Hard drive interfaces: SCSI, SAS, Firewire, IDE, SATA. New horizons: connecting IDE hard drives to SCSI adapters

In this article, we'll look into the future of SCSI and look at some of the advantages and disadvantages of SCSI, SAS, and SATA interfaces.

In fact, the issue is a bit more complex than just replacing SCSI with SATA and SAS. Traditional parallel SCSI is a tried and tested interface that has been around for a long time. Currently, SCSI offers a very fast data transfer rate of 320 Megabytes per second (Mbps) using the modern Ultra320 SCSI interface. In addition, SCSI offers a wide range of features, including Command-Tag Queuing (a method of optimizing I/O commands to increase performance). SCSI hard drives are reliable; in a short distance, you can create a daisy chain of 15 devices connected to a SCSI link. These features make SCSI an excellent choice for high performance desktops and workstations, up to and including enterprise servers, to this day.

SAS hard drives use the SCSI command set and have the same reliability and performance as SCSI drives, but use a serial version of the SCSI interface at 300 Mbps. Although slightly slower than 320 Mbps SCSI, the SAS interface is capable of supporting up to 128 devices over longer distances than the Ultra320 and can expand to 16,000 devices per channel. SAS hard drives offer the same reliability and rotation speeds (10000-15000) as SCSI drives.

SATA drives are a little different. Where SCSI and SAS drives focus on performance and reliability, SATA drives trade them off in favor of massive capacity increases and cost reductions. For example, a SATA drive has now reached a capacity of 1 terabyte (TB). SATA is used where maximum capacity is needed, such as data backup or archiving. SATA now offers point-to-point connections at speeds up to 300 Mbps, and easily outperforms the traditional parallel ATA interface at 150 Mbps.

So what will happen to SCSI? It works great. The problem with traditional SCSI is that it's just coming to the end of its useful life. Parallel SCSI at 320 Mb/s will not run much faster on current SCSI cable lengths. In comparison, SATA drives will reach 600 Mb/s in the near future, SAS have plans to reach 1200 Mb/s. SATA drives can also work with the SAS interface, so these drives can be used simultaneously in some storage systems. The potential for increased scalability and data transfer performance far exceeds that of SCSI. But SCSI isn't going away anytime soon. We will see SCSI in small and medium servers for a few more years. As hardware upgrades, SCSI will be systematically replaced by SAS/SATA drives to get faster and more convenient connections.

SCSI - Small Computer System Interface

Despite the seeming dominance of devices with IDE/EIDE interface, SCSI hard drives still account for about 27% of the market in terms of output. 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 IDE hard drives have achieved the performance of SCSI and are much cheaper. 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 costs 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 " tell me". I also partially classify myself as 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 debate "Which is better: IDE or SCSI" is one of the most common in many newsgroups. The number of messages and articles on this topic is very large. However, this question, like the famous "Windows NT or OS / 2 or Unix", in this formulation is unsolvable. The most frequent and correct reaction to them is “What for?”. After considering this issue in more detail, you will be able to make a decision for yourself about the need for SCSI for yourself.

Let's tell you in more detail what a simple SCSI controller can give compared to an 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 not difficult 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
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 also by a meter few!
you can use caching and RAID technologies to dramatically improve performance and reliability there used to be caching Tekrams, and now there are RAID for IDE it doesn't work and it's 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 connecting cables and their length, as a result of which the maximum connection length can be significantly reduced.

In order not to give 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, partly in the light of the above table:

  1. Price. Undoubtedly sometimes it 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 own cards.
  3. It is difficult to use a cable in a minitower case, it is longer than 80cm :)
  4. IDE HD is much easier to install, there is only one jumper, not 4-16 like on SCSI :)
  5. Most motherboards already have an IDE controller
  6. For 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 to ISA bus costs about $20, but now nobody needs such ones, so you can find cheaper ones. The next option is a controller on the PCI bus. The simplest version of FastSCSI costs about $40. However, there are now many motherboards that can install the Adaptec 7880 UltraWideSCSI for as little as +$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, in contrast to IDE, where it is difficult to come up with something new, for additional money, controllers can be expanded with the functions of a cache controller, RAID-0..5, hotswap, etc., so talking about the top controller cost limit is not quite correct.

And finally, about speed. As you know, today the maximum information transfer rate on the IDE bus is 33 Mb / s. For UWSCSI, a similar parameter reaches 40 Mb / s. The main advantages of SCSI are manifested when working in multitasking environments (well, not much in Windows95 :). Many of the benchmarks 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, to video processing) in which it is simply impossible to use the IDE. We will not talk about differences in internal architectures that also affect performance in this article, since there are too many special terms. We only note that while watching the development of the IDE, we are surprised to notice that it acquires many features of SCSI, but, let's hope, 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) is the 8-bit internal connector, often called 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 implies, you can and should connect external SCSI devices to it. The picture shows a mini-sub D connector with 50 pins.

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

(almost original dimensions)

Internal

Low Density 50-pin		 connection of internal narrow devices - HDD, CD-ROM, CD-R, MO, ZIP. (like IDE, only 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 one is built into the BIOS of the motherboard, for the rest, drivers for various operating systems are required. For SCSI devices, things are a little more complicated. To boot from a SCSI hard drive and work in DOS, you need your own SCSI BIOS. There are 3 options here.

  1. a microcircuit with SCSI BIOS is on the controller itself (as on VGA cards). When the computer is booted, 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 work under DOS.
  2. The SCSI BIOS image is flashed into the flash BIOS of the motherboard. Further on p.1. Usually, a SCSI BIOS is added to the motherboard BIOS for a controller based on the NCR 810 chip, Symbios Logic SYM53C810 (it is on the first picture) or Adaptec 78xx. You can control this process and change the SCSI BIOS to a newer version if desired. If there is a SCSI controller on the motherboard, this approach is used. This option is also more cost-effective :) - a controller without a BIOS chip is cheaper.
  3. There is no SCSI BIOS at all. All SCSI devices work only with operating system drivers.

