SCSI - Small Computer System Interface

Despite the seeming dominance of devices with IDE / EIDE interface, SCSI hard drives still have about 27% of the market in terms of production. This is usually explained by the fact that these interfaces are designed for different market segments - IDE for "popular and cheap systems", and SCSI for "high-performance workstations". However, many may argue that recently hard drives IDEs have achieved SCSI performance and are significantly less expensive. And the IDE controller, which is already the fastest, is usually located on the motherboard and does not require additional material costs, while a good SCSI controller needs to spend at least $ 100. But there are people who persistently prefer this interface with a hard-to-read name. By the way, SCSI is read and pronounced as “ tales". I also partially consider myself to be such and will try to attract at least a few more users to our side, as well as tell a little about SCSI itself.

SCSI vs IDE

The "Which is better: IDE or SCSI" controversy is one of the most common controversies in many newsgroups. The number of posts and articles on this topic is very large. However, this question, like the famous "Windows NT or OS / 2 or Unix", in such a setting is insoluble. The most frequent and correct reaction to them is “Why?”. Having considered this issue in more detail, you can make a decision for yourself about the need for SCSI for yourself.

We will tell you in more detail what a simple SCSI controller can give in comparison with IDE and why you should choose it or not.

SCSI offer	EIDE / ATAPI objections	SCSI response
the ability to connect 7 devices to one controller (to Wide - 15)	it is easy to install 4 IDE controllers and there will be 8 devices in total	for each IDE controller you need an interrupt! And only 2 will be with UDMA / 33. And 4 UWSCSI is 60 devices :)
wide range of connected devices	IDE has CDD, ZIP, MO, CD-R, CD-RW	Do you have drivers and programs for all this? and more? but for SCSI you can use any, including those included in the OS
the ability to connect both internal and external devices	? removable rack or LPT-IDE	:)
the total length of the SCSI cable can be up to 25 meters. In normal versions 3-6m *	if you do not overclock the PCI bus, you can even by a meter	few!
you can use caching and RAID technologies to dramatically improve performance and reliability	there used to be caching Tekram "s, but now there are also RAID for IDE	it doesn't work and is not serious at all

* It should be noted that in the case of using the Ultra or Ultra Wide SCSI interface, additional restrictions are imposed on the quality of the connecting cables and their length, as a result of which the maximum connection length can be significantly reduced.

In order not to get the impression that the IDE is very bad and you should be ashamed of using it, we also note the positive qualities of the IDE interface, partially in light of the above table:

1. Price. Sometimes it is undeniable very important.
2. Not everyone needs to connect 4 HDDs and 3 CDDs. Often two IDE channels are more than enough, and all sorts of scanners come with their cards.
3. It is difficult to use a ribbon cable longer than 80cm in the minitower case :)
4. IDE HD is much easier to install, there is only one jumper, and not 4-16 as on SCSI :)
5. Most motherboards already have an IDE controller
6. Have IDE devices the bus is always 16 bits and for models comparable in price, IDE wins in speed.

Now about the price. The simplest SCSI for the ISA bus costs about $ 20, but now they are simply not needed by anyone, so you can find it cheaper. The next option is a PCI bus controller. The simplest version of FastSCSI costs about $ 40. However, now there are many motherboards on which the Adaptec 7880 UltraWideSCSI can be installed for only + $ 70. Even the famous ASUS P55T2P4 and P2L97 have SCSI options. For UWSCSI cards, the price ranges from $ 100 to $ 600. There are also dual-channel (like IDE on Intel Triton HX / VX / TX) controllers. Their price is naturally higher. Note that in the case of SCSI, unlike IDE, where it is difficult to come up with something new, for additional money the controllers can be expanded with the functions of a cache controller, RAID-0..5, hotswap, etc., so talk about the top the boundary of the controller's cost is not entirely correct.

And finally, about speed. As you know, today the maximum data transfer rate on the IDE bus is 33 Mb / s. For UWSCSI, the same parameter reaches 40 Mb / s. The main advantages of SCSI appear when working in multitasking environments (well, not much in Windows95 :). Many tests given under WindowsNT show the undoubted advantage of SCSI. Perhaps this is the most popular OS today, for which the use of SCSI is more than justified. There may also be specific tasks (related, for example, with video processing) in which it is simply impossible to use the IDE. We will not talk about the differences in internal architectures that also affect performance in this article, since there are too many technical terms there. We only note that watching the development of the IDE, we are surprised to notice that it acquires many features of SCSI, but, hopefully, they will not merge at all.

What does a SCSI controller look like and what does it consist of?

Here is a picture of the simplest FastSCSI controller on the PCI bus.

As you can see, most of the space is occupied by connectors. The largest (and oldest) connector is for 8-bit internal devices, often referred to as narrow, it is similar to the IDE connector, only it has 50 pins instead of 40. Most controllers also have an external connector, as the name suggests, you can and should connect external SCSI devices to it. The picture shows a 50-pin mini-sub D connector.

For Wide devices, a similar one is used, but for 68 pins, the fastening is also not in the form of latches, but on screws - like COM mice and printers. It is even smaller than narrow due to the higher contact density. (By the way, despite the name, wide stub is also narrower than narrow). Sometimes you can find the old version of the external connector - just a centronix. The same (externally, but not functional :) you can find on your printer. Some devices, such as the IOmega ZIP Plus, as well as those designed for the Mac, use the usual 25-pin Cannon (D-SUB) like a modem. Mini-centronics is also used for external high-speed connections. Here is the complete table:

(dimensions are almost original)

Internal
Low-Density 50-pin	connection of internal narrow devices - HDD, CD-ROM, CD-R, MO, ZIP. (like IDE, only for 50 pins)
High-Density 68-pin	connection of internal wide devices, mainly HDD
External
DB-25	connection of external slow devices, mainly scanners, IOmega Zip Plus. most common on Mac. (like a modem)
Low-Density 50-pin	or Centronics 50-pin. external connection of scanners, streamers. usually SCSI-1
High-Density 50-pin	or Micro DB50, Mini DB50. standard external narrow connector
High-Density 68-pin	or Micro DB68, Mini DB68. standard external wide connector
High-Density 68-pin	or Micro Centronics. according to some sources it is used for external connection of SCSI devices

For the operation of any device, as you know, software support is required. For most IDE devices, the minimum is built into Motherboard BIOS boards, the rest require drivers for various operating systems. With SCSI devices, things are a little more complicated. For initial boot from SCSI hard disk and working in DOS requires its own SCSI BIOS. There are 3 options here.

1. there is a microcircuit with SCSI BIOS on the controller itself (like on VGA cards). When the computer boots up, it is activated and allows you to boot from a SCSI hard disk or, for example, CDROM, MO. When using a non-trivial operating system (Windows NT, OS / 2, * nix), drivers are always used to work with SCSI devices. They are also required for non-hard disk devices to run under DOS.
2. the SCSI BIOS image is flashed into flash BIOS motherboard. Further, according to claim 1. Usually in BIOS boards add SCSI BIOS for a controller based on the NCR 810 chip, Symbios Logic SYM53C810 (it is in the first picture) or Adaptec 78xx. This process can be controlled, if desired, and the SCSI BIOS version can be changed to a newer one. If there is a SCSI controller on the motherboard, this is exactly the approach. This option is also more profitable economically :) - a controller without a BIOS chip is cheaper.
3. There is no SCSI BIOS at all. The operation of all SCSI devices is provided only by the drivers of the operating system.

