the basis of what has since come to be known as SCSI-1.

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The original SCSI-1 specification permitted a maximum cable length of 6 meters only while using single-ended signaling and 25 meters using HVD. It allowed a maximum of 8 SCSI devices to be connected simultaneously.

SCSI-2. Work on the SCSI-2 specification started even before SCS1-1 was officially accepted by ANSI. SCSI-2 set out to double everything that SCSI-1 offered. Fast SCSI doubled the clock speed from 5 Mhz to 10 Mhz, thus doubling the throughput to 10 MB/s, but enforcing a halving of the cable length used in single-ended signaling to 3 meters. Another variant, called Wide SCSI, doubled the bus width to 16 bits, with the same result as Fast SCSI. Fast Wide SCSI combined both of them to quadruple the throughput (as compared to SCSI-1) to 20 MB/s with 3 meters as the maximum bus length.

s Max.

Bus
s


STA

Terms
Clock

Speed (MHz)
Bus

Speed,



MBytes/


Sec. Max
Bus Width



(in bits)
Lengths,meters (1) Max 



Device 


Support
Single 



ended
LVD HVD


SCSI-1

(2)
5 5 8 6 (3) 25 8


Wide

SCSI (2)
5 10 16 3 (3) 25 8


Fast

SCSI (2)
10 10 8 3 (3) 25 8


Fast

Wide SCSI
10 20 16 3 (3) 25 16


Ultra

SCSI (2)
20 20 8 1.5 (3) 25 8


Ultra

SCSI (2)
20 20 8 3 - - 4


Wide

Ultra SCSI
20 40 16 - (3) 25 16


Wide

Ultra SCSI
20 40 16 1.5 - - 8


Wide

Ultra SCSI
20 40 16 3 - - 4


Ultra2

SCSI (2,4)
40 40 8 (4) 12 25 8


Wide

Ultra2 SCSI (4)
40 80 16 (4) 12 25 16


Ultra3

SCSI (6) (AKA ULTRA 160)
80 160 16 (4) 12 (5) 16


Ultra320

SCSI
160 320 16 (4) 12 (5) 16
(1) The listed maximum bus lengths may be exceeded in Point-to-Point and engineered applications.



(2) Use of the word “Narrow”, preceding SCSI, Ultra SCSI or Ultra2 SCSI is optional.


(3) LVD (Low Voltage Differential) was not defined in the original SCSI standards for this speed. If all devices on the bus support LVD, then 12 m operation is possible at this speed. However, if any device on the bus is single-ended only, then the entire bus switches to single-ended mode and the distances in the single ended column apply.


(4) Single-ended is not defined for speeds beyond Ultra.


(5) HVD (High Voltage Differential, or Differential) is not defined for speeds beyond Ultra2.


(6) After Ultra2, all new speeds are “Wide” only.


This table is derived from a table at the website (www.scsita.org/aboutscsi/termsTermin.html). The numbers in parentheses denote a comment below the table




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SCSI-2 also saw passive termination (used in SCSI-1) giving way to the cleaner, more efficient active termination. SCSI-2 is designed to be backward compatible with SCSI-1.

SCSI-3. Work on the

SCSI-3 specification continues to progress. It divides the SCSI world into a number of different standards (and, thus, documents) and aims to take SCSI to the next level. Development on the SCSI-2 continues as Parallel SCSI, like the Ultra SCSI, which ups the clock speed to 20 MHz, thus giving throughputs of 20 MB/s with an 8 bit bus. It supports a maximum of 8 devices with a 1.5 meter cable length and four devices with a 3 meter length.

SCSI-3 has many other variations of Parallel SCSI, which push the clock speed upto limits of 160 Mhz with throughput going up to 320 MB/s (see table). 

The future. SCSI-3 also proposes to combine SCSI with some of the other popular modern day technologies. These include transporting SCSI over Fiber channel and FireWire (IEEE 1394) using FCP (Fiber Channel Protocol) and SBP (Serial Bus Protocol), respectively. Another very exciting development is the emergence of iSCSI, a standard that allows transport of SCSI commands over an IP network. This means that you can build SANs (Storage Area Networks) over your existing network. ISCSI-based products are already available.

Considering its past record and the newer initiatives that are being taken, who would bet against more success for SCSI in the future.

Kunal Dua

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