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What does scsi disk device mean? SCSI - what is it? A Brief History of the SCSI Standard

“We bravely step into uncharted territory” - IDE drives on SCSI controllers

With each new generation of drives, hard drive manufacturers pull new tricks out of their sleeves: the latest models are faster, quieter and larger than their predecessors. They have already reached 200 GB - and soon we will see 300 GB drives. But disks of this size are not available with a SCSI interface, and SCSI is the standard for the server market.

Powerful server systems must be reliable, fast, and have resources in terms of power and capacity. The first two parameters can be achieved without problems by using the best SCSI controllers and the best hard drives. But increasing storage capacity can cost a pretty penny.

So why don't we try using cheaper ones? IDE solutions- they do the same job as their more expensive SCSI counterparts. However, several arguments speak against the use of IDE disks: the maximum number of devices, the reliability of modern hard drives and lack of controller functionality.

Taiwanese manufacturer Acard has developed an adapter that allows IDE drives to work on SCSI controllers.

In fact, such problems do not affect home users. Even though SCSI systems are faster, they are not so attractive due to their high cost. In addition to the money you will pay for a modern hard drive, you will also need to buy a node controller. If you need a RAID controller, then be prepared to shell out at least the cost of a Pentium 4.

With two Ultra160 SCSI channels, the Adaptec 39160 provides a level of flexibility that is hard to beat.

Today, IDE drives are characterized by high speed and capacity. As for the price, SCSI is not a competitor to them.

But the server segment dictates completely different rules of the game. It's not about extra gigabytes - priority is given to maximum reliability and performance, since even a minor server downtime will cost serious money, and in the worst case, even call into question the existence of the company.

This is why SCSI solutions are so expensive: expensive development, high-quality components, and the market is relatively small.

However, not so long ago Maxtor announced its entry into the server segment of the market with a new line of drives with an IDE interface. With low minimum performance and adequate reliability, the goal is to achieve significantly increased capacity compared to SCSI drives (where the current maximum is 147 GB). Theoretically, the plan is good, because for the price of five Ultra320 SCSI drives, each 147 GB, you can buy 15 of the latest IDE drives, each 200 GB.

The only thing missing today is suitable controllers. There is little chance that manufacturers will release IDE versions of their high-end controllers. However, there are a huge number of SCSI node controllers on the market.

Besides the IDE2SCSI adapters below, Acard is primarily known for its SCSI and IDE controllers and related products, as well as unusual data solutions such as CD or DVD copy stations.

Also from Acard: dual-channel IDE RAID controller AEC-6880.

An unusual thing: IDE2SCSI adapter AEC7722, front view.

The adapter is equal in width to a 5.25" drive and connects directly to the IDE hard drive. However, the current on the IDE bus is not enough to power the controller, so an external power connection is required.

For tests we used HDD IBM (Hitachi).

As you can see, the connected adapter protrudes slightly to the left. Before purchasing an adapter, be sure to check that you have enough space in your computer case.

Be careful when connecting the adapter, as the board will bend slightly under pressure.

There are no components located on the back of the adapter. Only IDE connector.

According to Acard, the maximum speed of the adapter interface is 80 MB/s. Even though the peak transfer speed of modern drives may be higher, this throughput speed will be sufficient for most applications.

Chip, BIOS and jumpers (top). The last two are used to set the SCSI-ID.

The heart of the IDE2SCSI adapter: a controller made by Achip (ARC765-D).

Front and back views of the adapter.

Upside down: the SCSI node adapter from Adaptec looks for available drives. A 180 GB IDE disk from IBM was detected.

The SCSI connector has 80 small pins (top). In contrast, the IDE has only 40 pins.

A typical Ultra160 SCSI cable has three to five connectors for connecting drives. For more expensive versions, the number of connectors can reach up to 15.

The SCSI specification calls for termination at both ends of the bus, meaning there must be a special resistor there to prevent signals from being reflected.

Testing

Test system
CPU	Intel Pentium 4, 2.0 GHz 256 KB L2-Cache (Willamette)
Motherboard	Intel D845EBT, 845E chipset
Memory	256 MB DDR/PC2100, CL2, Infineon
Controller	IDE: i845E UltraDMA/100-Controller (ICH4) SCSI: Adaptec AHA-39160 Ultra160-SCSI
Video card	NVIDIA GeForce2 MX 400
LAN card	3COM 905TX PCI 100 MBit
OS	Windows XP Pro 5.10.2600, SP1
Tests
High-end applications	ZD WinBench 99 - Highend Disk Winmark 1.2
Performance	HD Tach 2.61, PC Mark 2002 (HD Test)
I/O Performance	Intel I/O Meter
Drivers and settings
Video driver	NVIDIA reference driver 29.42
IDE driver	Intel Application Accelerator 2.2.2
DirectX version	8.1
Permissions	1024x768, 16 bit, 85 Hz refresh

To see how a modern IDE hard drive would perform on a SCSI controller with a typical setup, we tested the IBM IC35L180 test drive in both configurations.

