What Does SCSI Mean? Small Computer Systems Interface

Computers are made up of components that are perfected over time. The constant search for better performance contemplates several characteristics, one being the speed of data transfer between devices. In this regard, the SCSI interface appears as one of the most known and traditional technologies to make more efficient the traffic of data coming from a hard disk. In this post, we will present the main features of the SCSI interface and explains the basics of its operation.

What is SCSI?

Acronym for Small Computer Systems InterfaceSCSI is basically a technology created to allow communication between computer devices in a fast and reliable way. Its application is more common in HDDs, although other types of devices have been launched taking advantage of this technology, such as printers, scanners and tape drives (usually used for backup).

This is an old technology. It was officially launched in 1986, but its development began at the end of the previous decade, with researcher Howard Shugart, considered the creator of the floppy disk, as the main name behind the project.

Pronounced as “iscãzi”, this technology proved extremely important in the following years, especially as the processors started getting faster and faster. With SCSI, the HDs and other devices could, in a way, keep up with this increase in speed.

The use of SCSI has always been more frequent in professional servers and applications that actually benefit from faster speeds. As for the home environment and the office as a whole, the IDE (now better known as PATA) interface, which emerged almost at the same time, dominated the market by being less complex and less expensive, despite offering fewer resources.

How does SCSI work?

SCSI technology is based on a device called Host Adapter, also known as a controller. In other words, it is the item responsible for allowing communication between a device and the computer through the SCSI interface. The controller may be present on the motherboard or be installed in it from a free slot, for example.

In addition to speed, SCSI technology also offers the advantage of allowing the connection of multiple devices from a single bus. However, only two devices can communicate at the same time. This limitation exists because a device to do the role of “initiator” ( initiator ) communication, while the other assumes the role of “recipient” ( target ).

Thus, it is possible, for example, to have five hard disks attached to the computer by means of a SCSI controller, but communication occurs only one at a time. For this communication to be possible, each device receives a unique identification ( SCSI ID ).

The maximum number of devices connected depends on the SCSI version, which will be dealt with later. In what is known as SCSI-1, you can have up to eight connected devices, one of which is the Host Adapter. Successor versions of SCSI support up to 16 connected devices.

The identification must be made by following the numbers 0 through 7 on the SCSI-1 or 0 to 15 in other versions. This configuration can be done manually from switching or jumpers (small parts with metal interior). Typically, the Host Adapter should receive the numbering 7. Obviously, if two or more devices receive the same SCSI ID, there will be communication conflicts.

For comparison purposes only, IDE technology allows the connection of only two devices on each bus, one identified as master and the other as slave. This configuration is also made by jumpers. In this case, the communication is carried out using flat cables that have three connectors: one is connected to the motherboard and the other to the HDDs (or other devices that use the interface, such as CD / DVD players). use a similar SCSI scheme – you can find SCSI cables that support up to 15 devices – it all depends on the application.

There is, however, a peculiarity in SCSI connections: these need a “termination” system, which is usually activated in the last device connected to the cable. This mechanism usually consists of a set of resistors that has the function of preventing the signals from the transmission from returning through the bus, as if it were a “back and forth” effect.

The signals are basically transmitted in three ways:

– Single-Ended (SE): In this mode, the signal is output by the controller to all devices connected via a single path. As the signal degrades along the path, it is recommended that the entire connection is no more than 6 meters. Because it is simple to implement, this signaling medium is widely used;

– High-Voltage Differential (HVD): In this mode, the signal is transmitted via two channels, a characteristic that makes it more resistant to interference problems, since it is possible to identify variations from the calculation of voltage differences of both. Here, the connected devices can receive a signal and retransmit it until reaching the destination. Thus, this type of signaling can be faster and can be used in longer cables, with up to 25 meters;

– Low-Voltage Differential (LVD): This mode is similar to HVD, but uses lower voltages. LVD connections should have cables up to 12 meters.

It is worth mentioning that SCSI technology can work with asynchronous and synchronous transmission modes. The first allows the initiator to send a command and wait for a response in all operations. The second works in a similar way, but is able to send multiple commands before receiving the response from the previous request. This feature may influence the speed of data transmission.

SCSI Versions

As previously reported, SCSI technology has gone through revisions over time that have resulted in new versions. The main features of each specification are listed below.

SCSI-1

The SCSI-1, ie the first version of SCSI, formally appeared in 1986. Their maximum rate of data transfer is 5 MB / s (megabyte per second), whereas a frequency (clock) of 5 MHz with 8 bits transferred at a time. Here, it is possible to use up to eight devices in one connection.

SCSI-2 (Fast SCSI)

The SCSI-2 (or Fast SCSI ) is a review launched in 1990 to circumvent some of the limitations of the SCSI-1. This specification includes features that, in the first release, were not necessarily mandatory, leading to compatibility issues. Among them are a set of approximately twenty instructions called the Common Command Set (CSS).

The second version of the SCSI is also characterized by working frequently up to 10 MHz and 8 bits, resulting in a maximum transfer rate of 10 MB / s. Here, it is only possible to work with up to eight devices on the same bus.

There is a variation implemented in 1994 called Wide Fast SCSI that also works with 10 MHz clock, but transferring 16 bits at a time, resulting in a speed of up to 20 MB / s, and support for up to sixteen devices.