In addition to supporting booting from SCSI devices, the BIOS usually has several other functions: configuring the adapter, checking the surface of disks, low-level formatting, setting SCSI device initialization parameters, 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 of hard drives. The SCSI BIOS often needs to store the configuration of the SCSI devices as well. This role is usually performed by a small chip like 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 preserved even when the power is turned off. In this SCSI chip, the BIOS can store both SCSI device settings and its own. In the general case, its presence is not associated with the presence of a chip with a SCSI BIOS, but, as practice shows, they are usually installed together.

On the next picture you can see the ASUSTeK UltraWide SCSI controller. It already has a SCSI BIOS chip. You can also see the internal and external Wide connectors.

The last (I couldn't find any more quickly :) picture shows a dual-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 is apparently 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
  • memory module connectors
  • floppy disk controller (mostly on older Adaptec boards)
  • IDE controller
  • sound card (on ASUSTeK cards for MediaBus)
  • VGA card

Other SCSI cards

Scanners and other slow SCSI devices often come with a simple SCSI controller. Usually this is a SCSI-1 controller on the ISA bus 16 or even 8 bits with one (external or internal) connector. It does not have a BIOS, eeprom, it often works without interruptions (polling mode), sometimes it supports only one (not 7) device. Basically, such a controller can only be used with your device, because. there are drivers for it. However, with a certain skill, you can connect to it, for example, a hard drive or streamer. This is justified only in the absence of money and the availability of time (or sports interest :), because a standard SCSI controller, as already mentioned, can be purchased for $20-40 and have an order of magnitude fewer problems and much more opportunities.

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 interface type (unipolar, differential, optics...). sometimes it can be called a type of connector to connect

The speed is affected mainly by the first two parameters. Usually they are written as prefixes to the word SCSI.

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

Let's take a look at the options for SCSI interfaces using Seagate hard drive designations as an example. In the model name, the last 1-2 letters indicate the interface, i.e. the same disk 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 everyday 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" is applied with the new Ultra2 protocol. "Single connector" are used mainly in hot-swap configurations, because 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. A 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 drive, 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 on-chip mass storage device, and IDE RAID, a box of 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 that one set of commands is used to work with them. Of course, as the SCSI physical layer evolved, so did the software interface. One of the most common today is ASPI. On top of this interface, you can use 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 the SCSI ID. For devices on a narrow SCSI bus, it can be from 0 to 7, on a wide one, respectively, from 0 to 15. The 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, CD-ROM device libraries, LUN is also used - the logical number of the device. If there are 8 CD-ROMs in the library, then it has a SCSI ID, for example, 6, and logically CD-ROMs differ in LUNs. 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 needed. The SCSI ID is usually set using jumpers (although there are new standards in SCSI similar to Plug&Play that do not require jumpers). They can also set the following parameters: parity check, terminator enable, terminator power supply, disk enable by controller command,

Installation

To install a SCSI controller and device, you need to have at least - themselves and also a SCSI cable :). Also useful is a free expansion slot in your PC, a free interrupt for that slot, 1-5 correct screws or screws, 2 to 8 different jumpers, a floppy drive or CD-ROM (already plugged in:) for driver media. More complex configurations may include external SCSI cables, external terminators (see below), Wide-Narrow adapters, and so on. Often there are questions about the ability to connect Fast / Ultra / Narrow / Wide devices in various combinations. For the most common devices, the general rule in this case is: if the connectors match, then you can connect. In other words, in this case it is important to distinguish between Narrow/Wide and ignore Fast/Ultra. (Leaves Ultra2 aside as it only exists in the LVD connector/interface variant). However, speed and reliability can drop significantly. See the SCSI Specifications / Interfaces section above for more details. In addition, there are various narrow-wide adapters, but their use is highly discouraged.

Controller

As already mentioned, the controller usually has SCSI ID=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 device on the SCSI bus, you can configure: parity, turn-on delay (so that not all 7 drives turn on at the same time), maximum device speed. For non-PnP controllers on the ISA bus, remember to set the interrupt it uses in the 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 this is often not important for the latest models.

Terminators

Perhaps someone remembers such a hard drive interface as ST506 (MFM / RLL), where the termination of the data cable on the last drive was just used. Terminators were also used in floppy disk drives, but for a very long time. The purpose of using terminators is to ensure signal level matching, 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 that have bus termination permanently enabled and external devices with a bus pass-through. Terminators options:

  1. internal. usually found on hard drives. switched on by setting one jumper
  2. automatic. most SCSI controllers have these. they decide whether to join or not
  3. in the form of resistor assemblies. on some CD-ROMs and CD-Rs they are. are turned off by deleting all assemblies from the panels.
  4. external. as in point 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 a cable to the next device in the chain.

In addition, terminators can be passive or active. Today, most are active, which 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, 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 separating 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 sounds like this: the SCSI bus must be terminated at both ends. Here we will consider the most common options for devices on the same SCSI bus (wide or narrow)

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

Variant with multiple internal devices. The 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 innermost devices.

There are internal and several external devices. Terminators on the internal and in 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-bit buses, which in fact are just the high and low bytes of the wide bus (in the descriptions and the SCSI BIOS, this is called High byte / Low byte). Now, following the above rules, you need to terminate both of these buses. Usually in such cases, the controller can independently terminate the high and low bytes of the wide bus. In this situation, the narrow bus is an extension of the low byte of the wide bus. Here is 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 better not to do so. Since the coexistence on the same bus of fast (wide is usually UltraWide SCSI) and slow devices (narrow is usually only Fast SCSI or even SCSI-1) is not good.

Homework: There are 3 connectors on the Wide controller: external and internal wide and internal 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 a wide connector 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 still use adapters, pay attention to termination! When connecting an external narrow device to the wide connector, the adapter must terminate high byte. If a narrow device is connected to the 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 drives is very simple, you only need to take care of two things - the terminator and the SCSI ID. Usually, a new disk has termination enabled, and the number is set to 6 or 2. Therefore, if you install the first disk, then there is nothing to worry about, and if not, then you need to check these settings. Another note about SCSI IDs is that older Adaptec controllers can only boot from 0 or 1.