In addition to supporting booting from SCSI devices, the BIOS usually has several other functions: configuring the adapter, checking disk surfaces, low-level formatting, configuring initialization parameters for SCSI devices, setting the boot device number, etc.

The next remark follows from the first. As you know, motherboards usually have CMOS. In it, the BIOS stores the board settings, including the configuration hard drives... For the SCSI BIOS, it is often necessary to store the SCSI device configuration as well. This role is usually performed by a small microcircuit such as 93C46 (flash). It connects to the main SCSI chip. It has only 8 legs and several tens of bytes of memory, but its contents are retained even when the power is turned off. In this chip, the SCSI BIOS can save both the parameters of SCSI devices and its own. In general, its presence is not associated with the presence of a microcircuit with a SCSI BIOS, but, as practice shows, they are usually installed together.

In the next picture you can see the UltraWide SCSI controller from ASUSTeK. It already has a SCSI BIOS chip. You can also see the inner and outer Wide connectors.

The last (I couldn't find any more :) picture shows a two-channel Ultra Wide SCSI controller. Its specification includes the following items: RAID levels 0,1,3,5; Failure Drive Rebuilding; Hot Swap and on-line Rebuilding; cache memory 2, 4, 8, 16, 32 Mb; Flash EEPROM for SCSI BIOS. The 486 processor is very clearly visible, which apparently is trying to manage all this stuff.

You can also find on the SCSI controller board

• SCSI bus activity LED and / or connector for its connection
• connectors for memory modules
• floppy disk controller (mostly on older Adaptec cards)
• IDE controller
• sound card(on ASUSTeK cards for MediaBus)
• VGA card

Other SCSI Cards

Often a simple SCSI controller is included with scanners and other low-speed SCSI devices. Usually it is a SCSI-1 controller on an ISA 16 or even 8 bit bus with one (external or internal) connector. It has no BIOS, eeprom, it often works without interrupts (polling mode), sometimes it only supports one (not 7) device. Basically, such a controller can only be used with your own device, because there is a driver only for him. However, with a certain skill, you can connect to it, for example HDD or streamer. This is justified only if there is no money and time (or sports interest :), since a standard SCSI controller, as already mentioned, can be purchased for $ 20-40 and have much less problems and much more features.

SCSI specifications

The main characteristics of the SCSI bus are

• its width is 8 or 16 bits. Or, in other words, "narrow" or "wide".
• speed (roughly - the frequency with which the bus is clocked)
• physical type of interface (unipolar, differential, optics ...). sometimes it can be called the type of connector to connect

the speed is mainly influenced by the first two parameters. They are usually prefixed to the SCSI word.

The maximum baud rate of the controller device is easy to calculate. To do this, you just need to take the bus frequency, and in the case of "Wide", multiply it by 2. For example - FastSCSI - 10Mb / s, Ultra2WideSCSI - 80Mb / s. Note that WideSCSI usually denotes WideFastSCSI, just like Ultra2 is known to me only in Wide version and only with LVD interface.

Using the example of Seagate hard drives, we will consider the options for SCSI interfaces. In the name of the model, the last 1-2 letters indicate the interface, i.e. the same disc can be produced with different interfaces, for example Baracuda 9LP - ST34573N, ST34573W, ST34573WC, ST34573WD, ST34573DC, ST34573LW, ST34573LC.

DC	80-pin Differential
FC	Fiber Channel
N	50-pin SCSI connector
ND	50-pin Differential SCSI connector
W	68-pin Wide SCSI connector
WC	80-pin Single connector SCSI
WD	68-pin Wide Differential SCSI connector
LW	68-pin Wide SCSI connector, low-voltage Differential
LC	80-pin Single connector SCSI connector, low-voltage Differential

In ordinary life, there are mainly interfaces designated N and W. Their "Differential" options provide increased noise immunity and increased allowable length of the SCSI bus. "Low-voltage" applies with the new Ultra2 protocol. "Single connector" is mainly used in hot-swap configurations, since combine SCSI power and ground signals in a single connector. Fiber Channel is more like a LAN interface than SCSI because it is a serial interface. The speed of 100Mb / s is quite common for him. Used in Hi-End configurations.

SCSI devices

It is not possible to list all SCSI devices, here are just a few of their types: hard disk, CD-ROM, CD-R, CD-RW, Tape (streamer), MO (magneto-optical drive), ZIP, Jaz, SyQuest, scanner. More exotic ones include Solid State disks (SSD) - a very fast mass storage device on chips and IDE RAID - a box with n IDE disks that pretends to be one large SCSI disk. In general, we can assume that all devices on the SCSI bus are the same and use the same set of commands to work with them. Of course as it develops physical layer SCSI also changed the programming interface. One of the most common today is ASPI. On top of this interface, you can apply drivers for scanners, CD-ROMs, MO. For example, the correct CD-ROM driver can work with any device on any controller as long as the controller has an ASPI driver. By the way, Windows95 emulates ASPI even for IDE / ATAPI devices. This can be seen, for example, in programs such as EZ-SCSI and Corel SCSI. Each device on the SCSI bus has its own number. This number is called SCSI ID. For devices on a narrow SCSI bus, it can be from 0 to 7, on wide, from 0 to 15, respectively. A SCSI controller, which is an equal SCSI device, also has its own number, usually 7. Note that if you have one controller, but Since there are both narrow and wide connectors, the SCSI bus is still one, and all devices on it must have unique numbers. For some purposes, such as CD-ROM device libraries, LUN is also used - the logical device number. If the library contains 8 CD-ROMs, then it has a SCSI ID, for example, 6, and logically CD-ROMs differ by LUN. For the controller, all this looks like SCSI ID - LUN pairs, in our example 6-0, 6-1, ..., 6-7. LUN support must be enabled in the SCSI BIOS if necessary. The SCSI ID is usually set with jumpers (although there are newer standards in SCSI like Plug & Play that do not require jumpers). They can also set parameters: parity check, turn on the terminator, power the terminator, turn on the disk at the command of the controller,

Installation

To install a SCSI controller and device, you need to have it as a minimum - they themselves and also a SCSI cable :). A free expansion slot in your PC may also come in handy, a free interrupt for this slot, 1-5 correct screws or screws, 2 to 8 different jumpers, a floppy drive or CD-ROM (already connected :) for the driver carrier. More complex configurations may include external SCSI cables, external terminators (see below), Wide-Narrow adapters, etc. There are often questions about the connectivity of Fast / Ultra / Narrow / Wide devices in various combinations. For the most common devices general rule in this case it is as follows: if the connectors match, then you can connect. In other words, in this case it is important to distinguish Narrow / Wide and ignore Fast / Ultra. (Aside, Ultra2 remains as it only exists in the LVD connector / interface variant). However, speed and reliability can drop dramatically. See SCSI Specifications / Interfaces above for details. In addition, there are various narrow-wide adapters, but their use is highly discouraged.

Controller

As already mentioned, the controller usually has a SCSI ID of 7. If you can think of a reason why this number needs to be changed, do it through the SCSI BIOS. You can also configure: support for ultra speeds, support for more than two drives, support for removable as a drive at boot time, etc. For each of the devices on the SCSI bus, you can configure: parity, power-on delay (so that all 7 disks are not turned on at the same time), maximum device speed. For non-PnP controllers on the ISA bus, do not forget to set the interrupt it uses in BIOS SETUP to "Legal ISA". For a PCI controller, check that it also got an interrupt, and it does not share it with anyone, although for latest models this is often not important.

