SCSI (Small Computer System Interface) Explained

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.

KEY TAKEAWAYS

• SCSI signifies the standard for parallel interfaces. This interface can transfer data at the rate of 8 bits per second and more, which is quite high in comparison to most regular parallel interfaces.
• The interface can support a minimum of up to 8 devices and a maximum of 16 devices with Wide SCSI, unlike its competing standards.
• The SCSI bus is used in several storage applications and devices to connect to the computer.
• There are different versions of SCSI standards and types of buses available such as SCSI-1, SCSI-2, and SCSI-3, as well as parallel SCSI, SAS, and Fiber Channel, respectively.
• The diverse, robust, and high-end components of the SCSI devices, along with the different types of equally good and efficient connectors, help a lot in the proper working of the various devices linked to the computer system.

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:

• Protocols
• Commands
• Optical interfaces
• Electrical signals
• Logical interfaces

Interfaces

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:

• Parallel SCSI, also known as SCSI Parallel Interface or SPI
• Serial Attached SCSI (SAS) that uses an improved SATA power cable
• Internet Small Computer System Interface (iSCSI) that uses TCP/IP as its transport mechanism and Gigabit Ethernet or speedier network links
• Fiber Channel Protocol for SCSI (FCP) that are usually hot-pluggable and used with optical fiber
• SCSI RDMA Protocol (SRP) that uses Remote Direct Memory Access (RDMA) connection to transport SCSI commands
• USB attached SCSI that uses the standard Universal Serial Bus
• Automation Drive Interface Transport Protocol (ADT)

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:

• Windows
• Mac OS
• Unix
• Linux

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:

• The inner layer – This is the protected layer through which the actual data is transmitted.
• The media layer – This layer contains wires to send the commands to control the devices.
• The outer layer – This layer contains the wires to carry parity information to determine whether or not the data is correct.

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

• DB-25 male external SCSI connectors
• Low-density 50-pin male external SCSI connectors
• High-density 50-pin male external SCSI connectors
• Low-density 50-pin male internal connectors
• High-density 68-pin male external SCSI connectors
• High-density 68-pin male internal SCSI connectors

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:

• You use a specific and shorter cable for noise reduction due to Electromagnetic Interference (EMI) and enhanced signals.
• The devices are configured either for differential transmission or single-ended transmission and not both on a single bus.

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:

• Passive termination – It is used for systems running at standard clock speed and supporting a distance of 3 feet (1 m) or less between the controller and the device.
• Active termination – It is used for Fast SCSI systems that support a distance of more than 3 feet.

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 SCSI-2 supports up to 16 devices.
• The Ultra SCSI supports 5 to 8 devices.
• The Ultra-320 SCSI supports up to 16 devices.
• The Fast SCSI supports up to 8 devices and a data transfer speed of 10 Mbps.
• The Fast Wide SCSI supports up to 16 devices and a data transfer speed of 20 Mbps.
• The Ultra Wide SCSI supports up to 16 devices and a data transfer speed of 40 Mbps.
• The Ultra2 Wide SCSI supports up to 16 devices and a data transfer speed of 80 Mbps.
• The Ultra3 SCSI supports up to 16 devices and a data transfer speed of 160 Mbps.
• The Ultra-320 SCSI supports up to 16 devices and a data transfer speed of 320 Mbps.
• The Ultra-640 SCSI supports up to 16 devices and a data transfer speed of 640 Mbps.

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:

• N (non-data)
• W (writing data from initiator to target)
• R (reading data)
• B (bidirectional)

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

• Test unit ready
• Inquiry
• Request sense
• Send diagnostic and receive diagnostic results
• Start/Stop unit
• Read capacity
• Format unit
• Four variants of Read
• Four variants of Write
• Log sense
• Mode sense
• Mode select

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.

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.

