SCSI Technology (Small Computer Systems Interface)


The computers are formed by components that areperfected with the passing of time. The constant search for better performance includes several features, being one of them is the speed of data transfer between the devices. In this regard, the SCSI interface appears as one of the most well-known and traditional to make more efficient the traffic data from a disk hard. In this text, the AbbreviationFinder presents the main characteristics of the interface SCSI , and explains the basic of its functioning.

What is SCSI?

Acronym for Small Computer Systems Interface, SCSI it is, basically, a technology created to allow communication between computing devices quickly and reliably. Your application is more common in the HDs (disks hard), although other types of devices have been released taking advantage of this technology, such as printers, scanners and tape drives (usually used for backup).

This is an old technology. Its arrival on the market happened officially in 1986, but its development was initiated at the end of the a decade earlier, having the researcher Howard Shugart, considered the creator of the floppy disk (diskette), as the main name for the back of the project.

Pronounced as “iscãzi”, this technology proved to be extremely important in the following years, especially because the processors were getting faster and faster. With the SCSI, Drives, and other devices were able to, somehow, keep track of this speed increase.

The use of SCSI has always been more frequent in servers and business applications, in fact, if benefit from greater speed. In relation to the environment home and to the offices of the general mode, the IDE interface (now more known as PATA), which came almost in the same season, dominated the market by being less complex and cheaper, although offer fewer resources.

How SCSI works?

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

In addition to the speed, the 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 need to do the role “” initiator (initiator) of communication, while the other assumes the role of “recipient” (the target).

Thus, it is possible, for example, have five hard disks connected to the computer through a SCSI controller, but the communication occurs only with one at a time. For this communication is possible, each device receives a unique id (SCSI ID).

The maximum amount of devices connected depends on the version of the SCSI, a subject that will be dealt with further front. In what is known as SCSI-1, you can have up to eight connected devices, one of them being the Host Adapter. Versions successor of SCSI support up to 16 devices connected.

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

Just for comparative purposes, the technology, the IDE allows the connection of only two devices on each bus, being a identified as master and the other as slave. This configuration is done by jumpers.In this case, the communication is performed by means of the flat cables that have three connectors: one is connected to the motherboard and the other to the HDs (or the other devices using the interface, such as CD/DVD players).You can use a schema similar in the SCSI – it is possible to find SCSI cables that support up to fifteen devices, it all depends on the application.

There is, however, a peculiarity in the SCSI connections: these need a system of “termination”, which is normally enabled 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 signs of transmission return by bus, as if it were an effect of “there-and-back”.

The signals are transmitted in basically three ways:

– Single-Ended (SE): in this mode, the signal is issued by the controller for all devices connected by means of a single via. As the signal degrades along the route, it is recommended that the connection to the whole has not more than 6 metres. For being simple to implement, this means of signaling is widely used;

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

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

Worth mentioning also that the SCSI technology you can work with the modes transmission – asynchronous and synchronous. The first allows the initiator to send a command and wait for a response at all operations. The second works in a similar way, but it is able to send several commands before receive the response from the previous request. This feature can influence the speed of the transmission of the data.

Versions of the SCSI

As reported before, the SCSI technology has passed by revisions over time that have resulted in new versions. The following are the key characteristics of each specification released.


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

SCSI-2 (Fast SCSI)

The SCSI-2 (or Fast SCSI) is a review launched in 1990 to work around some of the limitations of SCSI-1. This specification includes features that, in the the first version, were not necessarily mandatory, creating problems of compatibility. 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 work with frequencies up to 10 MHz and 8 bit, resulting in a maximum transfer rate of 10 MB/s. Here, it is also only possible to work with up to eight devices on the same bus.

There is a change implemented in 1994 called Wide SCSI Fast that also works with a clock 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 came to be recognized officially in 1995, but has the characteristic of being formed by several specifications. The first of them, call only SCSI-3 or Ultra SCSI, works with 8-bit and a frequency of 20 MHz, and may also transmit 20 MB/s. Here, you can connect up to eight devices.

In sequence came the Wide Ultra SCSI, which also has frequency of 20 MHz, but transfers 16-bits at a time, making this version has a speed of 40 MB/s and support up to sixteen devices.

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

Then appears the Wide Ultra2 SCSI, which works with 16-bit and frequency of 40 MHz, resulting in a maximum speed of 80 MB/s and supporting up to sixteen devices.

Ultra160 SCSI, Ultra-320 SCSI and Ultra640

The versions Ultra160 SCSI, Ultra-320 SCSI and Ultra640 SCSIhave emerged later. The numbers in the names refer the maximum rate of data transmission. Despite also, these versions are also part of the series revision of SCSI-3.

The Ultra160 SCSI also works with a frequency of 40 MHz and 16-bit, but performs two operations transfer per clock cycle instead of once, causing the specification has a speed of 160 MB/s.

The same happens with the Ultra-320 SCSI, with the difference that this version has a clock of 80 MHz, resulting in a rate maximum of 320 MB/s. finally, it appears that the Ultra640 SCSI, that if differentiates itself by having clock of 160 MHz, allowing for transfers of up to 640 MB/s.

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

Summary of the versions of SCSI

SAS (Serial Attached SCSI)

It is valid to point out that the SCSI also has other variations. One of them is the Serial Attached SCSI(SAS), which can reach speed 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 transmission scheme serial data (in the versions shown previously, the transmission is made in parallel) combined with a higher frequency.

The SAS stands out for considered, up to a certain point, a “rival” of standard SATA. In fact, both have similar features, being in some cases even possible to use SATA hard Drives in interfaces SAS, since it is common to use the same the connector in the two technologies.

The use of SAS is almost unique in computers and servers more and more sophisticated. HDs can not take advantage in terms of capacity compared to the SATA, on the other hand, it is common to find SAS drives focused on performance that can work with 10,000 or 15,000 RPM (revolutions per minute), for example.


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 the activation command of the SCSI from the networks IP.

With iSCSI it is possible, for example, to do with that particular server to access a data storage system (storage) existing in the same network optimally and reliable. Thus, it is not necessary connect the two machines directly, just take advantage of a already existing network.

Once that have the advantages of simplifying structures and save resources, solutions based on iSCSI are quite used until today.

Cables and connectors SCSI

Once the SATA technology has several specifications and can meet various types of devices, there are, as a consequence, various types of connectors. The following are some of them.


The SCSI technology lost space in the market after the arrival of the SATA standard in relation to the hard disks, in addition to technologies such as USB, FireWire , and Thunderbolt in relation to external hard Drives, scanners, printers and other. It is not for less: are technologies less complex,relatively cheaper, and that to meet expectations in relation to its features.

However, it is a mistake to believe that the SCSI is “dead”, after all, it is still possible to find utility for this technology in a number of applications. The mentioned standard SAS is an example. In addition, the SCSI Trade Association, association was created in 1996 to promote the technology, continues in full activity. On the occasion of the closing of this text, the authority worked including in the development of the specifications of the SCSI Express, a standard that seeking to take advantage of the combination of the technologies SCSI and PCI Express.

Scsi Technology (Small Computer Systems Interface) 1

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