Terminators

Perhaps someone remembers such a hard disk interface as ST506 (MFM / RLL), where the termination of the data cable on the last disk was just used. Terminators were also used in floppy drives, but for a very long time. The purpose of using terminators is to ensure matching of signal levels, to reduce attenuation and interference. They say that problems with terminators are the most common, but if you do everything carefully, they will not arise. Each SCSI device has the ability to enable or disable terminators. The exceptions are some scanners for which bus termination is permanently enabled and external devices with a pass-through bus. Terminator options:

1. internal. are usually found on hard drives. switched on by setting one jumper
2. automatic. most SCSI controllers have one. they themselves decide whether to turn on or not
3. in the form of assemblies of resistors. on some CD-ROMs and CD-Rs, they are. are disabled by removing all assemblies from the panels.
4. external. as in step 3, but more beautiful. for example, on an HP T4e tape drive. the device (usually external) has two SCSI connectors. one includes a cable to the controller, the other - a terminator or cable to the next device in the chain.

In addition, terminators can be passive or active. Today most are active, which provide great noise immunity and reliability at high speeds. You can usually determine which one is used on a SCSI device by the way it is turned on. If this is one jumper, or it is automatic, then it is most likely active. And if, to turn it off, it is necessary to pull out 1-2 resistor assemblies from the device, then it is passive. In principle, bus termination from different ends with different types of terminators is possible, but only at low speeds. By the way, this is another argument in favor of dividing slow and fast devices into different controllers or channels.

More details about terminators are written in the description of each device. Termination rules are often drawn in the adapter manual. The main thing is that the SCSI bus must be terminated at both ends. Here we will consider the most common options for devices on one SCSI bus (wide or narrow)

The simplest option: a controller and one device (external or internal - it doesn't matter). Terminators must be enabled on both the controller and the device (or device)

Option with multiple internal devices. Terminator is enabled only on the latter and on the controller.

There are both internal and external devices. Terminators are enabled on the outermost and outermost devices.

There are internally and several external devices. Terminators on the internal and the last external device

The situation is a little more complicated when narrow and wide devices are used simultaneously on the same controller (bus). Imagine that we have two 8 bits of the bus, which in fact are just the high and low bytes of the wide bus (in the descriptions and in the SCSI BIOS it is called High byte / Low byte). Now, following the above rules, both of these buses need to be terminated. Typically, in such cases, the high and low bytes of the wide bus can be independently terminated on the controller. In this situation, the narrow bus is a continuation of the low byte of the wide bus. Let's give one example:

Using Narrow and Wide devices on the same SCSI bus

In principle, this is possible, just pay attention to the termination. However, it is still better not to do this. Since fast (wide is usually UltraWide SCSI) and slow devices (narrow is usually only Fast SCSI or even SCSI-1) always coexist on the same bus is not good.

Homework: There are 3 connectors on the Wide controller: an outer and inner wide and an inner narrow. Three cables with devices can be connected to them. Question: on which devices do I need to enable terminators?

Using a Narrow device on a Wide controller (bus)

This option is quite workable. You only need to use a wide-narrow adapter, or it can be an external SCSI cable with a narrow connector on one end and wide on the other. Most often, such a need arises when connecting external narrow devices to a wide controller, since it usually has a wide external connector. If you do use adapters, pay attention to the termination! When connecting an external narrow device to the wide connector, the adapter should terminate high byte. If you connect a narrow device to an internal wide connector, then the adapter simply converts the connectors (i.e., reduces the number of wires from 68 to 50).

Hard drives

Connecting hard disks are very simple, you just need to take care of two things - the terminator and the SCSI ID. Usually, a new disc has termination enabled, and the number is set to 6 or 2. Therefore, if you put the first disc, then there is nothing to worry about, and if not, then you need to check these settings. Another note about SCSI IDs - older Adaptec controllers can only boot from 0 or 1.

The next stage of the installation is to format the disk. It is considered good practice to format the disc on a new controller before using it. This is due to the fact that different manufacturers of SCSI adapters use different sector translation schemes (can be compared with LBA, CHS, LARGE for IDE disks) and the disk may work poorly or not at all during the transfer. If the disk on the new controller does not work, try formatting it with the format command, and if it does not help, then from the SCSI BIOS (I personally have not seen such options).

If you are connecting more than two hard drives or drives larger than 2G, you may need to change the SCSI BIOS settings. When connecting removable devices such as the IOmega Jaz, you need to set the SCSI BIOS options to boot from them. The description of possible options is too long, maybe it will be given here later, but for now - read the descriptions, there is nothing terrible there :).

CD-ROM, CD-R, CD-RW

A driver is required for these DOS devices. It is usually installed on top of the ASPI driver. When working outside DOS, usually no drivers are required. Optionally, you can set the controller parameter to boot from CD. To work with CD-R / CD-RW devices in recording mode, you need special software (for example, Adaptec EZ-CD Pro).

Streamers

Similar to CD-ROM, SCSI tape drives can work with most operating systems with standard drivers. It is very fortunate that, for example, under WindowsNT, you can use the standard backup program, and not specialized software.

Scanners

Typically, scanners come with their own card. Sometimes it is completely "own", as, for example, in the Mustek Paragon 600N, and sometimes just the most simplified version of the standard SCSI. In principle, using a scanner with it should not cause problems, but sometimes connecting the scanner to another controller (if the scanner has such an opportunity) can be beneficial. Scanning A4 with 32-bit color at 600dpi is a picture of about 90 Mb and transferring this amount of information through the 8-bit ISA bus not only takes a lot of time, but also slows down the PC a lot, because drivers for this standard card are usually 16-bit (for example, Mustek Paragon 800IISP). A cheap FastSCSI PCI controller is usually used as an additional one. Less or more productive will not give you anything new. This option also has a note - you need to make sure that the scanner (or more importantly, its driver) can work with your new controller in your configuration. For example, Mustek Paragon 800IISP drivers are designed for your card or any ASPI compatible one.

When choosing a SCSI controller, you need to pay attention to several parameters (in random order and with great redundancy)

• your requirements and tasks
• compatibility
• popularity of the card manufacturer
• the prominence of the chip manufacturer
• availability of drivers
• technical support
• price
• advice from friends and acquaintances
• personal preferences
• appearance and equipment

FastSCSI PCI controller - Tekram DC-390. This controller is built on the basis of a well-known AMD chip, which guarantees operation under most operating systems with built-in drivers, but it can also be used from Tekram. There is a small and pretty SCSI BIOS.
Controllers based on the Symbios Logic SYM53C810 chip are well known to most operating systems. The SCSI BIOS for this is included in almost any AWARD BIOS for motherboards. Very cheap and nonetheless workable.

UltraWideSCSI PCI controller - Adaptec AHA2940UW. One of the most popular today, although it is already losing ground. However, it is still workable. Well, a little slow and expensive, but it works under all common operating systems.
Controllers on the Symbios Logic 53C875 chip. Many people note its speed and reliability.

Devices

The HDD - of course the Seagate Cheetah - is hard to argue with an RPM of 10,000. But this drive won't last long without additional cooling fans: (Other series of Seagate drives - Barracuda and Hawk - are also distinguished by their reliability.

The rest (CD-ROM, Tape, CD-R and others) - here everything is to your taste. SCSI devices are manufactured by many well-known companies. For example HP, Sony, Plextor, Yamaha.