• SCSI-1 – This is the original SCSI standard. It was developed way back in 1986 as ANSI X3.131-1986, with a data transfer speed of up to eight bits per second.
• SCSI-2 – This standard was approved in 1990. The standard included new features such as support for additional devices and Fast and Wide SCSI.
• SCSI-3 – This third type of SCSI standard was approved in 1996 as ANSI X3.270-1996.
• SCSI-5 – Also called Very High Density Connector Interface (VHDCI), this standard is the same as SCSI-3 but is designed for higher performance and for the next generation. It supports a smaller 0.8-millimeter connector for multiport applications, including Ultra SCSI Fast-20 and Low Voltage Differential Signal (LVDS) technology.

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

• The ‌Serial Attached SCSI (SAS)
• The SCSI over Fiber Channel Protocol (FCP)
• The USB Attached SCSI (UAS)

‌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:

• Single-ended devices – They come with a single data lead and ground lead for single-ended signal transmission through the bus. These devices are vulnerable to noise and need specific cable types and lengths.
• Differential devices – They use two data leads but none of these are ground leads. These are expensive devices and can function over distances beyond SCSI specs and are resistant to noise.

Advantages

• It comes with its own controller.
• It allows simple troubleshooting.
• It is less dependent on other devices to perform better.
• Data is transferred through a single cable.
• It allows streamlining the connection.
• It can support a wide variety of computers.
• It is highly accessible.
• It is easily replaceable.
• It allows upgrading older SCSI components.
• It supports faster read and write operations.
• It supports the system to boot up.
• It launches programs quickly.
• It is physically robust.
• It uses less power.
• It generates less heat.

Disadvantages

• Costlier drives
• Difficult to configure interface
• Need proper installation and management for best results
• Its performance reduces with time
• Lower capacity
• Lower storage cost per GB
• Lower number of data writes cycles
• Improper handling reduces lifespan

SCSI Vs SATA

• If capacity is your primary concern, the SCSI drives seem to lag behind the SATA hard drives in comparison.
• The SCSI drives are more affordable in comparison to the SATA drives, but fall behind in terms of specs to cost ratio.
• The Small Computer System Interfaces use parallel signals. On the other hand, Serial Advanced Technology Attachment uses, just as its name indicates, serial signals for transferring instructions, data, and information.
• The SCSI drives are mainly used in medium- to high-end server computers as well as in high-end workstations. On the other hand, SATA drives are more commonly used in low-end servers to compose Redundant Array of Independent Disks (RAID) and in personal computers.
• The SAS or Serial Attached SCSI drives have almost replaced the regular SCSI drives. On the other hand, SATA drives are still quite popular and used extensively.
• Usually, in the larger hard drives with storage capacities as large as 8 TB, 10 TB, 14 TB, and more, it is more likely that a SATA 3.0 interface will be used instead of SCSI.
• The SCSI can support a larger number of peripheral devices, up to eight or sixteen, including hard disk drives, tape drives, optical disc drives, network cards, scanners, printers, and lots more. The SATA interfaces support connecting only one device to one connector though they support optical disk drives.
• The SCSI drives are more reliable in comparison to the SATA drives because they are built better with high-quality parts. SATA drives are typically consumer-grade drives that are designed to be cheap and are not meant to handle server-level workloads.
• SCSI is an older technology that was launched around the 1980s and is more or less replaced by SAS and USB, limiting their use in personal computers due to their ATA HDD standards and low cost. SATA, on the other hand, is more modern, having been launched in 2003, and is more popular.
• The SCSI connectors are flat ribbon cables with 50 pins and the interfaces can be mounted physically on the motherboard or via plug-in adapters. The SATA interfaces, on the other hand, allow hot plugging and come with an open-source interface with 9 pins.
• The speed of data transfer on the current SCSI drives is up to 80 megabytes/second and is also backward compatible. On the other hand, when launched, SATA supported a data transfer rate of 1.5 Gbits/second, which was later enhanced up to 6 Gbits/second.

Questions & Answers:

Conclusion

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.