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.

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