In preparing the article, materials were used
companies IBM, Seagate, ASUSTeK, Tekram

What is SCSI?

A: To answer this question, see the [SCSI Basics] section.
What is SAS, which is better than SCSI or SAS, and how do they differ?
A: The [SAS or SCSI] section is devoted to the answer to this question.
What is eSATA?
A: eSATA is a SATA interface for connecting external SATA devices. It provides a 3 Gbps link to eliminate bandwidth bottlenecks inherent in modern devices external data storage.

What is Unified Serial?
A: All Unified Serial controllers allow you to connect SATA and SAS drives using a point-to-point interface. It uses an advanced SCSI command set to provide powerful data management, error handling, and performance.

The flexibility provided by support for SATA and SAS drives enables companies to easily standardize their I / O infrastructure for both primary storage of mission-critical data and secondary storage, depending on whether the drive is installed SATA or SAS. Customers can standardize their infrastructure with unified I / O controllers and storage systems, thereby reducing training and maintenance costs.

Can SATA drives be used with SAS controllers?

A: Yes, you can, while on one controller you can use both SAS and SATA disks at the same time. This allows you to start the transition to SAS technology now at a reasonable price.

Can SAS drives be used with SATA controllers?
Oh no.

Is it possible to connect SAS drives to the controller without using a hotswap basket?
A: Yes, you can. To do this, you need to use a special cable with an SFF-8482 connector on the side of the drives. The connector on the other end of the cable is identified 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 command set and the bus width (respectively, in speed).
SCSI-1 5MB / Sec 8 bit SCSI bus
SCSI-2 5MB / Sec 8 bit SCSI bus
SCSI-2 Fast 10MB / Sec 8 bit SCSI bus
SCSI-2 Fast Wide 20MB / Sec 16 bit SCSI bus
SCSI Ultra 20MB / Sec 8 bit SCSI bus
SCSI Ultra Wide 40MB / Sec 16 bit SCSI bus
Ultra2 Wide 80MB / sec 16 bit SCSI bus
Ultra160 160MB / sec 16 bit SCSI bus
Ultra320 320MB / sec 16 bit SCSI bus

When should you use a Low Voltage Differential (LVD) controller?
A: If:
Requires high data transfer speed - 80 - 320 MB / s
The surrounding area is very high in electromagnetic noise, which can interfere with data transmission. LVD mode provides much better noise immunity than Single Ended (SE) SCSI
It is necessary to ensure that SCSI devices are significantly removed from the computer. LVD devices can be removed from the SCSI controller at a distance of up to 12 meters (this is the maximum allowable length of an 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 there should be exactly two of them (terminators) for each SCSI bus. Most often, the first SCSI Terminator is the SCSI controller (as a rule, this function can be "disabled" in the controller BIOS, and by default it is enabled), and the second is the terminator connected to the last (from the SCSI controller) SCSI cable connector.

Some SCSI devices (legacy disks, floppy drives, tape drives) have a built-in terminator that can be enabled with a corresponding jumper on the device. In this case, make sure that the device with the terminator enabled is located at the very end of the SCSI bus.

And everything works for me without a SCSI terminator, maybe it will work like that?
A: For the time being, it may do, especially if you have only one disk and it is not used too intensively. But with an increase in the number of devices on the SCSI bus, or with an increase in the load on it, you, in the end, run the risk of losing data, so it is not worth saving on this.

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

The SCSI ID is assigned either automatically (for example, if hotswap cages that support this function are used), or by manually setting the appropriate jumpers on SCSI devices. SCSI ID has nothing to do with the physical order of devices on the SCSI bus (for example, a SCSI controller, as a rule, has a default SCSI ID of 7, although most often, but not always, it is located at the beginning of the SCSI bus), it is only important so that there are no devices with the same SCSI ID on the same SCSI bus.

SCSI ID values ​​can be:
from 0 to 15 (16 in total) for Wide (W) and UltraWide (UW, U2W, U160, U320) SCSI buses;
from 0 to 7 (8 in total) for Narrow (U, U2) SCSI bus;

What happens if two devices with the same SCSI ID are connected to the same SCSI channel?
A: Nothing good. In the best case, the SCSI controller will recognize one of these devices, but it will still not be able to work correctly with it, in the worst case, it will not "see" any of these devices. Neither the controller nor the disks will be damaged, but the risk of corrupting data on SCSI disks remains.

It should be borne in mind that the overwhelming majority of controllers do not report such an error in any way, so when connecting new devices to the SCSI bus, you should pay attention to the uniqueness of the SCSI ID.

Please note that the SCSI controller itself also has a SCSI ID (as a rule, it is 7, and can be changed to BIOS-e controller), so don't assign the same SCSI ID to the drives.

What is SAF-TE?
A: SAF-TE - SCSI Accessed Fault-Tolerant Enclosure - An "open" specification designed to provide a comprehensive and standardized method for monitoring and reporting the health of drives, power supplies, and cooling systems used in high reliability servers and storage subsystems. The technical requirements are independent of hardware I / O, operating systems, and server platform, because the chassis itself appears to be just another device on the SCSI bus. SAF-TE specifications have been adopted by many of the leading manufacturers of servers, storage devices and RAID controllers. Products that meet the SAF-TE specification reduce the cost of monitoring enclosure health, simplify the network administrator's job, and provide alarm notifications and equipment health information.

28. 07.2017

Dmitry Vassiyarov's blog.

SCSI is a fast and unusual interface

Hello.

From this article, you will learn the essentials about SCSI, what it is, where and why it is used, how many generations have passed since its inception, and how it is implemented in practice.

Read it - what if SCSI comes in handy for you too?

What does SCSI stand for?

This is a set capital letters from the phrase Small Computer Systems Interface. In Russian it sounds like "fairy tale", and the decryption is a system interface for small computers.

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

Other possibilities

In addition to simple hardware connection, the technology allows data exchange and defines a set of commands, which has become widespread. For example, in Windows it is used in a single stack for storage devices.

The most commonly used commands are writing, reading, checking devices, requesting their characteristics, setting new parameters for them or returning previous ones, etc.

There is also the implementation of commands over wires and controllers of other standards. When it comes to IDE, ATA or SATA, it is called ATAPI - ATA Packet Interface; if the top of the USB protocol is Mass Storage device. Thus, you can, for example, connect an external hard drive via normal USB and it will use the SCSI driver available in the OS.

Where is SCSI in demand?

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

But naturally, no one forbids you to put such tales of the device into your home computer. Or for example to a home server.

Technology in practice

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

The presence of a SCSI adapter means that some of the load is removed from the central processor, therefore, the hardware works faster.

Since this technology is consistent, devices should be connected accordingly. Moreover, each must have a unique ID, and they all have the same interface.

History of appearance

I want to tell you the story of creating an interface not out of my tediousness, but because through it you can understand more about the subject of our conversation.

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

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

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

After 2 years, he shared his development with the ANSI committee (American National Standarts Institute) - the same as GOST in our country. Based on this invention, ANSI specialists created SCSI.

Generations of the interface

It is noteworthy that the technology was created almost half a century ago, and we are still talking about it. All because she was constantly changing. Since its inception, 10 versions have been released. I will not bother you with details about each of them. I will only tell you what was originally, and what we have now.

SCSI-1

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

Ultra-640 SCSI

• The bus width has doubled, so you can connect up to 16 devices at the same time.
• Its frequency is 160 MHz DDR.
• The speed also cannot be compared with the first modification - now it reaches 640 Mb / s.
• The connector has 68 pins.
• The cable length is up to 10 m.

