This article will talk about what allows you to connect a hard drive to a computer, namely, about the interface hard disk. More precisely, talking about interfaces hard disksBecause the technologies for connecting these devices for all the time of their existence were invented the great set, and the abundance of standards in this area could lead to a confusion of an inexperienced user. However, about everything in order.

Hard disk interfaces (or strictly speaking, external drive interfaces, since they can act in their capacity not only, but also other types of drives, for example, drives for optical disks) are intended to exchange information between these external memory devices and motherboard. Hard drive interfaces are not less than the physical parameters of the drives, affect many performance of the drives and their performance. In particular, drive interfaces define their parameters such as the rate of data exchange between hard disk and the motherboard, the number of devices that can be connected to a computer, the ability to create disk arrays, the possibility of hot connection, support for NCQ and AHCI technology, etc. Also, the hard disk interface depends on which cable, cord or adapter to connect it to motherboard You will need.

SCSI - SMALL COMPUTER SYSTEM Interface

The SCSI interface is one of the oldest interfaces designed to connect drives in personal computers. This standard appeared in the early 1980s. One of his developers was Alan Shugart, also known as the inventor of disc drives for flexible disks.

Exterior view of the SCSI interface on the board and connection cable to it

SCSI Standard (Traditionally, this abbreviation is read in Russian transcription as "Skaz") was originally intended for use in personal computers, as evidenced by even the name of the format name - Small Computer System Interface, or a system interface for small computers. However, it happened that drives this type It was used mainly in the personal computers of the top-class, and later in the servers. This was due to the fact that, despite the successful architecture and a wide set of commands, the technical implementation of the interface was quite complex, and did not fit at the cost of mass PCs.

However, this standard has a number of opportunities inaccessible to other types of interfaces. For example, cord for connecting Small Computer System Interface devices may have maximum length 12 m, and the data transfer rate is 640 MB / c.

As a few later IDE interface, the SCSI interface is parallel. This means that tires transmitting information on multiple conductors are used in the interface. This feature He was one of the restraining factors for the development of the standard, and therefore a more perfect, serial standard SAS (from Serial Attached SCSI) was developed as its replacement.

SAS - Serial Attached SCSI

This is the interface of the SAS server disk.

Serial Attached SCSI was developed in the enhancement of a sufficiently old SMALL COMPUTERS SYSTEM Interface hard drive connectivity interface. Despite the fact that Serial Attached SCSI uses the main advantages of its predecessor, however, he has a lot of advantages. Among them should be noted as follows:

• Using a common tire by all devices.
• A serial data transmission protocol used by SAS allows you to use a smaller number of signal lines.
• There is no need for tire termination.
• Practically unlimited number of connected devices.
• Higher throughput (up to 12 Gb / c). In future implementations of the SAS protocol, it is intended to maintain the rate of data exchange of up to 24 Gbit / c.
• The ability to connect to the SAS drive controller with the Serial ATA interface.

As a rule, the SERIAL ATTACHED SCSI systems are built on the basis of several components. The main components include:

• Target devices. This category includes actual drives or disk arrays.
• Initiators - chips intended for generating requests for target devices.
• Data delivery system - Cables connecting target devices and initiators

SERIAL ATTACHED SCSI connectors may have different shape and size, depending on the type (external or internal) and from SAS versions. Below are the internal SFF-8482 connector and the external SFF-8644 external connector, designed for SAS-3:

On the left - the internal connector SAS SFF-8482; Right - external SAS SFF-8644 connector with cable.

Several examples of the appearance of cords and SAS adapters: HD-MINI SAS cord and SAS-Serial ATA-adapter cord.

Left - HD MINI SAS cord; Right - transition cord with SAS on Serial ATA

Firewire - IEEE 1394

Today, you can often find hard drives with the FireWire interface. Although any types of peripheral devices can be connected via the FireWire interface to a computer, and it cannot be called a specialized interface designed to connect exclusively hard drives, however, FireWire has a number of features that make it extremely convenient for this purpose.

FireWire - IEEE 1394 - Laptop view

The FireWire interface was designed in the mid-1990s. The start of development was put by the unsolicited firm Apple, in need of his own, other than USB, a bus for connecting peripheral equipmentFirst of all multimedia. The Specification describing the operation of the FireWire tire received the name IEEE 1394.

To date, FireWire is one of the most commonly used formats of high-speed consistent external tire. The main features of the standard include:

• The possibility of hot device connection.
• Open tire architecture.
• Flexible Topology Connecting Devices.
• Changing data transmission speed - from 100 to 3200 Mbps.
• The ability to transfer data between devices without computer participation.
• The possibility of organization local networks With the tire.
• Power transmission over bus.
• A large number of connected devices (up to 63).

To connect hard drives (usually by external hard drives) via the FireWire bus, as a rule, a special SBP-2 standard is used, using a set of Small Computers System Interface protocol commands. It is possible to connect FireWire devices to a conventional USB connector, but this requires a special adapter.

IDE - Integrated Drive Electronics

An IDE abbreviation is undoubtedly known to most personal computers users. The standard of the interface for connecting hard drives IDE was developed by a well-known firm producing hard drives - Western Digital. The advantage of the IDE compared to other interfaces that existed at that time, in particular, the Small Computers System Interface interface, as well as the ST-506 standard, was the lack of need to install a hard disk controller on the motherboard. The IDE standard meant the installation of the drive controller on the hull of the drive itself, and only the interface host adapter remained on the motherboard to connect IDE drives.

IDE interface on motherboard

This innovation made it possible to improve the work parameters of the IDE drive due to the fact that the distance between the controller and the drive itself decreased. In addition, the installation of the IDE controller inside the hard disk body made it possible to simplify both motherboards and the production of the Winchesters themselves, since the technology gave freedom to manufacturers in terms of the optimal organization of the logic of the drive.

