SCSI interface

In the early 1970s, an interface was developed for a minicomputer (at that time it was really a minicomputer) SCSI(read "skazi"), the name of which stands for Small Computer System Interface. Again, its “portability” is evident in the name. The original version assumed an exchange speed 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 transfer speed - today's SCSI controllers support speeds of 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 And the very first SCSI standard, of course, is already outdated and now almost no one remembers about it.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 look at each separately, but will summarize the main points in a table.

Bandwidth is easy to calculate: to do 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example, an UltraSCSI controller (often called Ultra SCSI-2) has a speed of 20 Mb/s. These tables are already somewhat outdated, since, according to it, the maximum is 80 Mb/s (Ultra2 Wide SCSI, or simply UltraWide SCSI, since Wide SCSI controllers other than Ultra2 are not produced now, and Ultra by default means Ultra2), and The speed of 160 Mb/s has already become widespread (this standard is called Ultra160 SCSI). These standards are only available with the LVD interface ( Low Voltage Differential), providing increased noise immunity and increased permissible SCSI cable length.

Here are typical connectors that can be found on SCSI controllers:

Domestic
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 HDDs
External
DB-25	Connecting external slow devices, mainly scanners, IOmega Zip Plus. Most common on Mac. (like a modem). Outdated
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, it is 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 (almost never used)

There is also another type of connector - CL, or Single Connector having 80 contacts. Single Connectors are used mainly in host-swap (when hot-swapping of the device may be required) 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 based on technology Fiber Channel. The devices are connected to the controller using a 6-wire cable and can communicate at speeds of 100 Mb/s or more. Serial SCSI controllers (also often called SCSI-3) have regular narrow and wide connectors, which allows you to connect standard SCSI devices. Fiber Channel is more of 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.

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

• The SCSI BIOS chip is on the controller itself (like on video cards). When the computer boots, it is activated and allows you to boot from a SCSI hard drive or, for example, a 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 necessary for running devices other than hard drives under DOS.
• The SCSI BIOS image is flashed into the motherboard's Flash-BIOS. Typically, SCSI BIOS is added to the BIOS of boards for controllers based on the most common chips. You can reflash it and thereby change the SCSI BIOS version to a newer one. If there is a SCSI controller on the motherboard, this is the approach used. This option is also more economically beneficial - a controller without a BIOS chip is cheaper.
• There is no SCSI BIOS at all. The operation of all SCSI devices is provided only by 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 intended in any case

In addition to supporting booting from SCSI devices, the BIOS usually performs several more functions: setting the adapter configuration, checking the disk surface, low-level formatting, setting initialization parameters for SCSI devices, setting the boot device number, and so on. The SCSI BIOS often also needs to store the configuration of SCSI devices. 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 retained even when the power is turned off (similar to CMOS on the motherboard). In this SCSI chip, the BIOS can save both the parameters of SCSI devices and its own settings. In general, its presence is not related to the presence of a microcircuit with a SCSI BIOS, but, as practice shows, they are usually installed together.

There are also powerful controllers for servers. In addition to mandatory support for the fastest modes, they usually have support for RAID, hot-swappable disks and an additional SCSI channel, which allows you to increase the number of connected devices. Often they also install a hardware cache of about 32, 64 or more megabytes. 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 stuff.

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

Various slow-moving devices (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, does not have a BIOS, works only with its driver and without interruptions (polling mode). From an economic point of view, this is quite justified, since it provides quite a lot of opportunities (it is clear that even the most primitive SCSI option is better than LPT or USB) at minimum wages. But, on the other hand, this is not good, because you won’t be able to use anything other than your own device with a 100% guarantee. Although no one really needs this - all the same, the operating speed of such cards is low and if there is a real need for SCSI, you will have to buy something more serious.

Devices are connected to the controller with an appropriate (narrow or wide) cable via a chain (similar to IDE). This also applies to external devices, only here we can draw an analogy with a serial connection of, say, a scanner and printer to a parallel port. There is no need to pay special attention to the speed indicators of the media, since in most cases the rule applies: “If the connector fits, it will work.” True, in this case the bus may slow down, so 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 a wide one, respectively, from 0 to 15. The SCSI controller, which is an equal SCSI device, also has its own number, usually it is 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, a 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 the CD-ROMs differ in LUN. For the controller, all this looks like ID - LUN pairs, in our example 6-0, 6- 1, ..., 6-7. If necessary, LUN support 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 similar to SCSI. Plug&Play, which does not require jumpers. You can also set parameters such as parity check (if the controller supports it), turning on the terminator, powering the terminator (see below), turning on the disk at the command of the controller, etc. Again, often all this can be done in software. way through the BIOS. The good thing about using ID is that it does not require interrupts. For SCSI to work, only one interrupt is needed (for the controller itself), which, unlike IDE, allows you to save this resource and therefore makes it possible to install more equipment in the system.