Serial Attached SCSI (SAS)

• Added support for connecting SATA devices.
• The speed of this interface has already grown to 12.0 Gbps.
• According to the developers, it is now possible to connect 16384 devices on one bus! In the previous generation, as described above, there were only 16.

Electrician

There are 3 ways to transmit information about an electrician:

• SE (single-ended) - asymmetrical view. Each signal is sent on a separate line.
• LVD (low-voltage-differential) is a low voltage differential standard. Signals "+" and "-" are forwarded on different wires. One twisted pair is assigned to each of them. They are transmitted under a voltage of ± 1.8 V.
• HVD (high-voltage-differential) is an analogue of the previous version, but with special transceivers and increased voltage.

The interface is loaded with terminators located at both ends of the bus. According to their electrical characteristics, they are divided into:

• Passive - simple 132 ohm resistors;
• Active - stabilizers that produce the required signal, and each power line 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 used in high frequency interfaces.

The 2nd model is most often used.

SCSI competitiveness

The SCSI standard has stood the test of time and is still popular today. Why?

• Has a high speed;
• You can create a chain of 15 devices;
• They are easy to manage;
• HDDs are highly reliable.

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

• High cost. But you need to understand that you are paying for quality. Although SATA hard drives offer more capacity for a lower price point, they don't have the same durability.
• Obsolescence. An improved competitor has appeared - SAS (Serial Attached SCSI) technology, which has more compact wires, does not need terminators, allows you to connect more devices and has better bandwidth.

That's all.

I am waiting for you on the blog pages as often as possible.

SCSI (Small Computer Systems Interface - System interface for small computers, pronounced in Russian as "skazi") is an interface designed to combine into unified system devices of various profiles: hard drives, scanners, streamers, CD-ROMs, etc. The essence of the interface is to provide a flexible mechanism for controlling these devices and the maximum speed of their operation as a single but divisible mechanism.

The roots of the SCSI interface go back to 1979, when the manufacturer of storage media M. Shugart was puzzled to find a universal interface standard for their disks, taking into account possible needs in the future. In the laboratories of M. Shugart, as a result, an interface was developed that supported logical and physical (head / cylinder / sector) addressing, based on the protocols of 8-bit parallel data transfer over an interface consisting of several lines. This interface was named SASI (Shugart Associates Systems Interface). The interface, in addition to describing the protocols, also included several 6-bit commands; the downside was that the interface was designed to use only one host-device pair.

Later, in 1981, M. Shugart submitted the documentation on the SASI interface to the ANSI committee (American National Standarts Institute, an analogue of GOST), which took it as the basis for working on the project, which was named SCSI. Most of the most important points from the SASI standard migrated to SCSI, for example, such important principles as device arbitration, bus release mechanisms, the ability to use more than one host adapter on the bus, etc. In 1984, the working documentation of the SCSI standard was submitted to ANSI, and, after numerous corrections and additions, in 1986 a document was adopted under the number X3.131-1986 - the first official SCSI standard, which is now called SCSI-1. In addition to the SASI standard, SCSI-1 is "overgrown" with such important functionality as 10-bit commands, synchronous and asynchronous data transfer protocol, the ability to connect to one host adapter up to 8 different devices. The standards that followed SCSI-1 evolved both in the direction of expanding the command language and increasing and complicating protocols, as well as increasing bus width, increasing speed and the number of devices connected to a single host adapter. For the 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 adopted by mainly hard drive manufacturers. In fig. Figures 1, 2 show some of the first examples of SCSI disks.

Rice. 1, 2. The first samples of SCSI drives - made by SONY (capacity 40 megabytes)
and Quantum (capacity 120 megabytes)

A brief history of the SCSI standard

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 advanced controls and at the same time is not focused on any particular type of device. Has an 8-bit data bus, the maximum transfer rate is up to 1.5 MB / s in asynchronous mode (by the "request-confirmation" method), and up to 5 MB / s in synchronous mode ("multiple requests - multiple confirmations" method) ... Parity can be used to detect errors. It is electrically implemented as 24 lines (unipolar or differential), although the vast majority of devices use unipolar signals.

SCSI-2 is a significant evolution of basic SCSI. Increased transfer speed (up to 3 MB / s in asynchronous mode and up to 10 MB / s in synchronous mode) - Fast SCSI. Added new commands and messages, made parity support mandatory. The possibility of expanding the data bus up to 16 bits (Wide SCSI) was introduced, which provided the speed up to 20 MB / s. A new 68-pin connector has been introduced. The subsequent specification, SCSI-3, not only introduced new baud rates, but also greatly expanded the command set. In addition, along with the traditional parallel bus interface, other parallel and serial protocols such as Fiber Channel, IEEE 1394 Firewire and Serial Storage Protocol (SSP) can be used as a transmission medium.

Ultra SCSI interface, uses a bus frequency of 20 MHz. The Ultra / Wide SCSI interface supports 16 devices and provides transfer rates up to 40 MB / s. Faster Ultra-2 Wide SCSI with transfer rates up to 80MB / 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 16-bit bus width, and up to 16 devices can also be connected to the interface. Comparative characteristics SCSI standards are shown in Table 1.

Table 1. Comparative characteristics of SCSI standards

Standard	Maximum bus speed, MB / sec.	Bus width	Maximum cable length, m			Maximum number of devices
			The only device	LVD	HVD
SCSI-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 SCSI	80	16	(4)	12	25	16
Ultra-3 SCSI,orUltra-160 SCSI	160	16	(4)	12	(5)	16
Ultra 320 SCSI	320	16	(4)	12	(5)	16
Ultra 640SCSI	640	16	(4)	(7)	(5)	16

What is a Host Adapter?

A host adapter is a device connected to a PC bus that provides a host (the meaning of the word "host" in relation to the standards describing data transfer interfaces (English host), most fully describes the phrase "bus master") communication with SCSI devices. The name "adapter" was not chosen by chance - this indicates that all the logic of device operation is located in peripherals on the bus; for devices called "controller" the logic is located in them.

The following manufacturers make or have released SCSI host adapters in the past:

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

Rice. 3. SCSI host adapter from Adaptec

Modern SCSI HDD manufacturers

Currently, the HDD market is undergoing rapid evolution - new, high-speed Serial ATA standards are replacing Parallel ATA. And, although the new SATA devices are already very close in terms of operating speed to SCSI devices, and in some places they outrun them, SCSI devices are still popular in High-End computers - servers and information arrays. This is due, first of all, to the high reliability of SCSI drives - both due to the relative simplicity of SCSI standards and a well-thought-out electrical interface, and in connection with the traditionally more thorough design and production study of devices. SCSI accounts for approximately 30 percent of the total hard disk drive market, and it is unlikely that it will ever step over this milestone: PC equipment with all the necessary cables, adapters, as well as the purchase of the host adapter itself will cost about $ 100, while drives will cost several times more their IDE counterparts. Today's SCSI disk manufacturers are:

Competition in the SCSI disk market is not great - most likely because the market is sufficiently full and does not develop as rapidly as the market for IDE devices - and this is primarily due to the fact that SCSI devices are used most often in servers. the demand for which is not so great. The convenience of SCSI devices is that they can be easily replaced on the fly, without shutting down and losing the server's performance. This is very important for servers, and not at all necessary for workstations. As a rule, servers (Fig. 4) are equipped with special sleds (Fig. 5), into which the disk in a special mount (Fig. 6) can be inserted very easily.

