SCSI Definition

March 22, 2022

Acronyms, Technology

According to abbreviationfinder, SCSI stands for Small Computer System Interface (interface Small Computer System) is a standard interface model parallel high speed created by the X3T9.2 committee of ANSI in 1986. Some professional castellanizan it as scuzzy or escosi, by the English pronunciation of its acronym, others on the contrary prefer to spell it.

Introduction

A SCSI interface is used to connect microcomputers to peripheral devices, such as hard drives and printers, and for connection to other computers and local area networks. Mounting a SCSI device on a computer requires that both the device and the motherboard have a SCSI controller. It is common for the device to come with a driver of this type, but this is not always the case, especially in the first devices. With a single SCSI connection (one port) and using sequential connections called daisy links, up to seven devices can be connected not including the computer, and up to 15 if working with the Ultra SCSI-3 standard in Wide mode; a computer using SCSI interface on both ports could be connected to 30 peripherals. Regarding the transfer speed of the SCSI bus, it depends on the standard and the way in which it is used; for SCSI-1 the speed was 5 Mbytes per second, while for Ultra SCSI-3 it reaches 80 Mbytes per second in Fast mode and up to 160 Mbytes per second in Wide mode. When a system has more than one SCSI card, each one is identified by a number called LUN (Logical Unit Number); the LUN card0 acts as a master card and the others act as slaves. Furthermore, an identifier ID is given to each of the devices connected to a card; the ID of each peripheral is set by means of microswitches or by jumpers, although it can also be configured by software. Each device has an address made up of the LUN and the ID that constitutes its priority number. Only one device can transmit through the port at a time; the device with the highest address has priority. Computers Apple Macintosh Plus, Macintosh SE, Macintosh II, IBM RS / 6000 and IBM PS / 2 Model 65 and others feature a serial SCSI port. It can also be attached to an IBM PC and compatible equipment as an expansion card. In the past, it was very popular with all kinds of computers. Today it continues to be popular in high-performance workplaces, servers, and high-end peripherals. Desktops and laptops typically use the slower IDE / SATA interfaces for hard drives and USB (USB uses a SCSI command set for some operations) as well as FireWire because of the cost difference between these devices.

Asymmetric and differential SCSI

There are two types of SCSI bus:

  • the asymmetric bus, known as SE (for Single-Ended or Single Termination), based on a parallel architecture in which each channel circulates on a wire, sensitive to interference. SCSI cables in SE mode have 8 wires for 8-bit transmission (called limited) or 16 wires for 16-bit cables (known as extended). This is the most common type of SCSI bus.
  • the differential bus carries signals over a pair of wires. The information is encoded by difference between the two wires (each transmits the opposite voltage) to displace the electromagnetic interruptions, allowing a considerable wiring distance (around 25 meters) to be obtained. In general, there are two modes: LVD (Low Differential Voltage) mode, based on 3.3 V signals, and HVD (High Differential Voltage) mode, which uses 5 V signals. Peripherals using this type of transmission are increasingly rare and usually carry the word “DIFF”.
  • 1. SCSI Bus 8 bits. Data transmission speed at 5 MBps. Its generic connector is 50 pins (Centronics connector) and low density. The maximum cable length is six meters. It allows up to 7 devices (including the controller), identified by addresses 0 to 6.
  • SCSI 2.!
    • Fast. With an 8 bus, it doubles the transmission speed (from 5 MBps to 10 MBps). Its generic connector is 50 pins and high density. The maximum length of the cable is three meters. It allows up to 7 devices (including the controller), identified by addresses 0 to 6.
    • Wide. Double the bus (go from 8 to 16 bits). Its generic connector is 68 pins and high density. The maximum length of the cable is three meters. It allows up to 16 devices (including the controller), identified by addresses 0 to 15.
  • SCSI 3.
    • 1 SPI (Parallel Interface or Ultra SCSI).
      • Ultra. 16-bit devices with execution speed of 20 MBps. Its generic connector is 34-pin high-density. The maximum cable length is 10 cm. Supports a maximum of 15 devices. It is also known as Fast 20 or SCSI-3.
      • Ultra Wide. 16-bit devices with 40 MBps execution speed. Its generic connector is 68 pins and high density. The maximum cable length is 1.5 meters. Supports a maximum of 15 devices. It is also known as Fast SCSI-3.
      • Ultra 2. 16-bit devices with execution speed of 80 MBps. Its generic connector is 68 pins and high density. The maximum cable length is twelve meters. Supports a maximum of 15 devices. It is also known as Fast 40.
    • 2 FireWire (IEEE 1394).
    • 3 SSA (Serial Storage Architecture). From IBM. Use full-duplex with separate channels.
    • 4 FC-AL (Fiber Channel Arbitrated Loop). Use fiber optic cables (up to 10 km) or coaxial (up to 24 m). With a maximum speed of 100 MBps.

SCSI

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