Now, as promised, about the terminators. In short, these are the things that are placed at the ends of the tire. The purpose of using terminators is to ensure matching of signal levels, 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 enabled forever, and external devices with a pass-through bus. Terminator options:

• Internal. Typically present on hard drives; enabled by installing one jumper
• Automatic. Most SCSI controllers have these. They decide for themselves whether to join or not.
• In the form of resistor assemblies, on some CD-ROMs and CD-Rs they are exactly like this. They are turned off by removing all assemblies from the panels.
• External. As in the previous paragraph, but more beautiful (for example, on the HP T4e streamer). The device (usually external) in this case has two SCSI connectors: one connects the cable to the controller, the other connects the terminator or cable to the next device in the chain.

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 SCSI device is being used by the way it is turned on. If it is one jumper, or it is automatic, then most likely it is active. And if to turn it off it is necessary to remove 1-2 resistor assemblies from the device, then it is passive. In principle, termination of a 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 outlined in the adapter manual. The main thing is this: the SCSI bus must be terminated at both ends. We will look at 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.

Option with several internal devices. Terminator is enabled only on the last 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 are internal and several external devices. Terminators are enabled on the internal and last external device.

The situation is a little more complicated when narrow- and wide-devices are used simultaneously on one controller (bus). Let's imagine that we have two 8-bit buses, which are actually just the high and low bytes of the wide bus (in the descriptions and SCSI BIOS this is called High byte/Low byte). Now, following the above rules, you need to terminate both of these buses. Typically, in such cases, the controller can independently terminate the high and low bytes of the wide bus. 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 necessary connector (both external and internal). You just 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, this need arises when connecting external narrow devices to a wide controller, since it usually has an external connector of the wide type. If you use 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 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 automatically, and problems should not arise; This information is provided as a guide rather than as practical information.

In various conferences and FAQs, questions like “Which is better: IDE or SCSI?” are often asked. It is very simple to answer, 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 speed

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

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 greater than that of IDE, and you can also install several SCSI controllers (usually no more than four)

All SCSI devices require only one interrupt

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

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 your goals. To work, say, in Microsoft Office, you do not need high speed at all. In addition, the benefits are strongly noticeable only in the case of active multitasking. You should also remember that for little money you can buy a much faster and more capacious hard drive with an IDE interface than with a SCSI one. But if you do video editing, CD burning, complex graphics, or just want maximum performance in your favorite Unreal (or whatever your favorite thing 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...

28. 07.2017

Blog of Dmitry Vassiyarov.

SCSI - fast and unusual interface

Hello.

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

Read it - what if SCSI is useful to you too?

What does SCSI mean?

This is a set of capital letters from the phrase Small Computer Systems Interface. In Russian it sounds like “skazi”, and the decoding is a system interface for small computers.

This standard was created to combine computer components for various purposes on one bus: hard drives, floppy drives, scanners, printers, etc. Why? To provide them with equally high speed of operation 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 options

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

The most commonly used commands are write, read, check devices, query their characteristics, set new parameters for them or return previous ones, etc.

There is also the implementation of commands over wires and controllers of other standards. If we are talking about IDE, ATA or SATA, it is called ATAPI - ATA Packet Interface; if on top of the USB protocol - Mass Storage device. Thus, you can, for example, connect an external hard drive via regular USB and the SCSI driver available in the operating system will be used for it.

Where is SCSI in demand?

On high performance servers and workstations. On servers belonging to the low price category, and especially at home, this interface is extremely rare; in such cases, the best option is the one we are used to.

But of course, no one forbids you to install such fabulous devices on your home computer. Or for example to a home server.

Technology in practice

All devices that you want to connect to one 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 the devices. The operating system recognizes and communicates with them as usual using .

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

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

History of appearance

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

So, in 1979, the inventor of 8-inch floppy disks and magnetic storage manufacturer Alan Shugart set himself the task of creating a universal interface for his products that would not lose its position taking into account 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 for 8-bit parallel sending of information along a path that included 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 (American National Standards Institute) committee - the same as GOST in our country. Based on this invention, ANSI specialists created SCSI.

Interface generations

It is noteworthy that the technology was created almost half a century ago, and we are still talking about it. This is because she was constantly transforming. Since its inception, 10 versions have been released. I won’t bore you with details about each of them. I’ll just tell you what it was initially and what we have now.