The new technology was originally obtained by the name Integrated Drive Electronics (built into the electronics storage). Subsequently, the standard describing its standard named ATA was developed. This name comes from the last part of the PC / AT computers family by adding the word Attachment.

To connect a hard disk or other device, for example, a drive for optical disks that supports Integrated Drive Electronics technology to a motherboard, a special IDE cable is used. Since ATA refers to parallel interfaces (therefore, it is also called Parallel ATA or PATA), that is, interfaces providing for simultaneous data transmission over multiple lines, its data cable has a large number of conductors (usually 40, and in recent versions The protocol had the ability to use an 80-core cable). Normal data cable for this standard It has a flat and wide view, but there are also cables of circular sections. Power Cable for ParalelLel ATA drives has a 4-pin connector and is connected to the computer power supply.

Below are examples of IDE cable and a circular data cord PATA:

Exterior type of interface cable: Sleva - flat, on the right in a round braid - PATA or IDE.

Thanks to the comparative cheapness of ParlelLel ATA drives, ease of implementation of the interface on the motherboard, as well as the simplicity of installing and configuring the PATA devices for the user, the drives type Integrated Drive Electronics for a long time have been outlined from the Winchester market for personal computers of the device of other types of interface.

However, the PATA standard has a number of shortcomings. First of all, this is a length limit that Parallel ATA data cable can have no more than 0.5 m. In addition, the parallel interface organization imposes a number of restrictions on the maximum data transfer rate. Does not support PATA standard and many advanced features that have other types of interfaces, such as hot connecting devices.

SATA - Serial ATA

View of the SATA interface on the motherboard

SATA interface (Serial ATA), as you can guess from the name, is an improvement in ATA. This is improved, first of all, in the alteration of the traditional parallel ATA (Parallel ATA) to the serial interface. However, these differences between the standard SERIAL ATA from the traditional are not limited. In addition to changing the type of data transfer from parallel to the serial, the connectors for transmitting data and power supply also changed.

Below is the SATA data cord:

Data cord for SATA interface

This made it possible to use a cord of much greater length and increase the data transfer rate. However, the minus was the fact that the PATA devices that before the appearance of SATA were present in the market in huge quantities, it became impossible to directly connect to new connectors. True, most new motherboards still have old connectors and support the connection of old devices. However, the reverse operation - the connection of the new type drive to the old motherboard usually causes much more problems. For this operation, the user usually requires the Serial ATA to PATA adapter. The power cable adapter typically has a relatively simple design.

SERIAL ATA TO PATA POINABLE:

Left general view of the cable; The appearance is enlarged by the appearance of the PATA and Serial ATA connector

It is more difficult, however, the situation is with such a device as an adapter to connect the device of the serial interface into the connector for the parallel interface. Usually the adapter of this type is made in the form of a small chip.

Exterior view of the universal bidirectional adapter between SATA - IDE interfaces

Currently, the Serial ATA interface almost displaced Parallel ATA, and the PATA drives can now be found mostly only in fairly old computers. Another feature of the new standard that has ensured its wide popularity has been support.

Type of adapter with IDE on SATA

About NCQ technology can be told a little more. The main advantage of NCQ is that it allows you to use ideas that have long been implemented in the SCSI protocol. In particular, NCQ supports the system of streamlining read / write operations entering several drives installed in the system. Thus, NCQ is able to significantly increase the performance of drives, especially the solids of hard drives.

Type of adapter with SATA on IDE

To use NCQ requires support for the hard disk technology, as well as the motherboard host adapter. Almost all adapters that support AHCI are supported and NCQ. In addition, NCQ supports some old proprietary adapters. Also, NCQ requires its support from the operating system.

eSATA - EXTERNAL SATA

Separately, it is worth mentioning the seemingly promising at one time, but never received widespread ESATA format (EXTERNAL SATA). As you can guess from the name, ESATA is a type of Serial ATA, intended for connecting exclusively external drives. Standard ESATA offers for external devices most of the standard features, i.e. Internal Serial ATA, in particular, the same system of signals and teams and as high speed.

ESATA connector on laptop

However, ESATA has some differences from the standard of the inner tire that has breeding it. In particular, ESATA supports a longer data cable (up to 2 m), and also has higher storage nutrition requirements. In addition, ESATA connectors are somewhat different from standard SERIAL ATA connectors.

Compared to other external tires, such as USB and FireWire, ESATA, however, has one significant disadvantage. If these tires allow power to power the device through the tire cable itself, the ESATA drive requires special power connectors. Therefore, despite the relatively high data transfer rate, ESATA is currently not very popular as an interface for connecting external drives.

Conclusion

Information stored on the hard disk can not be useful for the user and accessible to application programs until the computer processor has access to it. Rigid disk interfaces are a means for communication between these drives and the motherboard. To date, there are many different types of hard drive interfaces, each of which has its advantages, disadvantages and characteristic features. We hope that the information given in this article will largely be useful for the reader, because the selection of a modern hard disk is largely determined not only by its internal characteristics, such as the container, the amount of cache memory, the speed of access and rotation, but also by the interface, for which he was designed.

28. 07.2017

Dmitry Vasiairova Blog.

SCSI - fast and unusual interface

Hello.

From this article, you will find out the most necessary about SCSI what it is where and why is it used how many generations has come from the moment of appearance and as being implemented in practice.

Read - Suddenly, SCSI will come in handy and you?

What does SCSI mean?

This set capital letters From the phrase Small Computer Systems Interface. In Russian, he sounds like "Skaz", and the decoding is a system interface for small computers.

This standard is designed to combine computer components for various purposes on one bus: Winchesters, drives, scanners, printers, etc. Why? To provide them with the same high speed of work as a single, but at the same time divide mechanism. In addition, thanks to SCSI, you can use one device on multiple computers at once.

Other features

In addition to the simple iron connection, the technology allows you to exchange data and defines a set of commands that got widespread. For example, in Windows it is used in a single stack for information storage devices.

