Computers are made up of components that are refined over time. The constant search for better performance includes several characteristics, one of which is the speed of data transfer between devices. In this regard, the SCSI interface appears as one of the most well-known and traditional technologies to make data traffic from a hard drive more efficient. In this text, we present the main characteristics of the SCSI interface and explains the basics of its operation.

What is SCSI?

Acronym for Small Computer Systems Interface by abbreviationfinder, SCSI is basically a technology created to allow communication between computing devices in a fast and reliable way. Its application is more common in HDs (hard disks), although other types of devices have been launched taking advantage of this technology, such as printers, scanners and tape drives (generally used for backup).

It is an old technology. Its arrival on the market officially took place in 1986, but its development started at the end of the previous decade, with the researcher Howard Shugart, considered the creator of the floppy disk (diskette), as the main name behind the project.

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

The use of SCSI has always been more frequent in servers and professional applications that, in fact, benefit from greater speed. With regard to the home environment and offices in general, the IDE interface (currently better known as PATA), which appeared almost at the same time, dominated the market because it is less complex and cheaper, 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 can be present on the motherboard or be installed on it from a free slot, for example.

In addition to speed, SCSI technology also offers the advantage of allowing multiple devices to be connected 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 drives connected to the computer by means of a SCSI controller, but communication occurs only with one at a time. For this communication to be possible, each device receives a unique identification (SCSI ID).

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

The identification must be done following the numbers from 0 to 7 in SCSI-1 or from 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 be numbered 7. Obviously, if two or more devices receive the same SCSI ID, there will be communication conflicts.

For comparative 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 done by jumpers. In this case, the communication is carried out by means of flat cables that have three connectors: one is connected to the motherboard and the others to the HDs (or to 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 fifteen devices – it all depends on the application.

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

The signals are transmitted, basically, in three ways:

– Single-Ended (SE): in this mode, the signal is sent by the controller to all devices connected through a single route. As the signal degrades along the route, it is recommended that the entire connection be no longer than 6 meters. Since it is simple to implement, this means of signaling is widely used;

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

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

It is also worth noting 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 several commands before receiving the response from the previous request. This characteristic can influence the data transmission speed.

SCSI versions

As previously reported, SCSI technology has undergone revisions over time that have resulted in new versions. The main characteristics of each released specification are as follows.

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 a 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 version, were not necessarily mandatory, causing compatibility problems. Among them are a set of approximately twenty instructions called the Common Command Set (CSS).

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

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

SCSI-3 (Ultra SCSI)

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

Following was the Wide Ultra 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 support 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 a frequency of 40 MHz and 8 bits. Many know this version as SCSI-4. The number of devices supported is eight.

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

Ultra160 SCSI, Ultra320 SCSI and Ultra640

The Ultra160 SCSI, Ultra320 SCSI and Ultra640 SCSI versions came later. The numbers in the names refer to the maximum data transmission rate. Despite this, these versions are also part of the SCSI-3 revision series.

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

The same happens with the Ultra320 SCSI, with the difference that this version is clocked at 80 MHz, resulting in a maximum rate of 320 MB / s. Finally, there is the Ultra640 SCSI, which stands out for having a clock speed of 160 MHz, allowing transfers of up to 640 MB / s.

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

Summary of SCSI versions

The following table summarizes the main characteristics of the 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

Serial Attached SCSI (SAS)

It is worth noting that SCSI still has other variations. One of them is the Serial Attached SCSI (SAS), which can reach speeds of 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 considered, to some extent, a “rival” of the SATA standard. In fact, both have similar features, in some cases it is even possible to use SATA hard drives on SAS interfaces, since it is common to use the same connector in both technologies.

The use of SAS is almost exclusive on more sophisticated servers and computers. HDDs of the type may not have an advantage in terms of capacity over SATA, on the other hand, it is common to find SAS units focused on performance that can work with 10,000 or 15,000 RPM (revolutions per minute), for example.

iSCSI

As you already 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 it is possible, for example, to make a given server access an existing data storage system (storage) on the same network in an optimized and reliable way. Thus, it is not necessary to interconnect the two machines directly, just take advantage of an existing network.

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

SCSI cables and connectors

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

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

– HD50: has 50 pins divided into two rows. It 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 on hard drives, CD drives and other devices that are usually installed inside the computer;

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

Conclusion

SCSI technology lost space in the market after the arrival of the SATA standard in relation to hard drives, in addition to technologies such as USB, FireWire and Thunderbolt in relation to external hard drives, scanners, printers and others. Not for less: they are less complex technologies, relatively cheaper and that meet expectations regarding their functionality.

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. The mentioned SAS standard is an example. In addition, the SCSI Trade Association, an association created in 1996 to promote technology, remains in full swing. At the time of closing this text, the entity was also working on the development of SCSI Express specifications, a standard that seeks to take advantage of the combination of SCSI and PCI Express technologies.