The next step in the installation is formatting the drive. It is considered good practice to format the disk on a new controller before using it. This is due to the fact that different manufacturers of SCSI adapters use different sector translation schemes (it can be compared with LBA, CHS, LARGE for IDE disks) and during transfer, the disk may work poorly or not at all. 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, the SCSI BIOS options must be set to boot from them. The description of the possible options is too long, maybe it will be given here later, but for now - read the descriptions, there's nothing wrong there :).

CD-ROM, CD-R, CD-RW

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

streamers

Similarly, CD-ROM SCSI tape drives can run 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

Usually scanners come with their own card. Sometimes it is completely “its own”, like, for example, Mustek Paragon 600N, and sometimes it’s just the most simplified version of standard SCSI. In principle, using the scanner with it should not cause problems, but sometimes connecting the scanner to another controller (if the scanner has such a possibility) can be useful. Scanning A4 with 32bit color at 600dpi is a picture of about 90 Mb and transferring this amount of information through the 8bit 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 (example - Mustek Paragon 800IISP). A cheap FastSCSI PCI controller usually acts as an additional one. Less or more productive will not give anything new. This option also has a remark - you need to make sure that the scanner (or more importantly - its drivers) can work with your new controller in your configuration. For example, Mustek Paragon 800IISP drivers are designed for their own card or any ASPI compatible one.

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

  • your requirements and tasks
  • compatibility
  • reputation of the card manufacturer
  • reputation 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 the well-known AMD chip, which guarantees operability under most operating systems with built-in drivers, but 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. SCSI BIOS for it is included in almost any AWARD BIOS for motherboards. Very cheap and yet workable.

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

Devices

HDD - well, of course Seagate Cheetah - it's hard to argue with RPM 10000. But without additional cooling fans, this drive will not last long :(. Other series of Seagate drives - Barracuda and Hawk - are also distinguished by reliability.

The rest (CD-ROM, Tape, CD-R and others) are all to your liking. SCSI devices are produced by many well-known companies. For example HP, Sony, Plextor, Yamaha.

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


Leading Specialist at EPOS

Recently, a large number of various devices have appeared on our market that significantly expand the capabilities of a computer. First of all, these are Zip, Jaz and magneto-optics drives, these are various types of magnetic tape drives, as well as devices for single and multiple recording to CDs. Scanners have become very popular. Prices for hard drives have dropped to the point where a computer with two or three drives is no longer uncommon, and a server must contain a fault-tolerant disk array. In this regard, quite often the task of connecting new devices to the computer arises. This task can be solved most simply if a SCSI controller is installed in the computer.

Unlike the IDE, which supports a limited set of internal peripherals, the SCSI interface was designed to support many kinds of both internal and external devices.

What is a SCSI interface?

Basic SCSI (Small Computer System Interface, sometimes referred to as SCSI-1) is a universal interface for connecting various devices. In the basic standard, up to eight devices, including the controller, could be connected to one bus. The interface contains advanced controls and at the same time is not focused on any particular type of device. It has an 8-bit data bus, the maximum transfer rate is up to 1.5 Mb / s in asynchronous mode (according to the “request-acknowledgment” method), and up to 5 Mb / s in synchronous mode (multiple requests - multiple confirmations method) . Parity can be used to detect errors. Electrically implemented as 24 lines (unipolar or differential), although the vast majority of devices use unipolar signals.

In the process of development, the SCSI-2 standard was adopted - a significant development of the basic SCSI. Increased transfer rate (up to 3 Mb/s in asynchronous and up to 10 Mb/s in synchronous mode) - Fast SCSI. New commands and messages added, parity support made mandatory. The possibility of expanding the data bus to 16 bits (Wide SCSI) was introduced, which provided speeds 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 system. In addition, other parallel and serial protocols can be used as a transmission medium, along with the traditional parallel bus interface: Fiber Channel, IEEE 1394 Firewire and Serial Storage Protocol (SSP).

Currently, the most widely used interface is Ultra SCSI, which uses a bus frequency of 20 MHz. The Ultra/Wide SCSI interface supports 16 devices and provides data transfer rates up to 40 Mbps. But it is gradually being replaced by the faster Ultra-2 Wide SCSI, which provides transfer rates up to 80 Mb / s.

The continuous increase in the clock frequency of the bus 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. Therefore, with a further increase in the clock frequency, in accordance with the recommendations of SCSI-3, the number of bus wires, the technology of the bus itself, and the levels of signals transmitted over it have changed. The connector has remained the same as in the Ultra SCSI interface. However, the bus itself is now made with twisted wires (Fig. 1a, left, shows a photograph of an Ultra Wide cable, and Fig. 2b, right, of an Ultra-2 Wide cable).

Each signal on the Ultra-2 Wide bus is carried on two wires in opposite phase (differential). This is the so-called LVD (Low Voltage Differential), low voltage differential signaling. Thanks to differential signal transmission, the permissible length of the connecting cable has been increased to 12 m.

Comparison of various SCSI interfaces is given in the table:

StandardLength
cable, m		Speed,
Mb/s		Quantity
appliances
SCSI-16 5 8
SCSI-26 5...10 8 or 16
Fast SCSI-23 10...20 8
Wide SCSI-23 20 16
Fast Wide SCSI-23 20 16
Ultra SCSI-3, 8-bit1,5 20 8
Ultra SCSI-3, 16-bit1,5 40 16
Ultra-2SCSI12 40 8
Wide Ultra-2 SCSI12 80 16

Ultra SCSI devices can also work with a slower SCSI bus. It is also possible to use slow devices on a fast bus. In both cases, the bus runs at the speed of the slowest device. The highest data transfer rate can only be achieved if devices with the same interface are used.

Further development of the technology led to the emergence of the Ultra160/m SCSI standard. The transfer rate has been increased from 80 to 160 Mbps by using both edges of the challenge/acknowledge signal for data synchronization. The Ultra160/m SCSI standard uses a low-level differential interface (LVD) and allows cables up to 12 meters in length. A new component of the Ultra160/m SCSI interface is Environment Control. This intelligent technology is to check the storage subsystem, including connecting cables, backplanes, terminators, etc. If there is a risk of data loss, transmission occurs at lower speeds, a method widely used by modems and fax machines.