SCSI-1

• It is possible to connect a maximum of 8 devices to one bus, including the controller.
• The maximum speed was 1.5 Mb/s in the asynchronous variation (“request-confirmation”), and 5 Mb/s in the synchronous one - 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 supplied more often.
• The bus frequency was 5 MHz.
• The longest cable is 6 m, and for the differential HVD bus it is 25 m.

Ultra-640 SCSI

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

Serial Attached SCSI (SAS)

• Added support for connecting SATA devices.
• The speed of this interface has already increased to 12.0 Gbit/s.
• According to the developers, it is now possible to connect 16,384 devices to one bus! In the previous generation, as described above, there were only 16.

Electrics

There are 3 ways to transmit information regarding electrical:

• SE (single-ended) - asymmetrical look. Each signal is sent over a separate line.
• LVD (low-voltage-differential) - low voltage differential standard. The “+” and “-” signals are sent through different wires. Each of them is allocated one twisted pair. They are transmitted at a voltage of ±1.8 V.
• HVD (high-voltage-differential) - analogous to the previous version, but with special transceivers and increased voltage.

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

• Passive - simple 132 Ohm resistors;
• Active - stabilizers that produce the necessary signal, and each power line is connected to them with a resistance of 110 Ohms;
• FPT (Forced Perfect Terminator). The name speaks for itself - accelerated improved type. It has surge 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 high speed;
• You can create a chain of 15 devices;
• They are convenient to manage;
• HDDs are highly reliable.

Nevertheless, the share of such drives accounts for only about 30% of the modern market, since SCSI also has disadvantages:

• Expensive. But you need to understand that you are paying for quality. Although SATA hard drives have greater capacity at a lower price, they cannot boast of such durability.
• Obsolescence. An improved competitor has appeared - SAS (Serial Attached SCSI) technology, which has more compact wires, does not require terminators, allows you to connect more devices and has better throughput.

That's all.

I look forward to seeing you on the blog pages as often as possible.

External PC interfaces - SCSI bus

SCSI (Small Computer System Interface), pronounced “skazi”, is a system-level interface, standardized by ANSI, unlike interface ports (COM, LPT, IR, MIDI), it is a bus: the signal pins of many subscriber devices are connected to each other “ one to one."

The main purpose of the SCSI bus during the development of the first specification in 1985 was “to ensure hardware independence of devices of a certain class connected to a computer.”

Unlike hard expansion buses, the SCSI bus is implemented in the form of a separate cable loop, which allows the connection of up to 8 devices (SCSI-1 specification) of internal and external design. One of them is host adapter(Host Adapter) connects the SCSI bus to the computer’s system bus, seven others are free for peripherals.

Fig 1. SCSI adapter from ASUSTeK

The following can be connected to the bus:

• internal and external disk drives (CD-ROM, hard drives, removable hard drives, magneto-optical disks, etc.);
• streamers;
• scanners;
• photo and video cameras;
• other equipment used not only for IBM PC.

Each device connected to the bus has its own identifier SCSI ID, which is transmitted as a positional code over an 8-bit data bus (hence the limitation on the number of devices on the bus). A device (ID) can have up to 8 subdevices with their own LUNs (Logical Unit Number).

Any device can initiate communication with another target device(Target).

The SCSI bus exchange mode can be:

• asynchronous or
• synchronous with speed negotiation (Synchronous Negotiation), where data transfer is controlled by parity.

SCSI Specifications

SCSI-1 specification strictly defines the physical and electrical parameters of the interface and the minimum commands. Bus frequency - 5 MHz. Bus width is 8 bits. The ANSI standard was developed in December 1985.

SCSI-2 specification defines 18 basic SCSI commands (Common Command Set, CCS), required for all peripheral devices, and additional commands for CD-ROM and other peripherals. The devices support queues - they can accept chains of up to 256 commands and execute them in a pre-optimized order autonomously. Devices on the same SCSI bus can exchange data without CPU involvement. The ANSI standard was developed in March 1990.

Additional extensions to the SCSI-2 specification:

• Fast - doubling the synchronous transmission speed (bus frequency 10 MHz).
• Ultra - ultra-high-speed interface (bus frequency 20 MHz).
• Wide - increasing the bit depth to 16 bits, less often to 32 bits.

The maximum throughput depends on the frequency and bus width and is given in Table 1 for combinations of these extensions. 1.

