SCSI Drives

Objectives :

  • List the advantages of SCSI
  • List the different SCSI interfaces
  • Identify the SCSI variations
  • List the different types of SCSI signals
  • Use the SCSI ID
  • Work with SCSI connectors
  • Use the different SCSI cables
  • Work with SCSI drives
  • Install SCSI drives
  • Troubleshoot the SCSI drive

Advantages of SCSI :

Data transmission speeds reach about 320 MB per second between the system and the device

Allows to attach 15 devices to a single SCSI port

SCSI controller boosts the signals and enables the devices to be connected at a distance of about 25 meters from the system

Reduces the processing load on the microprocessor, as they have a micro-controller built into the device

Communication problems between the system and SCSI device are reduced as the ends of the SCSI have terminators

Types of SCSI Interface :

SCSI – 1 - Uses an 8-bit parallel bus to transfer data at a maximum speed of 5 MB per second. Can connect up to 8 devices with a cable length of 6 meters and uses passive terminators

SCSI – 2 - Uses a 16-bit bus to transfer data at a maximum speed of 20 MB per second. Can connect up to 16 devices and cable length of 3 meters. Uses 50 pin connector, command queuing feature and active terminator

SCSI – 3 - Uses a 16-bit bus to transfer data at a maximum speed of 40 MB per second. Can connect up to 32 devices and cable length is 3 meter. Uses active terminators

SCSI Variations :

Narrow SCSI – Transmits data over an 8-bit bus. Uses 50 pin connector cable known as A cable

Wide SCSI – Transmits data over a 16-bit bus. Uses 68 pin connector cable known as P cable

Fast SCSI – Transfers the data at the speed of 10 MB per second using the narrow SCSI

Wide Ultra – Transmits data at a speed of 20 MB per second for the narrow SCSI

SCSI Drive speeds – Available at speeds of 10,000 rpm to 15,000 rpm

Universal hard drive – Easy to upgrade the server hard drives and supports S.M.A.R.T feature

Serial Attached SCSI (SAS) – Allows much higher speed data transfer than previously available and is backward comaptible with SATA. Uses SCSI commands for interacting with SAS devices

Types of SCSI Bus Signaling :

Single-Ended (SE) – Transmits a positive voltage through a signal line to the receiver. Maximum cable length is 6 meter

Low-Voltage Differential (LVD) – Signals are transmitted using the difference in the voltage between the two wires. SCSI device adapter has a small transceiver built into it which boosts signal. Easily integrates up to 16 devices on SCSI bus using 12 meter of cable. Uses 3.3 voltage to represent logical 1 or 0

High-Voltage Differential (HVD) – Reduces problems of bouncing signals, signal interference problems, noise and crosstalk. Enables cable length up to 25 meters. It is very costly

Low-Voltage Differential/ Single - Ended Multi-mode (LVD/SE) – Compatible with LVD and SE signaling. However if there is a device that uses SE signaling on SCSI bus, then this will be set to use SE signaling

Working with SCSI Identification (ID) :

Jumper Block/ DIP Switch Settings – SCSI identification is set using software available with SCSI device, using the jumpers or switches on the SCSI device

Resolving Id Conflicts – SCSI identification specifies the priority in which the device has access to the bus. Host SCSI controller on the system is generally given the highest SCSI priority 7 to ensure that it gets first access to system resources. SCSI hard drives are factory set to have ID 0

Working with SCSI Connectors :

Internal Connectors – Located on SCSI devices such as hard disks and CD drives that are installed inside the system case

External Connectors – Located on the SCSI adapters and the SCSI devices such as printers, scanners and external tape drives

SCSI Cables :

Type :


Internal SCSI Cable – Flat ribbon cable

External SCSI Cables – Thick round cable

Termination Requirements :

  • Passive Terminator – Use a resistor to terminate the SCSI signals and if the distance between the last device on the SCSI chain and the SCSI controller is less than 1 meter
  • Active Terminator – Use a resistor and a voltage regulator and if the distance between the last device on the SCSI chain and the SCSI controller is more than 1 meter
  • Some devices also have terminator built into them or have jumpers to activate terminator

SCSI Drives :

Redundant Array of Inexpensive Disks (RAID) - RAID is a collection of hard disks that appear to the system as a single disk

  • RAID Level 0
  • RAID Level 1 – total disk space available is half of total capacity of disks
  • RAID Level 2
  • RAID Level 3 – requires minimum of 3 disks
  • RAID Level 4
  • RAID Level 5 - requires minimum of 3 disks
  • RAID 10
  • RAID 53 – implements features of RAID level 0 and RAID level 3
  • Linear RAID

Installing SCSI Drives :

Installing the Host Adapter – Given the highest priority over SCSI bus so that it can serve other devices properly

Installing the Internal SCSI Device – Connects to the host adapter bus using the SCSI cable

Installing the External SCSI Devices – Connects external devices such as SCSI scanners and printers to the system

Troubleshooting SCSI Drive :

Problem with the SCSI Daisy-Chain –

In a daisy chain any problem with any one device may cause all the devices to malfunction

Check that the power supply is switched on for all the devices

Check that the cables in the daisy-chain are properly connected and not loose

Check that both ends of the SCSI daisy-chain are terminated

Check that the SCSI ID is not repeated for two or more devices

Disconnect all the devices and reconnect them one by one

Reinstall the device drivers

The Connected Device is not Functioning Properly -

Check if the devices are connected properly

Ensure that the terminators are secured properly at the ends of the SCSI daisy-chain

Change the SCSI ID of the device

Reinstall the device driver

Summary :

Small Computer Systems Interface (SCSI) enables you to attach peripherals to the system

SCSI consists of controller, cables and the device

Advantages of SCSI :

  • Faster transfer of data
  • Attach multiple devices
  • Devices can be placed at a distance from the system
  • Reduced load on microprocessor
  • Reduced reflection of data signals

SCSI standards specify the features of SCSI devices

SCSI bus signaling specifies the method used by the SCSI devices to communicate with each other

Different SCSI signaling methods :

  • Single-Ended
  • Low-Voltage Differential
  • High Voltage Differential
  • Low-Voltage Differential / Single-Ended Multi-mode

The SCSI ID specifies the priority of the SCSI device on the SCSI bus

SCSI connectors enable you to affix the SCSI cable to the SCSI adapter or the SCSI device

The SCSI terminator is used to terminate the SCSI daisy-chain

Active terminators have a resistor and also a voltage regulator

RAID is a collection of hard disks that appear to the system as a single disk

Hot swap drives enable you to connect and disconnect drives to the system without shutting down or restarting the system

Installing SCSI host adapter, internal and external SCSI drives

#SCSI_Drives #Advantages_of_SCSI #Types_of_SCSI_Interface #SCSI_Variations #Types_of_SCSI_Bus_Signaling #SCSI_Identification #SCSI_Connectors #Internal_Connectors #External_Connectors #SCSI_Cables #Termination_Requirements #SCSI_Drives #Installing_SCSI_Drives #Troubleshooting_SCSI_Drive #Problem_with_the_SCSI_Daisy-Chain #The_Connected_Device_is_not_Functioning_Properly #Different_SCSI_signaling_methods #

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