Such an abundance of simultaneously used standards creates a certain confusion. In addition, it is not entirely clear why the transmission speed is continuously increasing. What devices can provide such a speed?

This issue requires special attention. Indeed, tests of even the most modern hard drives show that their speed characteristics are far from those of the bus transfer rate. However, the baud rate on the bus is extremely important. After all, the SCSI protocol is designed to support the simultaneous operation of several devices connected to the same bus. Data for one device (for definiteness, we will mean a hard disk) are sent over a common bus to the buffer memory of the disk. While the slow process of writing to disk continues, data for another device is being transferred, and so on. From the user's point of view, the recording is carried out, as it were, simultaneously on several disks. Therefore, the bus must provide a total transmission rate for all devices connected to the bus, and, taking into account the need to transfer service information, a much higher one. To evaluate the benefits of moving from Ultra Wide SCSI to Ultra-2 Wide SCSI, we measured data transfer rates for software RAID 0 on four IBM DDRS-39130 drives. The experiment was carried out on a computer with a TYAN board, NMC-6BCD + with an integrated Adaptec AIC-7890 controller, P-II 450 MHz processor. Operating system Windows NT 4 WS. Software RAID is created by means of the operating system. The disks selected for the experiment have an LVD or SE interface switch. Measured data transfer rate in a four drive system for Ultra-2 Wide SCSI (80 Mb/s) and Ultra Wide SCSI (40 Mb/s). In addition, the transfer rate for a single disk is measured. Measurements were taken using WinBench99. The results of the experiment are shown in the diagram (Fig. 2).

Rice. 2. Test results for Ultra and Ultra2 Wide SCSI interfaces

The transfer rate for a single drive was found to be the same in both Ultra and Ultra-2 modes (in diagram 1 SE). Software RAID level 0 in Ultra mode increased the performance of the disk system by about 2 times (4 SE). The same drives switched to Ultra-2 mode resulted in more than a 3x performance increase (4 LVDs).

To compare the efficiency of simultaneous operation of several devices with SCSI interface and IDE interface, we also assembled software RAID level 0 on four IDE disks. Although the performance of a single IDE drive was comparable to that of a SCSI drive (1 IDE), using RAID on four IDE drives did little to improve the performance of the drive system (4 IDE).

From the results of the experiment, it is clear that if it is necessary to connect only one device, then any interface will provide approximately the same efficiency. Performance will be determined only by the mechanical characteristics of the device itself. When connecting multiple devices (for example, multiple drives in a server), the SCSI interface, and especially Ultra-2, provides much better performance than, for example, IDE or earlier SCSI standards.

How to properly connect SCSI devices

All types of SCSI are (at least in theory) compatible with each other. Devices independently establish an acceptable exchange protocol. Therefore, the installation of devices comes down to setting the correct value for the device number (SCSI ID), physically connecting the device to the bus, and enabling terminators. However, quite often computer owners who connect SCSI devices to their computer on their own complain about their unstable operation. In most cases, this is due to incorrect connection of devices and, most often, terminators (sometimes for some reason they forget about these terminators).

What is a terminator?

At high clock frequencies of the data bus, if special measures are not taken to coordinate the loads, signal re-reflections occur (like an echo in the Carpathians), as a result of which the actual information exchange rate is significantly reduced. For load matching, BOTH ends of each SCSI bus line must be terminated with a resistance equal to the characteristic impedance of the line. In the simplest case, for this, load resistances are turned on at both ends of the line. This is the so-called passive matching. Currently, this method of matching is practically not used, especially in Ultra mode. Moreover, this is unacceptable in Ultra-2 mode. This is due to the difficulty of selecting load resistances that satisfactorily ensure matching with a large (and changing during operation) number of devices connected to the bus. Virtually all modern SCSI devices now use active negotiation. With active termination, auxiliary voltage sources (one or more) are used instead of resistive voltage dividers. These voltages are automatically adjusted in such a way as to provide optimal conditions for receiving signals transmitted over the bus. A variation of the described method is matching with forced signal clipping. To implement this method, clamping diodes are installed in the active terminator, which limit the maximum and minimum voltages of the input signals at certain levels. Signal levels, in turn, can be set by changing the reference voltages.

In most cases, both the controller and all SCSI devices have built-in active terminators that can be enabled or disabled. However, as a rule, it is better not to rely on the built-in terminator, but to connect an external one. It is desirable, of course, not to use a passive terminator. Modern terminators necessarily have a corresponding inscription in their designation (Fig. 3).

Rice. 3. Passive terminator

The most common are active terminators for the Ultra Wide SCSI bus (Fig. 4).

Rice. four. Active Ultra Wide SCSI terminator

Terminators for Ultra-2 Wide SCSI buses must have the abbreviation LVD in their designation (Fig. 5). Currently, universal SE / LVD terminators are also being produced, which automatically determine the type of interface and perform negotiation for this type of interface (Fig. 6).

Rice. 5. Terminator markings for Ultra2 Wide SCSI

How to connect terminators correctly?

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

If multiple devices are connected to the SCSI controller, terminators must be installed only at the ends of the SCSI bus. So, if all connected devices are internal, then the terminators must be enabled on the SCSI controller and on one (and only one) device that is physically connected to the last connector of the SCSI bus. Best results are obtained if an 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, recently 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 enabled on the controller and the last connected external device. It should be noted that the vast majority of external SCSI devices have two connectors, one of which is connected to the SCSI bus from the computer, and other SCSI devices can be connected to the other. In this case, it is advisable to turn off the internal terminators of all devices and use an active external terminator.

If it is necessary to connect both internal and external devices to one SCSI controller, then the controller is connected to the intermediate connector of the SCSI bus. Part of the SCSI bus is used to connect internal devices, and the other part ends with a connector for connecting external devices. In this case, the controller's internal terminator must be turned off. On the indoor unit connected to the last SCSI bus connector, the terminator must be on, and on the other indoor units, it must be off. An active external terminator must always be installed on the connector for connecting external devices. When connecting an external SCSI device, the external terminator is removed, an external device is connected to the SCSI connector, and the previously removed external terminator is connected to the additional connector of the external device (do not forget to set the number of the external device correctly, otherwise the computer will simply “freeze”).

