pctechguide.com

  • Home
  • Guides
  • Tutorials
  • Articles
  • Reviews
  • Glossary
  • Contact

SCSI Interface Evolution

SCSI-1, the original 1986 standard, is now obsolete. It used asynchronous transfer, where the host and the device, blind to the other’s maximum potential, slowly exchanged 8 bits at a time, offering a bandwidth of 3 MBps. SCSI-1 allowed up to eight devices – the host adapter and up to seven hard disks.

With synchronous transfer, the host and the device together determine the highest rate of transfer they can sustain and stick to it. Work started on SCSI-2 in 1986, the standard finally being approved by the American National Standards Institute (ANSI) in 1994. SCSI-2 featured synchronous transfer, raising the bandwidth to 5 MBps and added specifications for attaching devices other than hard disks, moving it into its role as a multiple-device interface.

SCSI-2 also added two optional speed improvements: doubling the signalling rate to 10MHz (Fast SCSI), and adding a second P cable to the SCSI bus, allowing 16-bit or 32-bit data transfers (Wide SCSI). These two options can be used separately or combined in Fast Wide SCSI, capable of a sustained data transfer rate of 20 MBps. Wide SCSI adapters may support up to 16 devices on a single chain, with IDs 0 to 15.

After SCSI-2 things get a little confusing. The SCSI-3 specification, drafted in 1996, splits SCSI into a number of specifications, including:

  • the SCSI Parallel interface (SPI), which defines the specification governing the workings of SCSI cables, and
  • the SCSI Interlock Protocol (SIP), which sets out the commands for all SCSI devices.

each document having its own revision level.

Importantly, SCSI-3 eliminates the need for a second cable for Fast SCSI or Wide SCSI and adds support for fibre-optic cable. Another major addition is SCAM (SCSI Configuration Auto-Magically), which addresses one of the common complaints about SCSI – that it was difficult to install and configure. A subset of Plug and Play, SCAM allows for self-configuring SCSI devices that select their own ID number, rather than the manual assignment of IDs in SCSI-1 and 2. It also allows autotermination.

UltraSCSI (also known as Fast-20) is an extension of SCSI-2 that doubles the signalling rate of the SPI specification to 20MHz, at the cost of shortening the length of the SCSI bus to 1.5m. In 1998 SPI-2 doubled the speed again to Fast-40 commonly, know as Ultra2 SCSI. By running the bus at 40MHz the 16-bit Wide implementation achieves a theoretical maximum bandwidth of 80 MBps.

The manner in which data is transmitted across a SCSI bus is defined by the method of signalling used. There are three types of SCSI Signaling that can be used: High-voltage differential (HVD), Low-voltage differential (LVD) and Single Ended (SE). HVD and SE have been around since the early SCSI standards, the latter’s popularity being largely because of the longer cable lengths it allows. LVD was introduced with the Ultra2 SCSI implementation and, in many ways, represents a compromise between its two predecessors. Using 3 volt instead of the standard 5 volt logic, it has all the advantages of 5V High Voltage Differential, but without the need for expensive transceivers. As well as being much less susceptible to noise interference, LVD allows cable lengths of up to 12m, even when the full 16 devices are attached.

LVD’s lower voltage also confers other advantages. The lower voltage and lower current requirements of LVD SCSI drivers means lower heat dissipation. That in turn means that the differential drivers can be included on the LVD SCSI interface ASIC, resulting in an interface with a smaller parts count, lower parts cost, a requirement for less real estate on the PCB and increased reliability.

Announced in late 1999, SPI-3 doubled the speed again to Fast-80. Commonly know as Ultra160 SCSI, this raised throughput to 160 MBps on a wide bus and offered three main improvements over Ultra2 in terms of the technology:

  • cyclic redundancy checking (CRC), which checks all transferred data, adding significantly to data integrity
  • domain validation, which intelligently verifies system configuration for improved reliability, and
  • double transition clocking, which is the main reason for the improved bandwidth.

2001 saw the announcement of Ultra320 SCSI, which built on the improvements realised by Ultra160 SCSI, adding features such as Packet Protocol and Quick Arbitration Select to further improve SCSI performance to 320 MBps.