Most often, the commands are used as recording, reading, checking devices, requesting their characteristics, installing new parameters for them or refund of the previous ones, etc.

It also happens to implement the commands over the wires and controllers of other standards. If we are talking about IDE, ATA or SATA, it is called ATAPI - ATA Packet Interface; If on top of the USB protocol - Mass Storage Device. Thus, you can, for example, connect a remote hard disk through a regular USB and will be used in the SCSI driver available in the Ourser.

Where is SCSI in demand?

On servers and workstations of high performance. On servers belonging to the low price category, and even more so at home, this interface is extremely rare; In such cases, the optimal option is familiar to us.

But naturally, no one forbars to put such a game of the device in their home computer. Or, for example, in the home server.

Technology in practice

All the devices you want to connect to one bus work through a special adapter, which, in turn, is inserted into the free slot on the motherboard. The controller has its own BIOS by which you can manage devices. The operating system recognizes and communicates with them, as usual, with the help.

The presence of the SCSI adapter means that with central processor A part of the load is removed, therefore, iron works faster.

As this technology It is consistent, then the devices should be connected accordingly. Moreover, each must have a unique ID, and all of them - the same interface.

History of appearance

I want to tell you the history of the creation of the interface not from its overseasness, but because through it you can understand more about the subject of our conversation.

So, in 1979, the inventor of 8-inch floppy disks and the manufacturer of magnetic drives Alan Shugart set itself the task to make a universal interface for its products, which would not lose its position with the development of technology.

And he managed to solve it by creating a standard that supports the logical and practical (head, cylinder, sector) addressing. It was based on the protocols of 8-bit parallel sending information along the path, including several lines.

The innovation was not very harder for the Russian-speaking population the name SASI (Shugart Associates Systems Interface), that is, a binding system interface, named after the founding father.

After 2 years, he shared his development with the ANSI Committee (American National Standarts Institute - the National Institute of Standardization of the United States) is the same as GOST in our country. On the basis of this invention, ANSI specialists created SCSI.

Interface generations

It is noteworthy that the technology has been created almost the population back, and we are still talking about it. All because she was constantly transformed. From the moment of appearance, 10 versions came out. I will not score your head for details about each of them. I will tell only what was originally, and what we have now.

SCSI-1.

• It is possible to connect the maximum of 8 devices to one bus, including the controller.
• The limit velocity was 1.5 MB / s in an asynchronous variation ("request-confirmation"), and 5 MB / s in synchronous - the same confirmations were returned to several requests.
• From the side of the electricians there were 24 lines, including differential and unipolar, although the second type signals were more often fed.
• The tire frequency was 5 MHz.
• The longest cable is 6 m, and for the differential HVD tire - 25 m.

Ultra-640 SCSI

• The busbar of the tire doubled, respectively, can be connected to 16 devices at the same time.
• Its frequency is 160 MHz DDR.
• The speed also does not go into any comparison with the first modification - now it reaches 640 MB / s.
• The connector consists of 68 contacts.
• The length of the cable reaches 10 m.

SERIAL ATTACHED SCSI (SAS)

• Support for connecting SATA devices.
• Speed this interface It has grown up to 12.0 Gbit / s.
• As developers declare, it is possible to connect 16384 devices for one bus! In the previous generation, as described above was only 16.

Electrician

There are 3 ways to transfer information regarding electricians:

• SE (single-ended) - asymmetrical appearance. Each signal is sent along a separate line.
• LVD (Low-Voltage-differential) is a differential standard with low voltage. Signals "+" and "-" are transferred by different wires. Each of them is given alone twisted steam. They are transmitted under the voltage of ± 1.8 V.
• HVD (High-Voltage-Differential) is an analogue of the previous version, but with special reception and increased voltage.

The interface load is distributed using terminators located at both ends of the tire. According to electrical characteristics, they are divided into:

• Passive - simple resistors at 132 ohms;
• Active - stabilizers producing the required signal, and each line of power is connected to them with a resistance of 110 ohms;
• FPT (Forced Perfect Terminator). The name speaks for itself - an accelerated improved type. It has emission limiters, and is applied in high-frequency interfaces.

Most often used 2nd model.

Competitiveness SCSI

Standard SCSI passed the test of time and is popular to this day. Why?

• Has high speed;
• You can create a chain of 15 devices;
• They are convenient to manage;
• HDD is distinguished by increased reliability.

Yet such drives account for only about 30% of the modern market, since SCSI has and disadvantages:

• Horregious. But you need to understand that you pay for quality. Although SATA winchesters have a greater capacity at a lesser price, they cannot boast such durability.
• Obsolescence. An improved competitor appeared - SAS technology (Serial Attached SCSI), which has more compact wires, does not need terminators, allows you to connect more devices and has better bandwidth.

That's all.

Waiting for you on the blog pages as often as possible.

What is SCSI?

A: The answer to this question is devoted to [SCSI Basics].
What is sas, what is better than SCSI or SAS and what do they differ?
A: A response to this question is dedicated to the [SAS or SCSI] section.
What is ESATA?
ABOUT: ESATA This is an SATA interface designed to connect external SATA devices. It provides a 3 Gbit / s channel bandwidth, which will exclude a delay in shortage bandwidthcharacteristic of modern external data storage devices.

What is Unified Serial?
A: All Unified Serial Controllers allow you to connect SATA and SAS discs using a point-to-point type interface. This uses an extended set of SCSI commands, providing powerful data management, error processing and performance.

The flexibility provided by the support of SATA and SAS discs provides companies with the ability to simply standardize I / O infrastructure for both primary storage of critical data and secondary storage, depending on which discs are installed - SATA or SAS. Customers can standardize their infrastructure through the use of unified I / O controllers and storage systems and, thereby reducing the cost of training and maintenance.

Is it possible to use SATA disks with SAS controllers?