SCSI-3 (Ultra SCSI)

The SCSI-3 has been recognized officially in 1995, but has the characteristic of being formed by various specifications. The first one, called SCSI-3 or Ultra SCSI only, works with 8 bits and 20 MHz frequency, and can also transmit 20 MB / s. Here, up to eight devices can be connected.

The next generation Ultra Wide SCSI, which also has a frequency of 20 MHz, but transfers 16 bits at a time, making this version have a speed of 40 MB / s and supports up to sixteen devices.

There is also the Ultra2 SCSI, which also has a maximum data transfer rate of 40 MB / s, but works with 40 MHz and 8 bit frequency. Many know this version as SCSI-4. The supported devices quantity is eight.

Next, the Wide Ultra2 SCSI appears, which works with 16 bits and 40 MHz frequency, resulting in the maximum speed of 80 MB / s and support for sixteen devices.

Ultra160 SCSI, Ultra320 SCSI and Ultra640

The Ultra160 SCSIUltra320 SCSI and Ultra640 SCSI versions came later. The numbers in the names refer to the maximum rate of data transmission. Nevertheless, these versions are also part of the SCSI-3 series of revisions.

The Ultra160 SCSI also works with 40 MHz and 16 bit frequency but performs two transfer operations per clock cycle instead of one, making the specification 160 MB / s.

The same happens with the Ultra320 SCSI, with the difference that this version has clock of 80 MHz, resulting in a maximum rate of 320 MB / s. Finally, the Ultra640 SCSI appears, which differs by having a clock of 160 MHz, allowing transfers of up to 640 MB / s.

Because these versions work with 16 bits, all allow up to sixteen devices on the same connection.

Summary of SCSI Versions

The following table summarizes the key features of SCSI versions:

Version Clock Bits Devices velocity
SCSI-1 5 MHz 8 8 5 MB / s
SCSI-2 (Fast SCSI) 10 MHz 8 8 10 MB / s
Wide Fast SCSI 10 MHz 16 16 20 MB / s
SCSI-3 (Ultra SCSI) 20 MHz 8 8 20 MB / s
Wide Ultra SCSI 20 MHz 16 16 40 MB / s
Ultra2 SCSI 40 MHz 8 8 40 MB / s
Wide Ultra2 SCSI 40 MHz 16 16 80 MB / s
Ultra160 SCSI 40 MHz 16 (2x) 16 160 MB / s
Ultra320 SCSI 80 MHz 16 (2x) 16 320 MB / s
Ultra640 SCSI 160 MHz 16 (2x) 16 640 MB / s

SAS (Serial Attached SCSI)

It is worth stressing that SCSI still has other variations. One of them is the Serial Attached SCSI (SAS), which can reach speeds up to 6 Gb / s (gigabits per second) and supports the connection of up to 128 devices. This is possible, among other reasons, because this variation uses a serial data transmission scheme (in the versions shown above, the transmission is done in parallel) combined with higher frequencies.

SAS stands out as being, to a certain extent, a “rival” of the SATA standard. In fact, both have similar features, and in some cases it is even possible to use SATA HDDs on SAS interfaces, since it is common to use the same connector in both technologies.

The use of SAS is almost exclusive to more sophisticated servers and computers. Type HDDs may not take advantage of capacity over SATA, on the other hand, it is common to find performance-focused SAS drives that can work at 10,000 or 15,000 RPM, for example.

iSCSI

As you may know, the SCSI standard can be used in conjunction with other technologies. The iSCSI ( Internet SCSI ) comes in this context: it is a specification that allows SCSI commands activation from networks IP.

With iSCSI is possible, for example, make certain server to access a data storage system ( storage ) on the same network optimally and reliably. Thus, it is not necessary to interconnect the two machines directly, just take advantage of an existing network.

Since they have the advantages of simplifying structures and saving resources, iSCSI-based solutions are widely used today.

SCSI cables and connectors

Since SATA technology has several specifications and can cater for different types of devices, there are, as a consequence, several types of connectors. Here are some of them.

– Centronics-50: one of the most popular connectors, has 50 lanes divided into two rows. Also for 8-bit connections;

– HD50: has 50 pins divided into two rows. Started to be used from SCSI-2 and works with 8-bit connections;

– IDC50: has 50 pins divided into two rows. It is quite common in hard drives, CD drives and other devices that are normally installed inside the computer;

– HD68: 68-way connector divided into two rows. It is quite found and can work with 16-bit connections. Its use is common with SCSI-3 specifications.

Finishing

SCSI technology has lost market share after the SATA standard on hard drives, as well as USB, FireWire and Thunderbolt technologies over external hard drives, scanners, printers, and others. Not for less: they are less complex technologies, relatively cheaper and meet expectations regarding their features.

However, it is a mistake to believe that SCSI is “dead”, after all, it is still possible to find utility for this technology in a number of applications. Said SAS standard is an example. In addition, the SCSI Trade Association, an association created in 1996 to promote technology, is still in full swing. At the time of writing, the organization was working on the development of SCSI Express specifications, a standard that seeks to take advantage of the combination of SCSI and PCI Express technologies.