Connecting terminators for devices with different interfaces

All of the above is true if all connected devices have the same interface (all Wide SCSI-2 devices or all SCSI-2 devices). If some of the devices have a Wide SCSI-2 interface, and at least one (usually a CD-ROM) has a SCSI-2 (Narrow) interface, then in some cases there are problems with the correct connection of the terminators. The problems are caused by the fact that the Wide and Narrow interfaces differ in the number of data lines in the bus.

The most common mistake is connecting several Wide SCSI-2 (or Ultra Wide SCSI-2) hard drives to the Wide SCSI-2 bus, and connecting to the last connector via a SCSI-2 CD-ROM adapter. Although a terminator will be included on the CD-ROM, this terminator will terminate 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 would be to connect devices with an 8-bit SCSI interface to intermediate bus connectors (terminators of 8-bit devices are disabled). Connect a Wide SCSI device with the terminator enabled (or an active external terminator) to the last connector. Of course, the presence of an adapter still worsens the performance of the system. This option should be avoided if possible (as well as the use of high-speed and slow devices on the same bus). However, in this situation, this is still the correct connection option. Ultra-2 SCSI controllers have a built-in interface converter, which allows you to connect all Ultra-2 devices to a separate bus without mixing them with slower devices.

Features of controllers with two connectors

Many SCSI controllers have 2 connectors: one for the SCSI interface and one for the Wide SCSI interface. These are just physically different connectors, the SCSI channel is the same. These different connectors avoid the use of any adapters, but do not eliminate the problem of connecting terminators. These controllers have "High On/Off" and "Low On/Off" switches. These are separate switches for active terminators for the high and low bytes of the bus, respectively. Moreover, the low byte ("Low") is the SCSI interface lines (Narrow), and the high byte is the interface extension lines to the Wide standard.

If devices of only one standard are connected to such a controller, then both switches are set to the "On" position. The SCSI (or Wide SCSI) bus is connected to the controller with one end connector, and a device with the terminator enabled is connected to the other end connector. The remaining devices with the terminators turned off are connected to the intermediate connectors.

If you need to connect multiple devices with different interfaces, two buses are used: SCSI and Wide SCSI. Both buses are connected with their end connectors to the corresponding connectors of the controller. Devices are connected to buses in accordance with the standard they support. Terminators are enabled only on the instrument connected to the end connector of the SCSI bus and on the instrument connected to the end connector of the Wide SCSI bus. On the controller, the terminator switches are set to the "High On" and "Low Off" positions.

Recently, controllers, including those installed on the motherboard, do not have such a switch (or the corresponding item in the BIOS menu). There is only "Terminator On/Off". In this case, we are talking only about the lower 8 bits of the bus. The higher bits are always terminated.

Power supply for active terminators

The active terminators currently in use require a supply voltage to operate. This voltage can be applied to the active terminator both from any SCSI device and from the controller. On modern SCSI devices, there is a special switch to select the power supply source of the active terminator built into these devices. Usually, the terminator is powered by the device itself ("Power from Drive") at the factory. If only one or several internal SCSI devices with the same interface are connected to the controller, then there are no problems.

If, under the conditions of normal bus termination, it is necessary to use an active external terminator, then you need to take care of supplying a supply voltage to it. To do this, on one of the devices connected to this bus, the mode of supplying voltage to the bus ("Power to SCSI Bus") must be enabled. If this is not done, then the external terminator simply will not work normally.

In all the cases discussed above, the best results are usually achieved when all terminators are powered from the same source. In order to supply voltage to all terminators from one source on one (any) device, the power supply mode of the terminator built into this device from the internal power source is switched on and at the same time the mode of supplying terminators supply voltage to the bus. To do this, jumpers (switches) on this device are set to the "Power to SCSI Bus and Drive" position. On other devices, on which it is necessary to enable termination, the terminator is powered from the SCSI bus (jumpers or switches are set to the "Power from SCSI Bus" position).

In the vast majority of cases, the system will work normally even if each terminator is powered from its own source. The main thing is that each terminator is supplied with voltage from at least one source. Moreover, nothing terrible will happen if several devices are set to supply voltage to terminators in the line. The power supply circuits of the terminators of all devices are protected against reverse voltage.

Dedicated SCSI Controllers

Scanners and some other slow SCSI devices often come bundled with a simple SCSI controller. Usually this is a SCSI-1 controller on the ISA bus 16 or even 8 bits, with one (external or internal) connector. There is no BIOS on it, it often works without interruptions (polling mode), sometimes it supports only one device (not 7). Basically, such a controller can only be used with your device. Other devices on such a controller most often will not work. Moreover, many devices (most often scanners) will not be able to work with a standard controller. Therefore, it is better not to rely on compatibility, but to connect standard SCSI devices to a separate standard controller.

General concepts

SCSI (Small Computer Interface) was founded in 1980. based on the industry standard ANSIX3T9.2 (transformed into the X3T10 specification) to unify the standard interface (later called SCSI-1). The data transfer rate was relatively low, depending on many factors, and averaged from about 1 to 2 MB / s, but still exceeded the fastest devices (hard drives), which could provide speeds of no more than 625 KB / s even using MFM encoding . The main advantage of SCSI over the IDE interface is that SCSI, originally designed as an interface for multitasking and multiuser operating systems, allows access to multiple devices almost simultaneously. SCSI has played a significant role in the creation of information and computing systems that require the connection of various types of devices. This interface provides a wide range of connected equipment, such as:

  • Hard drives - hard disks (DASD - Direct Access Storage Device)
  • Tape drives, tape drives, and other serial access devices
  • Magneto-optical drives, CD-ROM, CD-Recoder
  • I/O devices such as scanners

These devices are connected to the computer through a special SCSI adapter, and the operating system accesses them through the appropriate drivers. The presence of a native processor adapter on the SCSI card greatly reduces the load on the CPU during I/O operations. This circumstance is of great advantage when working in a network, as well as in multi-user and multi-tasking environments, due to the fact that the time for obtaining client access to the device is reduced. In desktop systems (desktop computers), the load of the central processor is not so critical for most user programs and applications, however, when working with graphics (especially when working with computer animation), the use of the SCSI subsystem allows you to increase system performance, since in this case most of the load on operations I / O will be transferred to the SCSI adapter.