A: Yes, you can, while on one controller you can simultaneously use both SAS and SATA discs. This allows you to start moving to SAS technology for reasonable money.

Is it possible to use SAS drives with SATA controllers?
Oh no.

Is it possible to connect SAS drives to the controller without using hotswap baskets?
A: Yes, you can. To do this, use a special cable with SFF-8482 connector from the drive side. The connector on the other end of the cable is determined by the SAS controller.

What is the difference between SCSI-1, SCSI-2, FAST, WIDE, ULTRA WIDE and ULTRA2 SCSI?
A: The main difference lies in the SCSI set of commands and the width of the tire (respectively - in speed).
SCSI-1 5MB / SEC 8 Bit SCSI Tire
SCSI-2 5MB / SEC 8 Bit SCSI Tire
SCSI-2 FAST 10MB / SEC 8 Bit Scsi Bus
SCSI-2 Fast Wide 20MB / SEC 16 Bit SCSI Tire
SCSI ULTRA 20MB / SEC 8 Bit Scsi Bus
SCSI ULTRA WIDE 40MB / SEC 16 Bit SCSI Tire
Ultra2 Wide 80MB / SEC 16 Bit SCSI Tire
Ultra160 160MB / SEC 16 Bit Scsi Tire
ULTRA320 320MB / SEC 16 Bit SCSI Tire

When do you need to use Low Voltage Differedial (LVD) Controller?
A: In case:
High data transfer rate is required - 80 - 320 MB / s
In the surrounding space, the level of electromagnetic noise affecting data transmission is very high. LVD mode provides much greater noise immunity than SCSI Single Ended (SE)
It is necessary to ensure a significant removal of SCSI devices from the computer. LVD devices can be removed from the SCSI controller to a distance of up to 12 meters (this is the maximum permissible length LVD SCSI cable.

What is SCSI Terminator and why is it needed?
A: SCSI Terminator is a small electronic device that should be located at both ends of the SCSI bus and their (terminators) should be two for each SCSI bus. Most often, the first SCSI Terminator-Oh is the SCSI controller (as a rule, you can "turn off" in the controller BIOS, and it is turned on by default), and the second is the terminator connected to the last (from the SCSI controller) of the cable SCSI connector.

Some SCSI devices (outdated discs, drives, streamers) have a built-in terminator, which can be enabled with an appropriate jumper on the device. In this case, it is necessary to ensure that the device with the terminator is placed at the very end of the SCSI bus.

And without SCSI Terminator, everything works, maybe it will come down and so?
A: For the time being, it may come down, especially if you have only one disk and it is used too intense. But with increasing the number of devices on the SCSI bus, or with an increase in the load on it, you, in the end, risk the loss of the data, so that it is not worth saving on it.

What is SCSI ID and why is it needed?
A: SCSI ID is a unique (within one SCSI bus) identifier (number) SCSI device. It is needed to provide addressing to devices on the SCSI bus.

A SCSI ID is assigned either automatically (for example, if HotSwaps are used for disk baskets that support such a function), or by manual installation of the corresponding jumpers on SCSI devices. SCSI ID is in no way connected with the physical order of the location of devices on the SCSI bus (for example, the SCSI controller, as a rule, has a default SCSI ID to 7-m, although most often, but not always, is located at the beginning of the SCSI tire), it is only important To do not have any devices with the same SCSI ID on the same SCSI.

SCSI ID values \u200b\u200bcan be:
from 0 to 15 (only 16) for WIDE (W) and Ultrawide (UW, U2W, U160, U320) SCSI tires;
from 0 to 7 (total 8) for Narrow (U, U2) SCSI tires;

What will happen if you connect two devices with the same SCSI channel to the same SCSI channel?
A: Nothing good. At best, the SCSI controller recognizes one of these devices, but it will not be able to work correctly with it, it will not "see" not one of these devices. Neither the controller will not be damaged, but the risk to spoil the data on SCSI disks remains.

It should be borne in mind that the overwhelming majority of the controllers do not report on such an error, so that when connecting new devices to the SCSI bus, it is necessary to pay attention to the compliance with the uniqueness of the SCSI ID.

Note that the SCSI itself controller also has a SCSI ID (as a rule, it is 7-mi, and can be changed to the BIOS-E controller), so it is not necessary to assign the same SCSI ID to disk.

What is Saf-Te?
ABOUT: SAF-TE - SCSI ACCESSED FAULT-TOLERANT ENCLOSURE (Available via SCSI Pavoropable Case) - "Open" technical requirementsDesigned to provide a comprehensive and standardized method for monitoring and output information about the status of drives, power sources and cooling systems used in high reliability servers and storage subsystems. Technical requirements are independent of the hardware of input - output, operating systems and server platforms, because the hull itself seems to be just another device on the SCSI bus. SAF-TE technical requirements have been accepted by many leading manufacturers of servers, data storage devices and RAID controller. Products satisfying SAF-TE specifications reduce the cost of cost control costs, simplify the operation of the network administrator, provide emergency notification and information about the status of equipment.

When you connect to the SCSI, the controller is only one instrument (for example, a hard disk), and on the controller, and the terminators must be enabled on the device. If this is an external device having an additional connector for connecting other external instrument SCSI (for example, an external SCSI CD-ROM), then you can use the external terminator (preferably active). In this case, the internal terminator of the device must necessarily be turned off.

If several devices are connected to the SCSI controller, the terminators must be installed only at the ends of the SCSI bus. So, if all connected devices are internal, then the terminators must be included on the SCSI controller and on one (and one) instrument that is physically connected to the last SCSI bus connector. The best results are obtained if the active external terminator is connected to the last connector, and the internal terminators on all devices (except the controller) are turned off. By the way, in recent times, many devices (for example, SE / LVD hard drives) do not have a built-in terminator at all.