Table 1. Data transfer rates, lengths and types of SCSI-1, SCSI-2 cables

SCSI-3 specification— further development of the standard aimed at increasing the number of connected devices, specification of additional commands, and support for Plug and Play. As an alternative to the parallel interface SPI(SCSI-3 Parallel Interface) it becomes possible to use a serial interface, including a fiber-optic interface with a data transfer rate of 100 MB/. SCSI-3 exists in the form of a wide range of documents defining individual aspects of the interface, and in many ways overlaps with the serial bus FireWire.

Terminators, connectors

By type of signals they differentiate linear(Single Ended) and differential(Differential) versions of SCSI, their cables and connectors are identical, but electrical compatibility there are no devices between them.

Differential the version for each signal uses a twisted pair of conductors and special transceivers, while a large total cable length becomes permissible while maintaining a high exchange frequency. The differential interface is used in powerful server disk systems, but is not common in ordinary PCs.

IN linear version, the signal must travel along its one conductor, twisted (or at least separate from the other in a flat cable) with a neutral (return) wire. Universal symbolic designations of versions are shown in Fig. 1.

SCSI devices are connected by cables chain(Daisy Chain), on the edge devices they connect terminators. Often one of the extreme devices is the host adapter. It can have both an internal and external connector for each channel:

Internal connectors
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 connectors
DB-25	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. Typically 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

When using the external and internal connectors of the host adapter simultaneously, its terminators are disabled. The correct use of terminators is essential - the absence of one of the terminators or, conversely, an extra terminator can lead to instability or loss of functionality of the interface.

In terms of execution, terminators can be either internal(placed on the device’s printed circuit board), and external(installed on cable or device connectors).

Based on their electrical properties, the following types of terminators are distinguished:

• Passive (SCSI-1) with an impedance of 132 Ohms are ordinary resistors. These terminators are not suitable for high-speed SCSI-2 modes.
• Active with an impedance of 110 Ohms - special terminators to ensure operation at a frequency of 10 MHz in SCSI-2.
• FPT (Forced Perfect Terminator) is an improved version of active terminators with emission limiters.

Active terminators require power, for which there are special TERMPWR interface lines.

Cables

The range of SCSI cables is quite wide. Main standardized cables:

• A-cable: standard for the 8-bit SCSI interface, a 50-wire internal loop (IDC-50 connectors) or an external shielded one (CENTRONICS-50 connectors).
• B-cable: The 16-bit SCSI-2 expander is not widely available.
• P-cable: 16-bit SCSI-2/3 68-wire with improved miniature shielded connectors, universal for internal and external cables of 8-, 16-, and 32-bit SCSI versions (8-bit pins 1-5, 31-39, 65 -68 are not used). The connectors for external connections look like a miniature version of Centronics with flat contacts, while the internal ones have pin contacts.
• Q cable: 68-wire expansion to 32 bits, used in conjunction with a P-cable.
• Cable with D-25P connectors- 8-bit, standard for Macintosh, used on some external devices (Iomega ZIP-Drive).

Various variations of adapter cables are possible.

The assignment of connector contacts using the example of a common A-cable is given in Table. 2.

Connector pin	Signal	Connector pin	Signal
1	GND	26	DB0#
2	GND	27	DB1#
3	GND	28	DB2#
4	GND	29	DB3#
5	GND	30	DB4#
6	GND	31	DB5#
7	GND	32	DB6#
8	GND	33	DB7#
9	GND	34	DBParity#
10	GND	35	GND
11	GND	36	GND
12	GND/Reserved	37	Reserved
13	Open	38	TERMPWR
14	Reserved	39	Reserved
15	GND	40	GND
16	GND	41	ATN#
17	GND	42	GND
18	GND	43	BSY#
19	GND	44	ACK#
20	GND	45	RST#
21	GND	46	MSG#
22	GND	47	SEL#
23	GND	48	C/D#
24	GND	49	REQ#
25	GND	50	I/O#

Table 2. SCSI A-cable connectors

Tire

Like the PCI bus, the SCSI bus assumes the ability to exchange information between any pair of devices. Of course, most often the exchange is between the host adapter and peripheral devices. “Smart” software can sometimes “cut corners” - copying data between devices without accessing the computer’s system bus. Smart host adapters with built-in cache memory have great potential here. In each exchange on the bus, his initiator(Initiator) and target device(Target). In table 3 shows the purpose of the bus signals.