SCSI specifications

Today there are several SCSI specifications:

  • SCSI-1: 8-bit data bus and 5MB/s synchronous data rate. Connector 25- or 50-pin;
  • SCSI-2 or Fast SCSI: Up to 10MB/s speed increase over 8-bit bus. Connector 50 pin;
  • Wide SCSI (Wide SCSI): increase in bus width to 16. The data transfer rate has increased from 10MB / s to 20MB / s. 68- or 80-pin connector (Single Connector), combining power and signal circuits;
  • Ultra SCSI (Fast-20) / Ultra Wide SCSI or SCSI-3: The data transfer rate has increased to 20MB/s on an 8-bit bus and up to 40MB/s on a 16-bit bus. SCSI-3 provides support for more devices (up to 15 per channel). 50 / 68- or 80-pin connector (Single Connector), combining power and signal circuits;
  • Ultra2 SCSI (LVD): Further increasing the speed of SCSI required the use of a Low Voltage Differential (LVD) low-voltage differential bus, in which signals are transmitted simultaneously on two wires, but in different polarity. Due to this, the noise immunity of the bus is sharply increased, it becomes possible to increase the data transfer rate over a 16-bit bus up to 80 MB / s and increase the length of the interface cable up to 12 m! Full implementation requires an Ultra2 SCSI adapter, an Ultra2 SCSI cable with an Ultra2 SCSI active terminator, and drives that support Ultra2 SCSI. In the absence of any of the listed components, the Ultra2 SCSI standard is automatically disabled and the system operates in one of the previous SCSI specifications. 68- or 80-pin connector (Single Connector), combining power and signal circuits;
  • Ultra3 SCSI (Ultra160 SCSI): Transfer rates can reach up to 160 MB per second with double data synchronization (data transfers twice as fast without increasing the clock speed), improved mechanism for optimizing the speed of data exchange with different devices, and the use of CRC algorithm instead of parity for improving the reliability of data transmission. The Ultra160 SCSI specification is fully compatible with Ultra2 SCSI in terms of cables, connectors, and terminators. An Ultra160 SCSI controller can support both Ultra160 SCSI and Ultra2 SCSI devices on the same bus at the same time, each running at maximum speed. 68- or 80-pin connector (Single Connector), combining power and signal circuits;
  • Ultra160+ SCSI: A modification of Ultra160 SCSI that implements Packetized SCSI - a packet method of information transfer (commands, data and status registers are transferred in one block at the same rate) and Quick Arbitration Select (QAS) a method for quickly transferring bus control from one SCSI device to another. As a result, delays are reduced and the integral data rate is increased.

Basic requirements for implementing a SCSI interface

· All drives and other SCSI devices must be connected to each other sequentially (daisy chain), forming a SCSI channel.

· Only those SCSI devices that have the same type of SCSI interface can be connected to one SCSI channel.

· Devices with a single-ended (single-ended) (unipolar) interface and devices with a differential (differential) (bipolar) interface must not be used on the same SCSI channel.

· One SCSI channel can simultaneously connect up to 8 for an 8-bit (narrow) data bus or up to 16 for a 16-bit (wide) data bus SCSI devices, including a SCSI controller. However, there are additional restrictions on the number of connected SCSI devices, depending on the length of the connecting cable and the data transfer rate.

· Each SCSI device, including the SCSI controller, must have a unique SCSI number (SCSI ID). Valid SCSI ID range: 0 to 7 for 8-bit (narrow) data bus or 0 to 15 for 16-bit (wide) data bus. All SCSI IDs are equal, however, by default, SCSI ID = 7 is set on SCSI controllers and it is not recommended to assign this number to other SCSI devices.

· Both ends of the SCSI channel must be terminated with a special matching device - terminator (Terminator). The terminator can be located inside the SCSI device, mounted on the end of the connecting SCSI cable or backplane, or made as a separate device that is connected to the last connector of the SCSI channel.

· All intermediate (not edge) SCSI devices must be unterminated. If these SCSI devices have built-in terminators, make sure that the switch (jumper) "Enable termination (terminator enable - TE)" is in the "Off / Disable" position.

· The connecting SCSI cable must meet the requirements of ANSI X3T10/1142D (section 6) for:

Characteristic impedance

Propagation Delay

Cumulative length

Permissible branch length

Interval between devices

To meet the characteristic impedance requirement, an unshielded flat cable or a twisted pair ribbon cable must be used. It is not allowed to use cables with different wave impedances on one SCSI channel. It is also not recommended to use shielded and unshielded cables on the same SCSI channel at the same time. This is especially important when implementing a SCSI interface according to the Ultra SCSI, Ultra2 SCSI and Ultra3 SCSI specifications.

What is the allowed length of the SCSI cable?

1) The total maximum cable length of a single-ended SCSI interface depends on several factors. The table below lists the maximum cable length for various SCSI specifications and configurations:

Specification Transfer rate Max. length of cable Max. number of devices
Fast SCSI 10 MB/s 3 meters 8
Wide SCSI 20 MB/s 3 meters 16
Ultra SCSI (8 bit Narrow) 20 MB/s 3 meters 5
Ultra SCSI (16 bit Wide) 40 MB/s 3 meters 5
Ultra SCSI (8 bit Narrow) 20 MB/s 1.5 meters 6-8
Ultra SCSI (16 bit Wide) 40 MB/s 1.5 meters 6-8
Ultra2SCSI 80 MB/s 1.5 meters 16

Note: While an Ultra SCSI (narrow or wide) interface should theoretically support up to 8 narrow or 16 wide devices, the X3T10/1071D specification does not allow the full number of devices to be supported when using a cable. To connect more than 4 devices, you must use a special connector board (backplane). But even so, the maximum data transfer rate will be achievable only when connecting no more than 8 devices. The length of the branch should be no more than 0.1 meters.