If all connected devices are external, then the terminators must be included on the controller and the last connected external device. It should be noted that external SCSI devices in the overwhelming majority have two connections, to one of which is connected to the SCSI bus from the computer, and other SCSI devices can be connected to another. In this case, it is advisable to disable the internal terminators of all devices and use the active external terminator.

If you need to connect to one SCSI controller, both internal and external devices, the controller is connected to the SCSI intermediate connector. The SCSI bus is used to connect internal devices, and the other part ends with the connector to connect external devices. In this case, the internal terminator of the controller must be turned off. On an internal device connected to the last SCSI bus connector, the terminator must be turned on, and on the other internal devices is turned off. On the connector for connecting external devices, an active external terminator must always be installed. When connecting an external SCSI device, an external terminator is removed, an external device is connected to the SCSI connector, and an external terminator has been connected to an additional external device connector (do not forget to correctly install the external number number, and then the computer will simply "hang").

Connecting terminators for devices with different interfaces

All of the above is right if all connected devices have the same interface (all WIDE SCSI-2 devices or all SCSI-2 devices). If part of the devices has a WIDE SCSI-2 interface, and at least one (usually CD-ROM) has an SCSI-2 interface (Narrow), then, in some cases, problems arise with proper connection Terminators. The problems are caused by the fact that Wide and Narrow interfaces are distinguished by the number of data lines in the bus.

The most common error is to connect to the WIDE SCSI-2 bus of several hard drives with WIDE SCSI-2 (or Ultra WIDE SCSI-2) interface, and the last connector connects via the CD-ROM adapter with the SCSI-2 interface. Despite the fact that the terminator will be enabled on the CD-ROM, this terminator will appreciate only 8 bus lines, while the remaining 8 lines used in the WIDE SCSI interface will be "hanging in the air."

A more correct solution will be connecting devices with an 8-bit SCSI interface to the intermediate bus connectors (Terminators of 8-bit devices are turned off). To the last connector, connect the WIDE SCSI device with the terminator enabled (or active external terminator). Of course, the presence of an adapter still worsens the system indicators. Such an option should be avoided (as well, however, as generally use on one tire of high-speed and slow devices). However, in this situation it is still the correct option of the connection. Ultra2 SCSI controllers have an integrated interface converter in their composition, which allows you to connect all Ultra2 standard devices to a separate bus without mixing them with less high-speed devices.

Features of controllers with two connectors

Many SCSI controllers have 2 connectors: one for the SCSI interface, the second for the WIDE SCSI interface. This is only physically different connectors, the SCSI channel is the same. These various connectors make it possible to avoid the use of any adapters, but do not eliminate problems with connecting the terminators. Such controllers have "HIGH ON / OFF" and "LOW ON / OFF" switches. These are separate switches of active terminators for senior and junior bus bytes, respectively. Moreover, the younger byte ("Low") is the SCSI (Narrow) interface lines, and the older byte - the interface extension lines to the WIDE standard.

If only one standard devices are connected to such a controller, then both switches are set to "ON". The SCSI (or WIDE SCSI) bus is connected by one end connector to the controller, the device with the terminator turned on to another end connector. The remaining devices with the terminators turned off are connected to intermediate connectors.

If you need to connect multiple devices with different interfaces, two tires are used: SCSI and WIDE SCSI. Both tires with their final connectors are connected to the corresponding controller connectors. The devices are connected to the tires in accordance with the standard supported by them. The terminators are included only on the device connected to the SCSI finite connector, and on the device connected to the WIDE SCSI bus termination. On the controller, the terminators switches are installed in the "High On" and "Low Off" position.

Recently, controllers, including those installed on the motherboard, do not have such a switch (or the corresponding item in menu Bios.). There is only "Terminator ON / OFF". In this case, we are talking only about the younger 8 tire discharges. Senior discharges are always stunned.

Nutrition of active terminators

The active terminators currently used require supply voltage to work. This voltage to the active terminator can be fed, both from any SCSI device and from the controller. On modern SCSI devices there is a special switch to select the source of the supply voltage of the firmware in these devices of the active terminator. Typically, the factory is set to the power supply of the terminator from the device itself ("POWER FROM DRIVE"). If only one or more internal SCSI devices with the same interface is connected to the controller, it does not occur.

If, according to the terms of normal tire harmonization, the use of an active external terminator is necessary, then you need to take care of the supply voltage on it. For this, on one of the devices connected to this bus, the voltage feed mode should be turned on ("Power to SCSI Bus"). If this is not done, then the external terminator will simply not work normally.

In all cases discussed above, the best results are usually achieved with the diet of all terminators from one source. To file a supply voltage to all terminators from one source on one (any) instrument, the power of the terminator built into this device from the internal power source and at the same time the terminator power supply voltage mode in the bus. To do this, on this jumper device (switches) are set to "Power to SCSI Bus and Drive". On the other devices in which the termination must be enabled, the terminator power supply is installed on the SCSI bus (jumpers or switches are installed in the "Power From SCSI Bus" position).

In the overwhelming majority of cases, the system will work normally and in the event that each terminator is powered by its source. The main thing is that every terminator served as a voltage of at least one source. Moreover, nothing terrible will occur if several devices will be set to the supply voltage mode of the terminators in the line. The supply chains of the terminators of all devices are protected from the counter-submitted voltage.

Specialized SCSI controllers

Often to scanners and some other slow SCSI devices included with a simple SCSI controller. This is usually SCSI-1 controller on the ISA 16 bus, or even 8, bits with one (external or internal) connector. There is no BIOS on it, it often works without interrupts (polling mode) sometimes supports only one device (and not 7). Basically, such a controller can be applied only with its device. Other devices on such a controller will not work most often. Moreover, many devices (most often scanners) will not be able to work with a standard controller. Therefore, it is better not to count on compatibility, and connect standard SCSI devices to a separate standard controller.