Rice. 4. Silver equipped with SCSI disks

Rice. 5. SCSI drive bay

Rice. 6. SCSI drive bracket used in hot-swappable servers

It is worth noting that very often server manufacturers relabel drives by giving them their brands. As an example, I will cite drives taken from Hewlett Packard and IBM e-Server servers (Fig. 7, 8), on which the real manufacturer of hard disk drives can be recognized only by the model name; I also saw discs extracted from Dell servers, which did not even have this information.

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

SCSI connector types

Rice. 9. Currently Used SCSI Connector Types

SCSI devices can have Various types connectors for their connection to the host adapter (see Fig. 9) - this is primarily due to design features the device itself. The most commonly used HDD connector is HD68 (Fig. 10), a little less often - SCA80 (Fig. 11). In the distant past, in the late 80s and early 90s, almost all SCSI drives were connected to the host via the HE50 connector (Figure 12). Currently, this connector is practically not found.

Rice. 10. HD68 connector.
Rice. 11. Connector SCA80.
Rice. 12. Connector HE50.

To connect devices of various connector configurations to the bus, specialized adapters are often required. Such adapters, for example, are produced by SCS (http://www.scaadapters.com), their cost ranges from $ 10 to $ 35 per piece. The complete set for working with any SCSI device is shown in Fig. 13, fig. 14 - 18 each adapter is shown separately

Rice. 13. Adapters required for connecting SCSI devices

Rice. 14 - 18. Same as fig. 13 separately.

How SCSI Works

To match the loads on the SCSI bus, terminators are used, which, according to their electrical properties, are divided into passive, active, and FPT terminators. Terminators must be powered, so the interface has Terminator Power lines. The passive terminators used in SCSI-1 devices are conventional 132 ohm resistors. Active terminators are a regulator that generates the desired signal - each line is connected to this regulator through a 110 ohm resistor. Currently, only active terminators are used, while auxiliary voltage sources are used - for these purposes, auxiliary diodes are usually used, which fix the voltage of the input signals at the required level. Finally, FPT (Forced Perfect Terminator) terminators are an improvement of active terminators, equipping them with emission limiters. Their use is in high frequency SCSI versions.

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

1. Standard initiator - standard performer
2. Extended initiator - extended executor
3. Standard initiator - extended performer
4. Extended Initiator - Standard Executor

When connecting standard performers to extended initiators, problems cannot arise - the extended standard supports all the functions of the standard, however, when connecting back, difficulties may arise with connecting terminators. In real life, these problems are easily solved by using adapters (see above).

SCSI bus states are usually divided into phases. There are only five such phases: the bus is free, arbitration (in this case, the initiator can get control of the bus), selection (in this case, the initiator who entered the arbitration phase first chooses the executor for further work), re-election (the executor confirms to the initiator that he was chosen by him for work and ready to work) and information phase (request-transfer of commands, data, messages). A block diagram of the phase sequence of one cycle of operation on the SCSI bus is shown in Fig. 19.

After the selection phase, the initiator can timeout, for which it can use two methods - perform a hard reset or go into the "bus free" phase. In any case, the end of the cycle of work on the SCSI bus will be setting the status "command completed" or sending the corresponding message with the release of the bus. Similar to the ATA standard, SCSI systems can use two protocols, a hard reset and a soft reset. In both cases, a one bit will be set on the Reset line, the differences in the types of resets lie in their mechanism and purposes - as a rule, a hard reset is performed to reset operations across the entire SCSI device system, while a software reset is used to reset only one device, not interfering with the work of others.

Rice. 19. Block diagram of the phase sequence of the SCSI bus

The SCSI bus uses nine control signals: BSY (Busy), SEL (Selection), C / D (Command / Data), I / O (Input / Output), MSG ( Message, REQ (Request), ACK (Acknowledge), RST (Reset), ATN (Attention). Busy, Select, and Reset signal sources can be both the initiator and the performer; only the performer can be the source of the Confirmation signal; the rest of the signals are the prerogative of the initiator. The types of information transfer are coded by combinations of bits set for the signals Message, Control / Data, I / O, as shown in Table. 2.

Table 2. Types of information transfer on the SCSI bus

The interface is driven by a message system. There are 28 of them, they can be single-byte, double-byte (one word) and extended. The message system is detailed in any SCSI standard.

For selection specific device an identifier bit exists on the SCSI bus. As a rule, SCSI devices have hardware configuration, that is, the system identifies the device by the jumpers installed on it. The limitation on the number of connected devices in the standard (8 bit) and extended (16 bit) SCSI version is imposed precisely by the existence of the identifier bit - in the 8 or 16 bit bus it is impossible to set more than 8 or 16 identification bits, respectively, and this also includes the identifier bit host adapter - that is, in other words, in addition to the host adapter on the bus, there can be 7 more devices for the standard SCSI, and 15 for the extended one.

SCSI commands

Command

Command code

CHANGE DEFINITION Compare (COMPARE) Copy (COPY) COPY AND VERIFY Format (FORMAT UNIT) Request (INQUIRY) LOCK-UNLOCK CACHE Log selection (LOG SELECT) Log Sensitivity (LOG SENSE) Mode selection (MODE SELECT) Mode Sensitivity (MODE SENSE) Preamplification (PRE-FETCH) Prevent permission to change media (PREVENT-ALLOW MEDIUM REMOVAL) Reading (READ) Read buffer (READ BUFFER) Show capacity (READ CAPACITY) Read defective data (READ DEFECT DATA) Read Long (READ LONG) Reassign block (REASSIGN BLOCK) Accept diagnostic results (RECEIVE DIAGNOSTIC RESULTS) Free (RELEASE) Request sensitivity (REQUEST SENSE) Reserve (RESERVE) Reset device (REZERO UNIT) Find the same data (SEARCH DATA EQUAL) Find senior data (SEARCH DATA HIGH) Find Lowest Data (SEARCH DATA LOW) Positioned (SEEK) Diagnostic request (SEND DIAGNOSTIC) Set limit (SET LIMIT) START STOP UNIT Synchronize cache (SYNCHRONIZE CACHE) Device readiness request (TEST UNIT READY) Verification (VERIFY) Write (WRITE) WRITE AND VERIFY Writing to the buffer (WRITE BUFFER) Long record (WRITE LONG) 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 above lists the basic SCSI standard commands applicable to hard disk drives. As in the ATA standard, for the SCSI standard there are both mandatory commands, that is, those that must be supported by any SCSI device, and optional, optional commands, which the device may not support. In addition to them, there are so-called vendor commands that are not described in the standard, specific to each manufacturer and often for each specific line of devices - commands that the manufacturer uses for the purpose of repairing or diagnosing a device. These commands are, as a rule, a trade secret of the manufacturer and are not published anywhere.

SE, LVD, HVD

Typically, you will find markings on a SCSI device like the one shown in Fig. 20. This marking indicates the type of data transmission at the electrical level. The first - SCSI SE (Single Ended), denotes this type of data transfer, when each signal on the bus is provided by one conductor. SCSI LVD (Low Voltage Differential) and SCSI HVD (High Voltage Differential) - low-voltage and high-voltage differential types - are physically organized in the same way: for each signal there are two conductors, one has a positive polarity signal, one negative. The difference between HVD and LVD is in the voltage in the conductors, for LVD it is lower than for HVD.

