Small Computer System Interface (SCSI)

What is SCSI (Small Computer System Interface)?

SCSI is the short form for Small Computer System Interface and refers to a parallel interface standard for hard disk drives. This is a standard that defines different command sets for different types of peripheral devices.

Technically, it denotes a set of standards planned for joining different peripheral devices physically and transferring data to and from them to the computer.

Understanding SCSI (Small Computer System Interface)

Understanding SCSI (Small Computer System Interface)

SCSI is the most commonly used interface for connecting different peripheral devices to the Central Processing Unit (CPU) of the computer through buses.

It is revolutionary in terms of efficiency and compatibility, which led to its widespread use.

The host adapter and all SCSI devices support a single daisy chain that allows connecting a group of nodes one after another by means of the hardware configuration.

The SCSI standard defines all of the following:


SCSI is available in different types of interfaces and uses a variety of cabling, some of which do not depend on SCSI standards completely, but still use the SCSI command protocol. These are:

Computers usually have these interfaces included in their design and are either integrated into the motherboard or through the plug-in adapters.

SCSI supports different operating systems such as:

What is SCSI

Cabling and Termination

There are also different types of SCSI connectors or cables that come with different features and capabilities that help in the working of the SCSI devices.

The cables typically consist of three layers such as:

Some of the most commonly used SCSI connectors used with different devices on computers are as follows:

For proper functioning of the SCSI, the cables must be terminated properly.

This will also help in recognizing the devices that are connected to the computer system.

Active and high-quality terminators should be used, preferably diagnostic terminators, so that issues in the signal and its quality can also be identified.

Also, make sure that:

Leaving the bus open is not recommended because it will reflect the electrical signals sent down and hinder communication between the SCSI controller and the devices.

Therefore, each end of the resistor circuit is to be closed. If both external and internal devices are supported by the bus, you will need to terminate the last device on every series.

Ideally, SCSI termination is grouped into two major sections as explained hereunder:

Devices Supported

Typically, SCSI-2 and above versions can support multiple devices via a single port connected to the system bus.

In fact, different SCSI interfaces support different types and a large number of devices depending on the version. For example:

The Serial Attached SCSI can support up to 16,256 addressable devices in each port, and allows transferring data at a speed of up to 3 Gbps.

Command Protocol

SCSI follows an extensive set of command protocols for data transmission between the target and the initiator.

The commands are sent to the Command Descriptor Block (CDB), which comprises a byte operation code that is trailed by five or more bytes for the command-specific parameters.

There are typically four different categories of SCSI commands such as:

There are nearly sixty different varieties of SCSI commands used, of which the most commonly used ones are as follows:

How Does SCSI Work?

SCSI usually uses an adapter for the cables to connect devices in a daisy chain and interface with and transfer data through the system bus. SCSI terminators are also used for the bus ends.

The different components of the SCSI help it work better. Some of the common components used in SCSI storage systems are:

An initiator – This is usually software-based and is integrated into the system board of the server or can be in the host adapter as well, and it issues requests and receives responses for services from the SCSI device.

A target – This typically refers to the physical storage device, though it can also be software-based. A target can be the whole storage array, just a hard disk, or even non-storage hardware that can perform like an SCSI target.

A service delivery subsystem – This specific mechanism allows communication through the cables between the target and the initiator.

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An expander – This is usually used only with SAS and allows several such devices to share one single initiator port.

SCSI uses three different types of bus signaling to send the electrical impulses through the wires and to affect termination as well. These are:

Single-ended (SE) – In this type, signals are generated and pushed by the controller out to all devices on the bus through a single data line, within a distance of about 10 feet and is commonly used in PCs.

High-voltage differential (HVD) – Usually used for servers, signals are sent in tandem in this approach through a data high line and low line.

The device on the bus with a signal transceiver is communicated to by the controller to receive and retransmit the signal until it reaches the target device, which may be up to 80 feet away.

Low-voltage differential (LVD) – A variation of HVD, smaller transceivers are used in this approach that are built inside the SCSI adapter of the devices. It is more affordable and uses less electricity during communication with a device up to 40 feet away.

How Many Types of SCSI are There?

Usually there are quite a few different types of SCSI standards, namely SCSI-1, SCSI-2, SCSI-3, and SCSI-5, with each having its own characteristics.

There are also several other types of SCSI buses, such as parallel SCSI, Serial Attached SCSI (SAS), parallel SCSI, and Fiber Channel.

In addition to the above, there are also several other physical versions of SCSI available such as:

‌These specific types of SCSI, however, break away from the conventional parallel SCSI bus and use serial communication to transfer data via the point-to-point links.

As for the SCSI devices, there are two major types such as:




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Questions & Answers:

How Many Devices Can SCSI Support?

The SCSI architecture can support at least 8 separate devices. This includes the controller, which is itself a device.

Therefore, you can say that SCSI can support a minimum of 7 other separate logical devices connected to it.

As for the maximum number of devices supported, theoretically, a single SCSI controller may support up to 64 addressable devices.

How Many Partitions Can Be Created Using SCSI?

You can have up to four separate primary partitions using SCSI, or three primary and one extended partition.

The extended partition, however, can support up to 15 SCSI logical partitions or 63 IDE logical partitions. On the other hand, the latest parallel SCSI standards support only up to 16 devices.

Is SCSI Still Used?

Typically, SCSI is largely replaced by the SAS and USB standards, and is not used as commonly any longer. However, it is not obsolete because it is still used in personal computers, enterprise and business server settings.

What is SCSI Used for?

SCSI is used to transfer data in personal computers and to communicate with different types of peripheral hardware, such as tape drives, disk drives, printers, scanners, CD-ROM drives, servers, RAID, Storage Area Network (SAN) and more.

Since it is more flexible, it is also used in an enterprise setting, especially in server computers for hard drive controllers.


The SCSI drives and interfaces are quite reliable and can transfer data at quite a fast rate.

These are quite robust, but they need proper installation, management, and handling to reap the maximum benefits.

Though the Small Computer System Interface has been largely replaced by SAS, it is still used in a lot of applications.