SCSI (Small Computer Systems Interface - System Interface for Small Computers, is pronounced in Russian as "Skazz") - Interface designed to merge into unified system Devices of various profiles: drives on hard magnetic carriers, scanners, streamers, CD-ROM, etc. The essence of the interface is to provide a flexible mechanism for managing these devices and the maximum speed of their operation as a single but divide mechanism.

The roots of the SCSI interface go to the distant 1979, when the manufacturer of information drives M. Shugart was puzzled to find the universal interface standard for his discs, given the possible future needs. In the laboratories, M. Sigarrt eventually developed an interface that supported the logical and physical (head / cylinder / sector) addressing, based on the 8-bit parallel data transmission protocols on the interface consisting of several lines. This interface was named SASI (Shugart Associates Systems Interface - a binding system interface of the shipart). Interface, in addition to describing the protocols, also included several 6-bit commands; The minus was that the interface was developed for using only one pair of host - device.

Later, in 1981, M. Shugart handed out the documentation on the SASI interface to the ANSI Committee (American National Standarts Institute - National Institute of Standardization of the United States, an analogue of GOST), which took it for the basic work on the project that was called SCSI. Most of the most important moments From the SASI standard moved to SCSI, for example, such important principles such as device arbitration, tire release mechanisms, the ability to use more than one host adapter, etc. In 1984, the working documentation of the SCSI standard was submitted for consideration by ANSI, and, after numerous adjustments and additions, in 1986 a document was adopted at number X3.131-1986 - the first official standard SCSI, which is now called SCSI-1. In addition to the SASI, SCSI-1 standard "Obros" such important functionalitylike 10-bit commands, synchronous and asynchronous data transmission protocol, the ability to connect to one host adapter to 8 various devices. The existed for SCSI-1, the standards developed both in the direction of the command language expansion and in increasing and complicating protocols and in increasing the width of the tire, increasing the speed and the number of devices connected to one host adapter. For current SCSI standards, the bus width is 16 bits, the number of connected devices is also 16.

The PC industry did not miss the emergence of a new standard, which was immediately taken by the manufacturers of NJMD. In fig. 1, 2 depict some of the first samples of SCSI disks.

Fig. 1, 2. The first samples of SCSI drives - company SONY (capacity 40 megabytes)
and Quantum (120 megabyte capacity)

SCSI Standard Brief History

The very first standard is SCSI-1; In this standard, it was possible to connect up to eight devices to one bus, including the controller. The interface contains developed controls and at the same time is not focused on any particular type of devices. It has an 8-time data bus, the maximum transmission rate is up to 1.5 MB / s in asynchronous mode (according to the "query-confirmation" method), and up to 5 MB / s in synchronous mode (multiple requests - a few confirmations ") . Parity control can be used to detect errors. Electrically implemented in the form of 24 lines (unipolar or differential), although unipolar signals are used in the overwhelming majority devices.

SCSI-2 is a substantial development of basic SCSI. Increased transmission rate (up to 3 MB / s in asynchronous and up to 10 MB / s in synchronous mode) - FAST SCSI. Added new commands and messages, support for parity control is made mandatory. The possibility of expanding the data bus to 16 digits (WIDE SCSI), which ensured the speed of up to 20 MB / s. A new 68-pin connecting connector has been introduced. Subsequent specification, SCSI-3, not only introduced new transmission rates, but also significantly expanded the command system. In addition, use, along with the traditional parallel interface, and other parallel and consecutive protocols, and other parallel and serial protocols are allowed as a transmission medium: Fiber Channel, IEEE 1394 Firewire and Serial Storage Protocol (SSP).

Ultra SCSI interface uses 20 MHz bus frequency. The Ultra / Wide SCSI interface supports 16 devices and provides data transfer rate up to 40 MB / s. More high-speed Ultra-2 WIDE SCSI, providing a transfer rate up to 80 MB / s. The following interfaces - Ultra-3 SCSI, ULTRA 320 SCSI, Ultra 640 SCSI - did not bring anything fundamentally new to the standard, except for speed. They also remain with a bus width of 16 bits, and up to 16 devices can also be connected to the interface. The comparative characteristic of the SCSI standards is shown in Table 1.

Table 1. Comparative characteristics of SCSI standards

Standard	Maximum tire speed, MB / s.	Tire bit	Maximum cable length, m			Maximum number of devices
			The only mouth	LVD.	HVD.
S.CSI-1.	5	8	6	(3)	25	8
SCSI-2.	10	8	3	(3)	25	8
Wide SCSI-2	20	16	3	(3)	25	16
SCSI-3.	20	8	1.5	(3)	25	8
WIDE SCSI-3	40	16	—	(3)	25	16
Ultra— 2 SCSI	40	8	(4)	12	25	8
Wide Ultra.-2 SCS.I.	80	16	(4)	12	25	16
Ultra-3 SCSI,or Ultra-160 SCSI	160	16	(4)	12	(5)	16
Ultra 320 SCSI	320	16	(4)	12	(5)	16
Ultra 640.SCSI	640	16	(4)	(7)	(5)	16

What is a host adapter?

The host adapter is a device connected to the PC bus providing the host (the value of the word "host" in relation to the standards describing the data transmission interfaces (eng. Host), the most fully describes the phrase "bus owner") communication with SCSI devices. The name "Adapter" is not chosen by chance - this indicates that all the logic of the devices is located in peripheral devices on the tire; For devices called "Controller" logic is located in themselves.

The following manufacturers are released or released in the past host adapters for SCSI devices:

An example of a host adapter can be the device shown in Fig. 3.