Rice. 20. Designations on SCSI devices that carry information about the electrical type of data transfer

It is logical that HVD and LVD devices are incompatible - if you connect an LVD device to the bus of an HVD device, the first one will inevitably die due to the overvoltage of the signal. The same can be said about SE and LVD devices - the cables for them are the same, but due to electrical characteristics they are not compatible. However, LVD devices can be connected to SE conductors, since they recognize bus voltages and if they receive a bipolar signal in one pair of conductors, they can switch to using it. As a rule, devices that can operate in both modes are marked with a special LVD / SE icon.

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).

The continuous increase in the bus clock speed has 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 inconvenient when using external high-speed SCSI devices, but more than enough to ensure the connection of devices inside the PC case.

Synopsis. Prospects and opportunities

The SCSI interface is very efficient and reliable, but it also has a lot of disadvantages. First of all, this is the high cost of the devices themselves - both drives and controllers. The next drawback is the complexity of configuration and management, which only trained people can handle. Finally, the last drawback of the interface, which makes it even less attractive to the user, is the impossibility of transferring the media to another PC if it is not equipped with a specialized SCSI adapter ...

The use of SCSI devices is impractical for the standard PC market for a very simple reason: the high price. However, manufacturers do not set themselves the goal of conquering the average consumer: it so happened historically that SCSI drives are mainly a server standard, and IDE is a workstation standard.

Meanwhile, SCSI drives are stepping on their heels latest standard IDE devices: SATA. The speed and performance of SATA devices are very high, and their use in servers is becoming more and more popular. The only drawback of SATA is a rather flimsy connector, which is associated with rather frequent failures of these devices. I think that the SCSI interface will undoubtedly win the battle with SATA in the field of server drives.

The development of the SCSI standard promises us in the future more high-speed devices of traditional SCSI reliability; there is no reason to predict a quick exit of SCSI devices from the market.

Serial Attached SCSI (SAS)

The latest trend in the world of SCSI devices is Serial Attached SCSI, an interface that uses three data transfer 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 actual transfer of data, the latter is intended to control the interface. Drives with this interface are now manufactured by Seagate, Samsung and Fujitsu.

A feature of this interface is that the signal is transmitted not over two (as in SATA), but over four conductors (one pair for receiving the signal, the other for sending it). The declared data transfer rates are 1.5 and 3.0 GB / s.

SCSI interface

In the early 1970s, an interface was developed for a mini-computer (at that time it was really a mini-computer) SCSI(read "tales"), the name of which stands for Small Computer System Interface. Again, the name shows its "portability". The original version assumed the exchange rate of 5 Mb / s, and the devices were connected using a 50-wire cable. Subsequently, various additions and improvements were made to SCSI to increase the exchange rate - today's SCSI controllers support speeds up to 160 Mb / s, that is, more than the standard PCI bus... And the very first SCSI standard, of course, is already outdated and now almost no one remembers about it. There are quite a few SCSI options, and they all have different and confusing names (the situation is almost the same as with IDE), so we will not consider each separately, but summarize the main provisions in a table.

Bandwidth easy to calculate: for this you just need to take the numerical value of the frequency, and in the case of Wide, multiply it by two. For example, an UltraSCSI controller (often referred to as Ultra SCSI-2) has a speed of 20 Mb / s. These tables are already outdated to some extent, since, according to it, the maximum is 80 Mb / s (Ultra2 Wide SCSI, or simply UltraWide SCSI, since now Wide SCSI controllers are not produced not Ultra2, and Ultra means Ultra2 by default), but the speed of 160 Mb / s has already become widespread (this standard is called Ultra160 SCSI). These standards are only available with an LVD interface ( Low Voltage Differential), providing increased noise immunity and increased allowable SCSI cable length.

These are the typical connectors found on SCSI controllers:

Internal
Low-Density 50-pin	Connecting slow internal devices - old HDDs, almost all CD / DVD-ROM, CD-R, MODD, ZIP, etc. (like IDE, only 50 pins)
High-Density 68-pin	Connecting internal wide devices, mainly HDD
External
DB-25	Connecting external slow devices, mainly scanners, IOmega Zip Plus. Most common on Mac. (like a modem). Deprecated
Low-Density 50-pin	Or Centronics 50-pin. External connection of scanners, streamers, usually SCSI-1 (the very first SCSI option). Like SCSI-1 itself, it is already deprecated
High-Density 50-pin	Or Micro DB50 or Mini DB50. Standard external connector for connecting scanners, external CD-ROMs, old HDDs, etc.
High-Density 68-pin	Or Micro DB68, Mini DB68. Standard external wide connector, mainly for HDD connection
High-Density 68-pin	Similar to the previous one (practically not used anywhere)

There is also another type of connector - CL, or Single Connector having 80 contacts. Single Connectors are used primarily in host-swap configurations (when hot swapping may be required) because they combine the SCSI power and ground signals in a single connector.

In addition to the above interfaces, there is also the so-called Serial SCSI technology-based Fiber Channel... The devices are connected to the controller using a 6-core cable and can be exchanged at speeds of 100 Mb / s or more. Serial SCSI controllers (also often referred to as SCSI-3) also have regular narrow and wide connectors, allowing standard SCSI devices to be connected. Fiber Channel is more like a networking standard than a media interface; it uses serial data transmission. For more information on Fiber Channel technology, see the article Fiber Channel Technology.

For the operation of any device, as you know, software support is required. For most IDE devices, it is built into the motherboard BIOS; for the rest, drivers are required for various operating systems. For SCSI devices, things are a little more complicated. To boot from a SCSI hard disk for the first time and work in DOS, you need your own SCSI BIOS. There are 3 options here:

• The microcircuit with SCSI BIOS is on the controller itself (like on video cards). When the computer boots up, it is activated and allows you to boot from a SCSI hard disk or, for example, CD-ROM, MO. When using a non-trivial operating system (Windows NT, OS / 2, * nix), drivers are always used to work with SCSI devices. They are also required for non-hard disk devices to run under DOS.
• The SCSI BIOS image is flashed into the Flash BIOS of the motherboard. Typically, a SCSI BIOS is added to the BIOS of the board for controllers based on the most common chips. It can be flashed and thereby change the SCSI BIOS version to a newer one. If there is a SCSI controller on the motherboard, this is exactly the approach. This option is also more cost-effective - a controller without a BIOS chip is cheaper.
• There is no SCSI BIOS at all. All SCSI devices are supported only by operating system drivers. Loading from them, of course, is impossible. This approach is used when creating your own controller for an external device (for example, a scanner), that is, when downloading from a device does not make sense and the use of drivers is supposed in any case.

In addition to supporting booting from SCSI devices, the BIOS usually performs several other functions: configuring the adapter, checking disk surfaces, low-level formatting, configuring initialization parameters for SCSI devices, setting the boot device number, and so on. The SCSI BIOS often needs to store the configuration of SCSI devices as well. This role is usually performed by a small microcircuit such as 93C46 (flash). It connects to the main SCSI chip. It has only 8 legs and several tens of bytes of memory, but its contents are retained even when the power is turned off (similar to CMOS on a motherboard). In this chip, the SCSI BIOS can store both the SCSI device parameters and its own settings. In general, its presence is not associated with the presence of a microcircuit with a SCSI BIOS, but, as practice shows, they are usually installed together.