Signal	Source: I=Initiator, T=Target	Purpose
DBx#	-	Inverse data bus with parity bits
TERMPWR	-	Power supply for terminators
ATN#	I	Attention
BSY#	I, T	Bus is busy
REQ#	T	Request for data transfer
ACK#	I	Reply to REQ#
RST#	I, T	Reset
MSG#	T	Target conveys a message
SEL#	I/T	Selecting a target device by the initiator or Reselecting the initiator by the target device
C/D#	T	Control(0) / data(1) on bus
I/O#	T	Direction of transmission relative to the initiator or phase Selection(1)/Reselection(0)

Table 3. SCSI bus signal assignments

SCSI Device Configuration Options

All devices on the bus must be configured in a consistent manner. They require setting the following basic parameters programmatically or using jumpers:

Device ID— SCSI ID — address 0-7 (addresses 0-15 are valid for Wide-SCSI), unique for each device on the bus. Typically, the host adapter that should have the highest priority is assigned ID 7. The factory assignment of device IDs is shown in Table. 4, although it is not mandatory. Devices are addressed by a positional code (although the ID is specified by a 3-4-bit code), which ensures compatibility between addressing 8 and 16-bit devices on the same bus.

Table 4: Factory Default Device IDs

Specification currently under development PnP for SCSI devices, allowing you to automate the process of assigning identifiers. The specification provides the possibility of coexistence of traditional (Legasy SCSI) devices, the identifiers of which are specified by jumpers, with automatically configured PnP devices.

Parity control- SCSI Parity. If at least one device on a bus does not support parity, it must be disabled on all devices on that bus. Parity control, especially for disk devices, is a means of protecting against corruption of data during transmission.

Enabling Terminators- Termination. Modern devices use active terminators, which can be turned on by a single jumper or even controlled by a software signal. Terminators should be enabled only on the extreme devices in the chain. Modern host adapters allow you to automatically turn on your terminator if they are extreme, and turn it off if the internal and external channel connectors are used. This allows you to connect and disconnect external devices without worrying about switching terminators. In older adapter models, when making such switches, you had to open the case and rearrange the jumper. In older devices, passive terminators had to be installed in special sockets (and removed from there). In the absence of internal terminators, it was necessary to use external ones installed on the cable.

Power supply for terminators - TerminatorPower. Power supply to terminators by jumper or by software must be turned on on at least one device when active terminators are used (for modern devices this means “always”).

Synchronous communication speed matching- SCSI Synchronous Negotiation. The synchronous exchange mode, which provides high performance, is enabled by mutual agreement of the devices. However, if at least one device on the bus does not support it, negotiation must be disabled on the host adapter. Moreover, if the exchange is initiated by a synchronous device, the host will support this mode.

Start on command - Start on Command, or delayed start - Delayed Start. When this option is enabled, the device engine starts only upon a command from the host adapter, which reduces the peak load of the power supply at the moment of switching on. The host will launch devices sequentially.

Shutdown permission - Enable Disconnection. Selecting this option allows devices to disconnect from the bus when data is not ready, which is very effective in multitasking mode with several peripheral devices on the bus.

Host adapter

SCSI Host Adapter is the most important interface node that determines the performance of the SCSI device subsystem. There is a wide range of adapters, starting from the simplest ones, to which you can only connect devices that are not performance critical. Such adapters are sometimes included with scanners, and connecting a drive to them can be an insurmountable task. High-performance adapters have their own dedicated processor, large amounts of buffer memory, and use highly efficient direct bus control modes for memory access.

Configuring SCSI host adapters from the point of view of the SCSI bus is no different from configuring other devices (see above). For modern adapters, software configuration is used instead of jumpers. The configuration utility is usually included in the BIOS extension (on the adapter card) and is prompted to run during initialization during POST.

Like any expansion card, the host adapter must also be configured in terms of the expansion bus to which it connects. SCSI adapters exist for all buses: ISA (8-16 bits), EISA, MCA, PCI, VLB, PCMCIA. Parallel port adapters are available. Some newer motherboards have a built-in SCSI adapter.

System resources for the SCSI bus adapter include:

• Memory area for BIOS ROM expansion needed to support device configuration and disk functions. If several host adapters of the same type are installed in the system, the ROM BIOS for them is used from one adapter. It may turn out that it will not be possible to get several different types of host adapters to work together on one computer.
• I/O Port area.
• IRQ - interrupt request.
• DMA is a direct memory access channel (for ISA/EISA buses), often used to capture bus control (Bus-Mastering).

SCSI devices

“It is not possible to list all SCSI devices; we will list only a few of their types: hard drive, CD-ROM, CD-R, CD-RW, Tape (streamer), MO (magneto-optical drive), ZIP, Jaz, SyQuest, scanner. Among the more exotic ones, we note Solid State disks (SSD) - a very fast mass memory 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 the same set of commands is used to work with them.