There have been seven generations of SCSI since the first true SCSI interface standard was approved by ANSI in 1986. During that time the protocol has evolved from an 8-bit, single-ended interface transferring data at 5 MBps to a 16-bit, differential interface transferring data at 320 MBps:

Max. Bus Speed

(MBps)

Bus Width

(Bits)

Max. Bus Length

(Metres)

Max. Device Support
Single-ended LVD HVD
SCSI-1 5 8

(Narrow)

6 – 25 8
Fast SCSI 10 8 3 – 25 8
Fast Wide SCSI 20 16

(Wide)

3 – 25 16
Ultra SCSI 20 8 1.5 – 25 8
Ultra SCSI 20 8 3 – – 4
Wide Ultra SCSI 40 16 – – 25 16
Wide Ultra SCSI 40 16 1.5 – – 8
Wide Ultra SCSI 40 16 3 – – 4
Ultra2 SCSI 40 8 Not defined for speeds beyond Ultra 12 25 8
Wide Ultra2 SCSI 80 16 – 12 25 16
Ultra3 SCSI or Ultra160 SCSI 160 16 – 12 Not defined for speeds beyond Ultra2 16
Ultra320 SCSI 320 16 – 12 – 16

SCSI is entirely backward compatible, with ancient SCSI-1 devices operating on the latest host adapters. Of course, to exploit the potential of faster, more recent SCSI devices, a matching host adapter is required. Similarly, the fastest host won’t speed up an old, slow SCSI device.

SCSI has become the accepted standard for server-based mass storage and the Ultra2 LVD implementation is often seen teamed up with Redundant Array of Independent Disks (RAID) arrays to provide both high speed and high availability. However, its dominance of server storage is coming under increasing pressure from the Fibre Channel standard.

  • What Is The System Bus?
  • ISA Bus – Industry Standard Architecture
  • Local Bus Interfaces
  • PCI Bus Interfaces
  • What is AGP and AGP Pro?
  • Internal Interfaces Summary
  • PCI-X Interfaces
  • PCI Express Interfaces
  • IDE Interfaces
  • EIDE Interfaces
  • Hard Disks – What IS ATA and Ultra ATA?
  • Serial ATA (SATA) interface guide
  • SCSI Explained – With Pictures
  • SCSI Interface Evolution
  • Fibre Channel Interfaces
  • Hard Disks – What is Serial Storage Architecture?
  • I/O Interface Standards
  • How It Works: The Idea and Technology Behind USB
  • IEEE 1394 Interfaces
  • USB 2.0 Intefaces
  • FireWire 800 Interfaces

Share this:

  • Click to share on Twitter (Opens in new window)
  • Click to share on Facebook (Opens in new window)

Related

Filed Under: Interfaces

Latest Articles

File Recover Plus Review

File Recover Plus Review

File Recover Plus ESupport is selling File Recover Plus, for reasons I just can't even imagine. I didn't even want to post a review on this product, but I thought I had better let people know what they were getting if they did happen to purchase File Recovery Plus. My experience with this product … [Read More...]

Transfer VHS to DVD Method

Transfer VHS to DVD Method

Transferring  a VHS tape  to a DVD sounds like something hard to do but in reality it's not.  It's basically taking what is on tape and converting it to a digital file that can be burned onto a DVD or any media source for backup and preservation purposes. The best way to preserve and copy your … [Read More...]

Robotic Tape Storage Applications

There are all kinds of storage device that can be used to back up data, from high density super-floppy disks to DVD drives. But when it come to providing the speed, capacity and reliability needed to protect shared network data, most … [Read More...]


4 Very Useful CAD Hacks You Might Not Be Using

The market for CAD design is growing around 7% a year. It is expected to be worth $7.3 billion by 2022. CAD experts have a lot of job security, … [Read More...]

What Are the Best eSports Games to Watch in 2020?

The esports industry is growing at a remarkable rate.  According to Statistica, the global market for esports is expected to reach $1.79 billion … [Read More...]

Choosing the Safest Crypto Wallet for the New Year

 There are a number of excellent benefits of cryptocurrencies.  They are extremely convenient and offer a tremendous amount of privacy. … [Read More...]

Guidelines on Troubleshooting Python Code

Whenever we write a computer program we need to verify that it works as expected. Usually we execute the code, if necessary we provide some inputs, … [Read More...]

Developing a Python to Extract Data from Your Smartphone

Many times we ask ourselves: "What part of the universe is this phone from?", even if we already know where it is, but we don't know which operator. … [Read More...]

Most Demanding Heroes to Play in Dota 2

Dota 2 is one of the most popular multi-player online combat games. It had an average player count of 485,000 throughout most of 2018. The peak number … [Read More...]

Guides

  • Computer Communications
  • Mobile Computing
  • PC Components
  • PC Data Storage
  • PC Input-Output
  • PC Multimedia
  • Processors (CPUs)

Recent Posts

PC Speed Maximizer Review

PROS: As a stand alone registry cleaner, PC Speed Maximizer is easy to you and gives accurate registry repairs that can be counted on. CONS: PC … [Read More...]

Digital Video Convergence

Internet-based TV seems inevitable because the potential services are compelling, and because the newcomers to broadcasting from the … [Read More...]

Getting the Most out of Windows 10 with Cortana

Have you ever wondered what it would be like to have your computer actually assist you? If you have Windows 10, it is possible with the new service … [Read More...]

Return to top of page

Copyright © 2019 About | Privacy | Contact Information | Wrtie For Us | Disclaimer | Copyright License | Authors