2) The maximum total cable length of the High Voltage Differential (HVD - High Voltage Differential) SCSI interface is 25 meters. The high-voltage differential SCSI interface must use a twisted-pair cable. The length of the branch should be no more than 0.2 meters. The spacing between devices on the main SCSI bus must be at least three times the length of the spurs. But, despite this limitation, up to 16 SCSI devices can be connected to the high-voltage differential SCSI interface, which can be addressed via a 16-bit SCSI bus.

3) The maximum total cable length of the low voltage differential (LVD - Low Voltage Differential) SCSI interface is up to 25 meters for 2 devices or up to 12 meters for more than 2 devices. The rest of the requirements are similar to those of the high-voltage differential SCSI interface.

Is it possible to determine the type of SCSI interface by the appearance of a SCSI device?

Unfortunately, based on the appearance of a SCSI device, we can only say about the SCSI interface: "Narrow" or "Wide". Below is the view from the side of the connectors of some SCSI devices:

Narrow device with SCSI-1, SCSI-2 or Ultra SCSI interface.

Wide device with SCSI-2, Ultra SCSI, Ultra2 SCSI, or Ultra3 SCSI interface.

Wide SCA device with SCSI-2, Ultra SCSI, Ultra2 SCSI, or Ultra3 SCSI interface.

Additional information can be found on the manufacturer's website on the model designation of the SCSI device.

?"> What does it mean?

The SCA interface was designed to provide a standard connection for systems using hot swappable drives. Drives with SCA interface are connected to a special SCSI backplane (SCSI backplane), which provides power supply, SCSI ID setting and SCSI bus termination. A distinctive feature of drives with an SCA interface is an 80-pin connector, which combines an interface connector, a power connector, and pins for SCSI ID.

How to connect an SCA drive to a SCSI controller with a standard 50 or 68 pin SCSI interface?

To connect a drive with an SCA interface to a standard SCSI controller, you need a special SCA adapter. The SCA adapter must have a 50- or 68-pin interface connector, a power connector, and, if the drive does not have this, a terminator, and a device for setting the SCSI ID.

The SCSI device installed in the computer is not working (not recognized). What is the reason?

Try the following:

· Make sure the SCSI controller to which the SCSI device is connected is recognized and working correctly. A sign of this is a message about loading the BIOS of the SCSI controller after loading the motherboard BIOS (if the SCSI controller has its own BIOS) and a message about the successful loading of the SCSI controller drivers (under DOS) or a message about the normal functioning of the SCSI controller (under Windows). If not, check the setting of the interrupt number, the I/O addresses for the SCSI controller card, and the correct version of the drivers for the type of SCSI controller and operating system.

· Make sure the SCSI cable and power cable are of good quality and the connectors are inserted properly.

· Make sure that each SCSI device has a different SCSI ID. The SCSI ID for SCSI devices can be anything except 7, which is usually reserved for the SCSI controller.

· Make sure that the SCSI bus termination is set correctly: enabled (set) only on the edge SCSI devices of the chain and disabled (removed) on all intermediate SCSI devices of the chain.

· If the SCSI controller has its own BIOS, make sure that the parameters by which the SCSI controller accesses SCSI devices (baud rate, data buses, parity, etc.) match the characteristics of the connected SCSI devices.

What is needed for the computer to be able to boot from a SCSI drive.

To boot from a SCSI drive, the following conditions must be met:

· The motherboard must have a BIOS that allows booting the OS from SCSI devices. In this case, the presence of IDE drives in the system is allowed. If the motherboard is old (the BIOS does not allow booting from SCSI devices), all IDE drives should be disabled. As a last resort, it is allowed to have IDE drives with all partitions formatted as (Extended).

· The SCSI controller must have its own BIOS. Make sure that in the SCSI controller parameters, in the section, the number of the corresponding SCSI device is set.

· The boot partition of the SCSI drive must be formatted as (Primary) and (Active).

What is needed to fully realize the capabilities of the LVD SCSI interface?

For the normal functioning of the LVD SCSI interface, in addition to the standard requirements of the SCSI interface (unique SCSI ID, termination of the SCSI bus), specific requirements for the LVD must be met:

SCSI controller must support LVD interface

Both ends of the SCSI chain must have active LVD terminators

All SCSI devices on the bus must support the LVD interface

Failure to meet any of these requirements will result in the SCSI system only being able to function on more than the SCSI specification.

How compatible are LVD devices with previous spec SCSI devices?

The LVD SCSI interface is fully compatible with the single-ended SCSI interface. Thanks to a unique feature of the LVD SCSI interface, known as multi-moding, a special input/output circuitry (DiffSens) automatically detects the type of SCSI bus to which the device is connected (LVD or single-ended) and adapts to the corresponding capabilities of this bus. Therefore, LVD devices will work with SCSI-1 and SCSI-2 interfaces. Conversely, SCSI-1 and SCSI-2 single wire devices will operate on the LVD bus. Compatibility is an important feature of SCSI, but when using SCSI devices of different manufactures on the same SCSI bus, all peripheral devices on that bus will operate on the SCSI specification supported by ALL devices on that bus. For example, if a single-ended device is connected to an LVD bus with LVD devices, then all devices on that bus will operate in single-ended mode.

High Voltage Differential (HVD) devices require a dedicated controller and are not compatible with LVD or single-ended devices.

SCSI interface

In the early 1970s, an interface was developed for a minicomputer (at that time it really was a minicomputer) SCSI(read "tell"), whose name stands for Small Computer System Interface. Again, its "portability" is visible in the name. The initial version assumed an 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 that increased 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 variants of SCSI, 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 will summarize the main points in a table.

The bandwidth is 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. The data in the table is already somewhat outdated, since, according to it, the maximum is 80 Mb / s (Ultra2 Wide SCSI, or simply UltraWide SCSI, since Wide SCSI controllers other than Ultra2 are not released now, and Ultra means Ultra2 by default), but 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 length of the SCSI cable.