Of course, as the SCSI physical layer developed, the software interface also changed. 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 SCSI ID. For some purposes, for example, CD-ROM device libraries, a 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 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 number 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 parameters: parity check, turning on the terminator, powering the terminator, turning on the disk at the controller’s command.

All SCSI devices require special drivers. A basic disk drive driver is usually included in the host adapter's BIOS. Extensions such as ASPI (Advanced SCSI Programming Interface) are downloaded separately.

Hard drives

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

The next installation step is formatting the disk. Before using a disk on a new controller, it is considered good practice to format it on it. This is due to the fact that different SCSI adapter manufacturers use different sector translation schemes (can be compared with LBA, CHS, LARGE for IDE drives) and when transferred 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 that 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 IOmega Jaz, you need to set the SCSI BIOS options 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 with it :) .

CD-ROM, CD-R, CD-RW

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

Streamers

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

Scanners

Typically, scanners come with their own card. Sometimes it is completely “our own”, as, for example, in the Mustek Paragon 600N, and sometimes it is just the most simplified version of 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 this capability) can be beneficial. Scanning A4 with 32-bit color at 600 dpi is a picture of about 90 Mb and transferring this amount of information via the 8-bit ISA bus not only takes a lot of time, but also greatly slows down the PC, since the drivers for this standard card are usually 16-bit ( example - Mustek Paragon 800IISP). An additional one is usually a cheap FastSCSI PCI controller. Less or more productive will not give anything new. This option also has a caveat - 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 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
• 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
• recommendations (personal and subjective)

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 can also be used from Tekram. There is a small and nice SCSI BIOS.
Controllers on the Symbios Logic SYM53C810 chip are well known to most OSes. SCSI BIOS specifically for this purpose is included in almost any AWARD BIOS for motherboards. Very cheap and yet functional.

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 a chip Symbios Logic 53C875. Many people note its speed and reliability.

Devices

HDD - of course Seagate Cheetah- It's hard to argue with RPM 10,000. But without additional cooling fans, this drive will not last long :(. Other Seagate drive series - Barracuda and Hawk - are also reliable.

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

This article was prepared based on materials from the book Mikhail Guk"IBM PC Hardware" (Peter Publishing House)

In this article, we take a look at 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 little more complex than simply replacing SCSI with SATA and SAS. Traditional parallel SCSI is a tried and tested interface that has been in use for a long time. Currently, SCSI offers very fast data transfer rates 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; over a short distance it is possible to create a daisy chain of 15 devices connected to a SCSI link. These features make SCSI an excellent choice for productivity desktops and workstations, all the way up to enterprise servers today.

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, with a speed of 300 MB/sec. Although it is slightly slower than SCSI at 320 Mbps, the SAS interface is capable of supporting up to 128 devices over longer distances than 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 sacrifice these in favor of significantly increasing capacity and reducing cost. For example, the SATA drive has now reached a capacity of 1 terabyte (TB). SATA is used where maximum capacity is needed, such as for data backup or archiving. SATA now offers point-to-point connections at speeds of up to 300 Mbps, and easily outpaces the traditional ATA parallel interface's 150 Mbps.

So what will happen to SCSI? It works great. The problem with traditional SCSI is that it is simply reaching the end of its life. Parallel SCSI, which has a speed of 320 MB/sec, will not be significantly faster on current SCSI cable lengths. For comparison, SATA drives will reach speeds of 600 MB/sec in the near future, SAS has plans to reach 1200 MB/sec. SATA drives can also work with a SAS interface, so these drives can be used simultaneously in some storage systems. The potential for increased expandability and data transfer performance far exceeds that of SCSI. But SCSI isn't going away anytime soon. We'll continue to see SCSI in small and midsize servers for several years to come. As hardware is upgraded, SCSI will be systematically replaced by SAS/SATA drives for faster speeds and easier connections.

This article will talk about what allows you to connect a hard drive to a computer, namely, the hard drive interface. More precisely, about hard drive interfaces, because a great many technologies have been invented for connecting these devices throughout their existence, and the abundance of standards in this area can confuse an inexperienced user. However, first things first.

Hard drive interfaces (or strictly speaking, external drive interfaces, since they can be not only drives, but also other types of drives, for example, optical drives) are designed to exchange information between these external memory devices and the motherboard. Hard drive interfaces, no less than the physical parameters of the drives, affect many of the operating characteristics of the drives and their performance. In particular, drive interfaces determine such parameters as the speed of data exchange between the hard drive and the motherboard, the number of devices that can be connected to the computer, the ability to create disk arrays, the possibility of hot plugging, support for NCQ and AHCI technologies, etc. . It also depends on the hard drive interface which cable, cord or adapter you will need to connect it to the motherboard.