Fig. 3. SCSI Adaptec host adapter

Modern manufacturers of NGMD SCSI

Currently, the NJMD market is experiencing a turbulent evolution - new, high-speed SERIAL ATA standards come to replace Parallel Ata. And, although the new SATA devices already closely approached the speed of operation to SCSI devices, and somewhere they overtake them, the SCSI devices remain still popular in high-end computers - servers and information arrays. This is due, first of all, with high reliability of SCSI drives - both due to the relative simplicity of SCSI standards and the well-thought-out electrical interface and in connection with the traditionally more thorough design and production profile of devices. SCSI accounts for approximately 30 percent of the entire NGMD market, and it is unlikely to ever post this line: PC equipment by all the necessary cables, adapters, and the purchase of the host adapter will cost approximately $ 100, the drives will cost several times more Their IDE-fellow. Modern manufacturers of SCSI discs are:

Competition in the SCSI disc market is small - most likely, because the market has sufficient content and does not develop so violently as the IDE device market - and is due, first of all, with the fact that SCSI devices are used most often in the servers, The demand for which is not so great. The convenience of SCSI devices is that they can be easily replaced by the work, without shutting down and loss of server performance. It is very important for servers, and not necessarily for workstations. As a rule, the server (Fig. 4) is equipped with special sleds (Fig. 5), in which the disk in a special mount (Fig. 6) is inserted very easily.

Fig. 4. Severcher equipped with SCSI discs

Fig. 5. SCSI disc compartment

Fig. 6. Fastening for SCSI disks used in servers with support for the "Hot Replacement" function

It costs to notice that very often server manufacturers are dispersed with drives, giving them their brands. As an example, I will give the drives seized from Hewlett Packard and IBM E-Servers (Fig. 7, 8), on which the real manufacturer of HMD can be found only by the name of the model; The author also saw discs extracted from Dell servers, on which even this information was absent.

Fig. 7, 8. Modern SCSI disks used in servers

SCSI connectors types

Fig. 9. Currently used SCSI connectors

SCSI devices may have different types Connectors for connecting them to the host adapter (see Fig. 9) - primarily this is due to the design features of the device itself. Most often for HDD, the HD68 connector is used (Fig. 10), slightly less often - SCA80 (Fig. 11). In the distant past, in the late 80s - early 90s, almost all SCSI drives were connected to the host by means of the non50 connector (Fig. 12). Currently, this connector is practically not found.

Fig. 10. HD68 connector.
Fig. 11. SCA80 connector.
Fig. 12. NE50 connector.

Specialized adapters may often require specialized adapters to connect various configuration connector connector. Such adapters, for example, manufactures SCS (http://www.scaadapters.com), their cost ranges from $ 10 to $ 35 per piece. A complete set for working with any SCSI device is depicted in fig. 13, in fig. 14 - 18 each adapter is depicted separately

Fig. 13. Adapters required for connecting SCSI devices

Fig. 14 - 18. Same as Fig. 13, separately.

How SCSI works

To negotiate loads on the SCSI bus, terminators are used, which by electrical properties are divided into passive, active and FPT terminators. Terminators must be powered by, therefore, the interface has terminator power lines (Terminator Power). Passive terminators were used in SCSI-1 devices, are conventional resistors with a resistance of 132 ohms. The active terminators are a stabilizer generating the desired signal - each line is connected to this stabilizer through a resistance resistor 110 ohms. Currently, only active terminators are used, while the auxiliary voltage sources are used - auxiliary diodes are usually used for these purposes, which fix the input signals voltage at the required level. Finally, FPT Terminators (Forced Perfect Terminator - an accelerated Improved Terminator) The essence of the improvement of active terminators, the equipment of their emission limiters. Their use is in high-frequency versions of SCSI.

All SCSI devices are customary to be divided into initiators and performers. It should be borne in mind that the tire can be standard (8 bits) or extended (16 bits) of the bit. Considering all this, the entire number of possible combinations of device connection can be reduced to four:

1. Standard initiator - standard performer
2. Advanced Initiator - Advanced Contractor
3. Standard initiator - advanced performer
4. Advanced Initiator - Standard Contractor

When connecting standard performers to advanced initiators, problems cannot - the Advanced Standard supports all the functions of the standard, however reverse connection Completions may occur with the connection of terminators. In real life, these problems are easily solved by the use of adapters (see above).

SCSI tire status is made to divide the phases. There are only five such phases: the bus is free, arbitration (the initiator can receive the bus control), the choice (while the initiator entered into the phase of the arbitration is first, selects the artist for further work), re-election (the Contractor confirms the initiator that it is chosen for them for Works and ready for work) and information phase (query-transmission of commands, data, messages). A block diagram of the phase sequence of one cycle of work on the SCSI bus is presented in Fig. nineteen.

After the selection phase, the initiator can produce a timeout, for which it can use two methods - perform a hardware reset or go to the "Tire Free" phase. In any case, the end of the work cycle on the SCSI bus will be the status of the "Command" status or the transfer of the corresponding message with the release of the tire. Similarly, the SCSI standard in SCSI systems can be used to reset the device for two protocols - on the hardware reset protocol (Hard Reset) and software reset protocol (Soft Reset). In both cases, a bit of the RESET line will be set to the unit, the differences in the types of discharges consist in their mechanism and purposes - as a rule, hardware reset is carried out to drop operations throughout the SCSI system system, the software is used to drop only one device, not Mixing the rest.

Fig. 19. SCSI Tire Phase Scheme Scheme

On the SCSI bus, nine control signals are used: BSY (Busy, Busy), SEL (Selection, Selection), C / D (Command / Data, Control / Data), I / O (Input / Output, I / O), MSG ( Message, message), REQ (Request, request), ACK (Acknowledge, confirmation), RST (Reset, Reset), ATN (Attention, Attention). Signal sources are busy, the choice and reset can be the initiator, and the performer; Only the performer may be a source of a confirmation signal; The remaining signals are prerogative of the initiator. Types of information transfer are encoded by bits combinations displayed for signals. Message, control / data, input / output, as shown in Table. 2.

Table 2. Types of information transfer via SCSI bus

The interface is controlled by the message system. There are 28 of them 28, they can be single-way, double-byte (one word) and extended. The message system is described in detail in any SCSI standard.