Here are the typical connectors found on SCSI controllers:

Internal

Low Density 50-pin		 Connecting internal slow devices - old HDDs, almost all CD/DVD-ROMs, CD-Rs, MODDs, ZIPs, etc. (like IDE, only 50 pins)

High Density 68-pin		 Connecting internal wide devices, mainly HDD
External

DB-25		 Connecting external slow devices, mostly 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 version of SCSI). Like SCSI-1 itself, already obsolete

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 connecting HDD

High Density 68-pin		 Similar to the previous one (practically never used)

There is also another type of connector - CL, or Single Connector having 80 contacts. Single Connectors are mainly used in host-swap (when a device may need to be hot-swapped) configurations, as they combine SCSI power and ground signals in one connector.

In addition to the above interfaces, there is also the so-called Serial SCSI powered by technology fiber channel. The devices are connected to the controller using a 6-wire 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 the usual narrow and wide connectors, which allows you to connect standard SCSI devices. Fiber Channel is more like a networking standard than a media interface; it uses serial data transmission. For more information about 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 BIOS of the motherboard, for the rest, drivers are required for various operating systems. For SCSI devices, things are a bit more complicated. To boot from a SCSI hard drive 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 (as on video cards). When the computer is booted, 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 drive devices to work under DOS.
  • The SCSI BIOS image is embedded in the flash BIOS of the motherboard. Usually, SCSI BIOS for controllers based on the most common chips is added to the board BIOS. It can be flashed and thus change the SCSI BIOS version to a newer one. If there is a SCSI controller on the motherboard, this approach is used. This option is also more economical - a controller without a BIOS chip costs less.
  • There is no SCSI BIOS at all. All SCSI devices work only with operating system drivers. Downloading from them, of course, is impossible. This approach is used when creating your own controller for some external device (for example, a scanner), that is, when loading from the device does not make sense and the use of drivers is expected anyway

In addition to supporting booting from SCSI devices, the BIOS usually performs several other functions: configuring the adapter, checking the disk surface, low-level formatting, setting the 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 chip like 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 preserved even when the power is turned off (similar to CMOS on a motherboard). On this SCSI chip, the BIOS can store both SCSI device settings and its own settings. In the general case, its presence is not associated with the presence of a chip with a SCSI BIOS, but, as practice shows, they are usually installed together.

There are also powerful controllers for servers. In addition to the obligatory support for the fastest 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, a hardware cache of 32, 64 or more megabytes is also installed. In the picture on the left you can see such a controller from ASUSTeK (a good company, by the way). The 486th processor is very clearly visible on its board, which, apparently, is trying to manage all this goodness.

Also on the SCSI controller board, you can find the SCSI bus activity LED and / or a connector for connecting it, and if there is cache support, then sometimes slots for memory modules. It happens that they put an additional IDE controller, a sound card or a VGA card. On very old controllers, you can sometimes find connectors for connecting floppy disk drives.

Various devices that are not too hasty (usually scanners) 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, 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 version of SCSI is better than LPT or USB) at minimal salaries. But, on the other hand, this is not good, because nothing but your own device with a 100% guarantee can not be used. Although no one really needs this, the speed of such cards is still low, and if you really need SCSI, you will have to buy something more serious.

Devices are connected to the controller with an appropriate (narrow or wide) cable in a chain (similar to IDE). This also applies to external devices, only here you can draw an analogy with a serial connection, say, a scanner and a printer to a parallel port. Don't pay too much attention to media speeds, because in most cases the rule applies: "If the connector fits, then it will work." True, in this case, the bus may slow down, therefore, if possible, it is better to connect slow devices to one connector, fast devices 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 a wide one, respectively, from 0 to 15. The 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, CD-ROM device libraries, LUN is also used - the logical device number. If there are 8 CD-ROMs in the library, then it has a SCSI ID, for example, 6, and logically CD-ROMs differ in LUNs. 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 to Jumper-free Plug&Play You can also set options such as parity (if the controller supports it), terminator enable, terminator power (see below), disk enable on controller command, etc. Again, this can often be done in software. way through the BIOS. The principle of using ID is good because it does not require interrupts. For SCSI to work, only one interrupt is enough (for the controller itself), which, unlike the IDE, saves this resource and therefore makes it possible to install more equipment.

Now, as promised, about terminators. In short, these are things that are placed at the ends of the tire. The purpose of using terminators is to ensure signal level matching, 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 that have bus termination permanently enabled, and external devices with a bus through. Terminators options:

  • Internal. Usually present on hard drives; switched on by setting one jumper
  • Automatic. Most SCSI controllers have these. They decide for themselves whether to join or not.
  • In the form of assemblies of resistors, on some CD-ROMs and CD-Rs they are just that. They are turned off by deleting 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: the cable to the controller is connected to one, the terminator or cable to the next device in the chain is connected to the other.

The last two types, however, are already outdated and are not used. 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 at low speeds. By the way, this is another argument in favor of separating 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 sounds like this: the SCSI bus must be terminated at both ends. We will look at the most common options for devices on a single 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.

Variant with multiple internal devices. The terminator is enabled only on the last device and on the controller.

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

There are internal and several external devices. Terminators are enabled on the internal and on 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-bit buses, which in fact are just the high and low bytes of the wide bus (in the descriptions and the SCSI BIOS, this is called High byte / Low byte). Now, following the above rules, you need to terminate both of these buses. 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 an extension of the low byte of the wide bus. Here is one example:

Narrow devices can be used on a wide bus even when the controller does not have the necessary 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 a wide connector 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 connector. If you use adapters, pay attention to termination. When connecting an external narrow device to a wide connector, the adapter must terminate the high byte. If a narrow device is connected to the internal wide connector, then the adapter simply converts the connectors (that is, 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 for reference rather than practical information.

In various conferences and FAQs, a question like "Which is better: IDE or SCSI?" is often asked. It is very simple to answer it, 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
  • Cable length can be 3-6 meters
  • Generally higher reliability than IDE for both controllers and SCSI devices
  • Ability to use external devices
  • The maximum number of devices (up to 15) is much more than the 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 RAID and host-swap technologies. True, similar IDE controllers have also begun to appear recently, but they are certainly not as good as those of SCSI.

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

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