SCSI - Small Computer System Interface

The SCSI interface is one of the oldest interfaces designed for connecting storage devices in personal computers. This standard appeared in the early 1980s. One of its developers was Alan Shugart, also known as the inventor of the floppy disk drive.

Appearance of the SCSI interface on the board and the cable connecting to it

The SCSI standard (traditionally this abbreviation is read in Russian transcription as “skazi”) was originally intended for use in personal computers, as evidenced by even the name of the format - Small Computer System Interface, or system interface for small computers. However, it so happened that drives of this type were used mainly in top-class personal computers, and subsequently in servers. This was due to the fact that, despite the successful architecture and a wide set of commands, the technical implementation of the interface was quite complex and was not affordable for mass PCs.

However, this standard had a number of features that were not available for other types of interfaces. For example, the cord for connecting Small Computer System Interface devices can have a maximum length of 12 m, and the data transfer speed can be 640 MB/s.

Like the IDE interface that appeared a little later, the SCSI interface is parallel. This means that the interface uses buses that transmit information over multiple wires. This feature was one of the limiting factors for the development of the standard, and therefore a more advanced, consistent SAS standard (from Serial Attached SCSI) was developed as its replacement.

SAS - Serial Attached SCSI

This is what the SAS server disk interface looks like

Serial Attached SCSI was developed as an improvement to the rather old Small Computers System Interface for connecting hard drives. Despite the fact that Serial Attached SCSI uses the main advantages of its predecessor, it nevertheless has many advantages. Among them it is worth noting the following:

• Use of a common bus by all devices.
• The serial communication protocol used by SAS allows for fewer signal lines to be used.
• There is no need for bus termination.
• Virtually unlimited number of connected devices.
• Higher throughput (up to 12 Gbps). Future implementations of the SAS protocol are expected to support data transfer rates of up to 24 Gbit/s.
• Possibility of connecting drives with Serial ATA interface to the SAS controller.

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

• Target devices. This category includes the actual drives or disk arrays.
• Initiators are chips designed to generate requests to target devices.
• Data delivery system - cables connecting target devices and initiators

Serial Attached SCSI connectors come in different shapes and sizes, depending on the type (external or internal) and SAS versions. Below are the SFF-8482 internal connector and the SFF-8644 external connector designed for SAS-3:

On the left is an internal SAS connector SFF-8482; On the right is an external SAS SFF-8644 connector with cable.

A few examples of the appearance of SAS cords and adapters: HD-Mini SAS cord and SAS-Serial ATA adapter cord.

On the left is the HD Mini SAS cable; On the right is an adapter cable from SAS to Serial ATA.

Firewire - IEEE 1394

Today you can often find hard drives with a Firewire interface. Although the Firewire interface can connect any type of peripheral devices to a computer, and it is not a specialized interface designed exclusively for connecting hard drives, Firewire nevertheless has a number of features that make it extremely convenient for this purpose.

FireWire - IEEE 1394 - view on a laptop

The Firewire interface was developed in the mid-1990s. The development began with the well-known company Apple, which needed its own bus, different from USB, for connecting peripheral equipment, primarily multimedia. The specification describing the operation of the Firewire bus is called IEEE 1394.

Firewire is one of the most commonly used high-speed serial external bus formats today. The main features of the standard include:

• Possibility of hot connection of devices.
• Open bus architecture.
• Flexible topology for connecting devices.
• Data transfer speeds vary widely – from 100 to 3200 Mbit/s.
• The ability to transfer data between devices without a computer.
• Possibility of organizing local networks using a bus.
• Power transmission via bus.
• A large number of connected devices (up to 63).

To connect hard drives (usually via external hard drive enclosures) via the Firewire bus, as a rule, a special SBP-2 standard is used, which uses the Small Computers System Interface protocol command set. It is possible to connect Firewire devices to a regular USB connector, but this requires a special adapter.

IDE - Integrated Drive Electronics

The abbreviation IDE is undoubtedly known to most personal computer users. The interface standard for connecting IDE hard drives was developed by a well-known hard drive manufacturer - Western Digital. The advantage of IDE over other interfaces that existed at the time, in particular the Small Computers System Interface, as well as the ST-506 standard, was that there was no need to install a hard drive controller on the motherboard. The IDE standard implied installing a drive controller on the drive itself, and only a host interface adapter for connecting IDE drives remained on the motherboard.