For selection specific device The SCSI bus exists a bit identifier. As a rule, the SCSI devices have hardware configuration, that is, the system identifies the device on the jumpers installed on it. Restrictions on the number of connected devices in the standard (8 bits) and the extended (16 bits) of the SCSI execution is superimposed by the existence of the bit identifier - in the 8 or 16 bit bus, it is impossible to set more than 8 or 16 bits of identification, and the bit identifier also includes The host adapter is, that is, in other words, besides the host adapter on the tire can exist for standard SCSI - another 7 devices for extended - 15.

SCSI teams

Team

Team Code

Change Definition (Change Definition) Compare (Compare) Copy (Copy) Copy and verify (Copy and Verify) Format Unit) Request (Inquiry) Lock-unlock Cache Log Select Log Sensitivity (Log Sense) Mode Select Mode Sense (Mode Sense) Pre-Fetch (Pre-Fetch) Disable permission to change the medium (Prevent-Allow Medium Removal) Reading (read) Read buffer (read buffer) Show Capacity Read defective data (Read Defect Data) Long reading (read long) Reassign Block (REASSIGN BLOCK) Take Diagnostic Results (Receive Diagnostic Results) Release) Request Sensitivity (Request Sense) Reserve Rezero Unit (Rezero Unit) Find the same data (Search Data Equal) Find Senior Data (Search Data High) Search Data Low SEEK) Diagnostic Request (Send Diagnostic) Set limit (Set Limit) Stop Stop Unit Synchronize Cache (Synchronize Cache) Device readiness (Test Unit Ready) Verify (verify) Recording (WRITE) Write and Verify WRITE BUFFER (WRITE BUFFER) Long entry (Write Long) Record the same (WRITE SAME)

40h. 39h. 18h. 3ah 04h. 12h. 36h. 4CH 4dh. 15h, 55h 1Ah, 5ah 34h. 1eh. 08h. 28h, 3ch. 25h. 37h. 3eh 07h. 1CH 17h. 03h. 16h. 01h. 31h. 30h. 32h 0bh. 2BH, 1DH 33h. 1bh. 35h. 00h 2fh. 0ah 2ah 2eh. 3bh. 3fh. 41h.

The table below lists the main SCSI-standard commands applicable to the NGMD. As in the Ata Standard, the SCSI standard exists as mandatory commands, that is, those that must be supported by any SCSI device and commands are optional, optional, the support of which the device may not be provided. In addition to them, there are indescribed in the standard specific for each manufacturer and often for each specific device line of devices so-called vendor commands - commands that use the manufacturer for repair purposes or diagnostics. These teams are usually the commercial mystery of the manufacturer and are not published anywhere.

SE, LVD, HVD

Usually, on the SCSI device, you can detect a marking similar to those depicted in Fig. 20. This marking indicates the type of data transmission at the electric level. The first - SCSI SE (Single Ended) denotes such a type of data transfer when each signal on the bus is provided with one conductor. SCSI LVD (Low Voltage Differential) and SCSI HVD (High Voltage Differedial) - low-voltage and high-voltage differential type - are physically organized the same: for each signal there are two conductors, one by one there is a positive polarity signal, otherwise negative. The differences in HVD and LVD are in voltage in conductors, for LVD it is lower than for HVD.

Fig. 20. Designations on SCSI devices carrying information about the electrical data transfer type

It is logical that HVD and LVD devices are incompatible - if you connect the LVD device to the HVD device bus, the first will die inevitably due to the excess of the signal voltage. The same can be said about SE and LVD devices - cables for them are the same, but due to electrical characteristics are not compatible. However, the LVD devices can be connected to SE conductors, as they recognize voltages over the bus and if a bipolar signal is obtained in one pair of conductors, they can switch to its use. As a rule, devices that can operate in both modes are designated by a special LVD / SE icon.

The compatibility of all types of devices on one bus is usually not required, but if such a need arises, the use of specialized adapters solves this problem quite easily (see above).

Continuous increase in the tire clock frequency led to the need to limit the maximum length of the connecting cable in the Ultra SCSI interface to one and a half meters. This is quite uncomfortable when using external high-speed SCSI devices, but more than enough to ensure the connection of devices inside the PC housing.

Synopsis. Prospects and opportunities

The SCSI interface is very productive and reliable, but it also has a considerable number of flaws. First of all, this is the high cost of the devices themselves - both drives and controllers. The following disadvantage is the complexity of configuration and management, which only trained people can cope with. Finally, the last lack of an interface that makes it even less attractive for the user is the impossibility of carrying a carrier to another PC, if it is not equipped with a specialized SCSI adapter ...

The use of SCSI devices is inappropriate for the standard PC market for a very simple reason: high price. However, manufacturers do not set a goal to win an ordinary consumer: it has been historically developed that SCSI drives are mainly server standard, and the IDE standard of workstations.

Meanwhile, SCSI drives comes on heels newest Standard IDE devices: SATA. The speed and performance of SATA devices is very high, and their use in servers is becoming increasingly popular. The only minus SATA is a pretty harvester connector, with which there are quite frequent failures of these devices. I think that the battle with SATA on the field of server drives interface SCSI will win undoubtedly.

The development of the SCSI standard promises to us in the future more high-speed devices traditional for SCSI reliability; Follow the fast care of SCSI devices from the market does not have.

SERIAL ATTACHED SCSI (SAS)

The last edge in the world of SCSI devices is a Serial Attached SCSI, an interface that uses three data transmission protocols (SSP-Serial SCSI Protocol, STP - Serial Ata Tunneled Protocol, SMP - Serial Management Protocol). As you can see from the names of the protocols, the first two are intended for the transmission of data, the latter is designed to control the interface. Drives with this interface are now producing Seagate, Samsung and Fujitsu.

A feature of this interface is that the signal is transmitted not by two (as in SATA), but by four conductors (one pair - to receive a signal, other - to send it). The stated data transfer rates are 1.5 and 3.0 GB / s.