There are also powerful server controllers. In addition to the mandatory support of the highest speed modes, they usually have support for RAID, hot-swap drives and an additional SCSI channel, which allows you to increase the number of connected devices. Often they also install a hardware cache of a kind of a megabyte of 32, 64 or more. In the picture on the left you can see such a controller from ASUSTeK (a good, by the way, firm). The 486 processor is very clearly visible on its board, which, apparently, is trying to control all this stuff.

You can also find a SCSI bus activity LED and / or a connector for its connection on the SCSI controller board, and if there is cache support, then sometimes slots for memory modules. It happens that they also install an additional IDE controller, sound card or VGA card. Very old controllers sometimes have connectors for floppy disk drives.

Various not too hasty devices (usually scanners) in the kit often come with their own SCSI controller. As a rule, it has an extremely simplified configuration: it is designed for only one device and works only with it, does not have a BIOS, it works only with its own driver and without interruptions (polling mode). From an economic point of view, this is quite justified, since it provides a lot of opportunities (it is clear that even the most primitive SCSI version is better than LPT or USB) with minimal salaries. But, on the other hand, this is not good, because you cannot use anything except your own device with a 100% guarantee. Although no one really needs it - all the same, the speed of operation of such cards is not high, and if you really need SCSI, you will have to buy something more serious.

Devices are connected to the controller with the appropriate (narrow or wide) cable along the chain (similar to IDE). This also applies to external devices, only here you can draw an analogy with a serial connection of, say, a scanner and a printer to a parallel port. It is not necessary to pay special attention to the speed indicators of the media, since in most cases the rule applies: "If the connector fits, then it will work." However, in this case, the bus may slow down, therefore, if possible, it is better to connect slow devices to one connector, fast ones to another. Naturally the controller must somehow distinguish between devices connected to the same cable in order to establish communication with them. To do this, each device has its own logical number, which is called SCSI ID... For devices on a narrow SCSI bus, it can be from 0 to 7, on wide, from 0 to 15. A SCSI controller, which is a peer SCSI device, also has its own number, usually 7. Note that if you have one controller, but there are both narrow and wide connectors, then the SCSI bus is still one, and all devices on it must have unique numbers. For some purposes, for example, for CD-ROM device libraries, LUN is also used - the logical device number. If the library contains 8 CD-ROMs, then it has a SCSI ID, for example, 6, and logically CD-ROMs differ by LUN. For the controller, all this looks like ID - LUN pairs, in our example 6-0, 6- 1, ..., 6-7. LUN support, if necessary, must be enabled in the SCSI BIOS. The SCSI ID number is usually set using jumpers or through the same BIOS (automatically or manually), since there are new standards in SCSI, similar Plug & Play, which do not require jumpers. You can also set the parameters for the type of parity check (if the controller supports it), enable the terminator, power the terminator (see below), turn on the disk at the command of the controller, etc. Again, often all this can be done in software. way through the BIOS. The principle of using ID is good because it does not require interrupts. For SCSI operation, only one interrupt is enough (for the controller itself), which, unlike IDE, allows you to save this resource and therefore makes it possible to install more equipment.

Now, as promised, about the terminators. In short, these are gizmos that are placed at the ends of the tire. The purpose of using terminators is to ensure matching of signal levels, to reduce attenuation and interference. They say that problems with terminators are the most common, but if you do everything carefully, they will not arise. Each SCSI device has the ability to enable or disable terminators. The exceptions are some scanners, for which bus termination is enabled forever, and external devices with a pass-through bus. Terminator options:

• Internal. Usually present at hard drives; switched on by setting one jumper
• Automatic. Most SCSI controllers have these. They decide for themselves whether to turn on or not.
• In the form of resistor assemblies, this is what some CD-ROMs and CD-Rs have. Disabled by removing all assemblies from the panels.
• External. As in the previous paragraph, but more beautiful (for example, on an HP T4e streamer). The device (usually external) in this case has two SCSI connectors: one connects the cable to the controller, the other - a terminator or cable to the next device in the chain.

The last two types, however, are already outdated and do not apply. In addition, terminators can be passive or active. Today almost all are active, they provide greater noise immunity and reliability at high speeds. You can usually determine which one is used on a SCSI device by the way it is turned on. If this is one jumper, or it is automatic, then most likely it is active. And if, to turn it off, it is necessary to pull out 1-2 resistor assemblies from the device, then it is passive. In principle, termination of the bus from different ends with different types of terminators is possible, but only on low speeds... By the way, this is another argument in favor of dividing slow and fast devices into different controllers or channels.

More details about terminators are written in the description of each device. Termination rules are often drawn in the adapter manual. The main thing is that the SCSI bus must be terminated at both ends. We will consider the most common options for devices on the same SCSI bus.

The simplest option: a controller and one device (external or internal - it doesn't matter). Terminators must be enabled on both the controller and the device.

Multiple option internal devices... Terminator is only enabled on latest device and on the controller.

There are both internal and external devices. Terminators are enabled on the outermost internal and external devices, but disabled on the controller.

There is an internal and several external devices. Terminators are enabled on the internal and the last external device.

The situation is a little more complicated when narrow- and wide-devices are used simultaneously on the same controller (bus). Imagine that we have two 8 bits of the bus, which in fact are just the high and low bytes of the wide bus (in the descriptions and in the SCSI BIOS it is called High byte / Low byte). Now, following the above rules, both of these buses need to be terminated. Usually, in such cases, the high and low bytes of the wide bus can be independently terminated on the controller. In this situation, the narrow bus is a continuation of the low byte of the wide bus. Let's give one example:

Narrow devices can also be used on a wide bus even when the controller does not have the required connector (both external and internal). You only need to use a wide-narrow adapter, or it can be an external SCSI cable with a narrow connector on one end and wide on the other. Most often, such a need arises when connecting external narrow-devices to a wide-controller, since it usually has an external wide-type connector. If you are using adapters, pay attention to the termination. When connecting an external narrow device to a wide connector, the adapter must terminate high byte. If a narrow device is connected to an internal wide connector, then the adapter simply converts the connectors (that is, it reduces the number of wires from 68 to 50). True, as already noted, termination is often performed by the controller itself and devices in automatic mode, and problems should not arise; this information is provided as a guide rather than practical information.

In various conferences and FAQs, a question like "Which is better: IDE or SCSI?" Is often asked. The answer to it is very simple, but with a small and very important addition: "It depends on what for." Here are the main advantages of SCSI over IDE:

Higher data transfer rate

Simultaneous work with all devices is possible, wherever they are and no matter how they are connected

The cable length can be 3-6 meters

Generally higher reliability compared to IDE for both controllers and SCSI devices

Ability to use external devices

The maximum number of devices (up to 15) is significantly more than that of IDE, in addition, you can install several SCSI controllers (usually no more than four)

All SCSI devices need only one interrupt

To improve reliability and performance, you can use caching and technologies RAID and host-swap. True, recently similar IDE controllers have begun to appear, but they are certainly not as good as those of SCSI.

However, for all its glory, SCSI is an expensive interface. This applies to both controllers and devices. Before deciding if you need SCSI, you need to understand your goals. For work, say in Microsoft Office, you do not need high performance at all. In addition, the benefits are only very noticeable in the case of active multitasking. It should also be remembered that for a little money you can buy a much faster and more capacious hard disk with IDE interface than with SCSI. But if you do video editing, CD recording, complex graphics, or just want maximum performance in your favorite Unreal (or whatever is your favorite), then SCSI, of course, is worth the money spent. In general, decide for yourself. if your finances are in a deplorable state, then there is nothing special to decide ...