IDE interface on motherboard

This innovation has improved the operating parameters of the IDE drive due to the fact that the distance between the controller and the drive itself has been reduced. In addition, installing an IDE controller inside the hard drive case made it possible to somewhat simplify both motherboards and the production of hard drives themselves, since the technology gave freedom to manufacturers in terms of optimal organization of the logic of the drive.

The new technology was initially called Integrated Drive Electronics. Subsequently, a standard was developed to describe it, called ATA. This name is derived from the last part of the name of the PC/AT family of computers by adding the word Attachment.

An IDE cable is used to connect a hard drive or other device, such as an optical drive that supports Integrated Drive Electronics technology, to the motherboard. Since ATA refers to parallel interfaces (therefore it is also called Parallel ATA or PATA), that is, interfaces that provide for simultaneous data transmission over several lines, its data cable has a large number of conductors (usually 40, and in recent versions of the protocol it was possible to use 80-core cable). A typical data cable for this standard is flat and wide, but round cables are also available. The power cable for Parallel ATA drives has a 4-pin connector and is connected to the computer's power supply.

Below are examples of IDE cable and round PATA data cable:

Appearance of the interface cable: on the left - flat, on the right in a round braid - PATA or IDE.

Thanks to the comparative low cost of Parallel ATA drives, the ease of implementation of the interface on the motherboard, as well as the ease of installation and configuration of PATA devices for the user, Integrated Drive Electronics type drives have for a long time pushed out devices of other interface types from the market of hard drives for budget-level personal computers.

However, the PATA standard also has a number of disadvantages. First of all, this is a limitation on the length that a Parallel ATA data cable can have - no more than 0.5 m. In addition, the parallel organization of the interface imposes a number of restrictions on the maximum data transfer speed. It does not support the PATA standard and many of the advanced features that other types of interfaces have, such as hot plugging of devices.

SATA - Serial ATA

View of the SATA interface on the motherboard

The SATA (Serial ATA) interface, as the name suggests, is an improvement over ATA. This improvement consists, first of all, in converting the traditional parallel ATA (Parallel ATA) into a serial interface. However, the differences between the Serial ATA standard and the traditional one are not limited to this. In addition to changing the data transmission type from parallel to serial, the data and power connectors also changed.

Below is the SATA data cable:

Data cable for SATA interface

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

Serial ATA to PATA power adapter:

On the left is a general view of the cable; On the right is an enlarged view of the PATA and Serial ATA connectors

However, the situation is more complicated with a device such as an adapter for connecting a serial interface device to a parallel interface connector. Typically, an adapter of this type is made in the form of a small microcircuit.

Appearance of a universal bidirectional adapter between SATA - IDE interfaces

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

Type of adapter from IDE to SATA

You can tell us a little more about NCQ technology. The main advantage of NCQ is that it allows you to use ideas that have long been implemented in the SCSI protocol. In particular, NCQ supports a system for sequencing read/write operations across multiple drives installed in a system. Thus, NCQ can significantly improve the performance of drives, especially hard drive arrays.

Type of adapter from SATA to IDE

To use NCQ, technology support is required on the hard drive side, as well as on the motherboard host adapter. Almost all adapters that support AHCI also support NCQ. In addition, some older proprietary adapters also support NCQ. Also, for NCQ to work, it requires support from the operating system.

eSATA - External SATA

It is worth mentioning separately the eSATA (External SATA) format, which seemed promising at the time, but never became widespread. As you can guess from the name, eSATA is a type of Serial ATA designed for connecting exclusively external drives. The eSATA standard offers most of the capabilities of the standard for external devices, i.e. internal Serial ATA, in particular, the same system of signals and commands and the same high speed.

eSATA connector on a laptop

However, eSATA also has some differences from the internal bus standard that gave birth to it. In particular, eSATA supports a longer data cable (up to 2 m) and also has higher power requirements for drives. Additionally, eSATA connectors are slightly different from standard Serial ATA connectors.

Compared to other external buses, such as USB and Firewire, eSATA, however, has one significant drawback. While these buses allow the device to be powered via the bus cable itself, the eSATA drive requires special connectors for power. Therefore, despite the relatively high data transfer speed, eSATA is currently not very popular as an interface for connecting external drives.

Conclusion

Information stored on a hard drive cannot be useful to the user or accessible to application programs until it is accessed by the computer's central processing unit. Hard drive interfaces provide a means of communication between these drives and the motherboard. Today, there are many different types of hard drive interfaces, each of which has its own advantages, disadvantages and characteristic features. We hope that the information provided in this article will be largely useful to the reader, because the choice of a modern hard drive is largely determined not only by its internal characteristics, such as capacity, cache memory, access and rotation speed, but also by the interface for which it was developed.