Conclusion: useful but expensive

The test result is clear: the difference between a hard drive running on IDE and on an Adaptec 39160 SCSI controller is negligible in all important tests.

The slightly reduced I/O performance is due to the need to convert interface protocols, which is quite important in a server environment. Each disk access operation is processed by the Achip controller. So the IDE hard disks with the adapter should not be used in disk-intensive applications (i.e. databases or web servers). In these areas, SCSI drives have a clear advantage over their IDE counterparts because they can provide more I/O operations per second.

SCSI adapters and IDE hard drives with adapters are interesting in applications that require large-capacity hard drives. If you install large hard drives, you can equip your data storage with fewer disks, and more importantly, it will cost you much less than the SCSI option. Even if you install several backup drives in case of IDE hard drive failure (in a large RAID cluster), you will still save a significant amount of money. Of course, the transition to such a configuration is, first of all, a matter of trust in the hard drive manufacturer.

If you are interested in the IDE2SCSI adapter, then we will disappoint you a little: it is by no means cheap. On the Acard website, prices start at $69 - a fairly significant price for a controller designed for budget-friendly solutions.

Therefore, using an Acard adapter only makes sense in cases where you will save a lot of money by abandoning SCSI drives and switching to large IDE drives, without taking into account additional expensive security measures (redundancy, mirroring, hot-swap bays).

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

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

Later, in 1981, M. Shugart transferred documentation on the SASI interface to the ANSI committee (American National Standards Institute, analogue of GOST), which accepted it as the basis for work on the project, which was called SCSI. Most most important points from the SASI standard migrated to SCSI, for example, such important principles as device arbitration, bus release mechanisms, the ability to use more than one host adapter on the bus, etc. In 1984, the working documentation of the SCSI standard was submitted to ANSI, and, after numerous adjustments and additions, document number X3.131-1986 was adopted in 1986 - the first official SCSI standard, which is now called SCSI-1. In addition to the SASI standard, SCSI-1 has acquired such important functionality, as 10-bit commands, synchronous and asynchronous data transfer protocol, the ability to connect to one host adapter up to 8 various devices. The standards that followed SCSI-1 developed both in the direction of expanding the command language and increasing and complicating the protocols, as well as increasing the bus width, increasing the speed and number of devices connected to one host adapter. For current SCSI standards, the bus width is 16 bits, the number of connected devices is also 16.

The PC industry did not miss the emergence of a new standard, which was immediately adopted mainly by HDD manufacturers. In Fig. 1, 2 show some of the first samples of SCSI disks.

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

A Brief History of the SCSI Standard

The very first standard is SCSI-1; in this standard, it was possible to connect up to eight devices, including the controller, to one bus. The interface contains advanced management tools and at the same time is not focused on any specific type of device. It has an 8-bit data bus, the maximum transfer speed is up to 1.5 MB/s in asynchronous mode (according to the "request-acknowledgement" method), and up to 5 MB/s in synchronous mode ("several requests - several confirmations" method) . Parity can be used to detect errors. Electrically implemented in the form of 24 lines (unipolar or differential), although the vast majority of devices use unipolar signals.

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

Ultra SCSI interface, uses a bus frequency of 20 MHz. The Ultra/Wide SCSI interface supports 16 devices and provides data transfer rates of up to 40 MB/s. Faster Ultra-2 Wide SCSI, providing transfer speeds up to 80 MB/s. The following interfaces - Ultra-3 SCSI, Ultra 320 SCSI, Ultra 640 SCSI - did not bring anything fundamentally new to the standard except speed. They also remain with a 16-bit bus width, and up to 16 devices can also be connected to the interface. Comparative characteristics SCSI standards are given in Table 1.

Table 1. Comparative characteristics of SCSI standards

Standard	Maximum bus speed, MB/sec.	Bus width	Maximum cable length, m			Maximum number of devices
Standard	Maximum bus speed, MB/sec.	Bus width	The only device	LVD	HVD	Maximum number of devices
SCSI-1	5	8	6	(3)	25	8
SCSI-2	10	8	3	(3)	25	8
Wide SCSI-2	20	16	3	(3)	25	16
SCSI-3	20	8	1.5	(3)	25	8
Wide SCSI-3	40	16	—	(3)	25	16
Ultra— 2 SCSI	40	8	(4)	12	25	8
Wide Ultra-2 SCSI	80	16	(4)	12	25	16
Ultra-3 SCSI,orUltra-160 SCSI	160	16	(4)	12	(5)	16
Ultra 320 SCSI	320	16	(4)	12	(5)	16
Ultra 640SCSI	640	16	(4)	(7)	(5)	16

What is a host adapter?

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

The following manufacturers produce or have produced host adapters for SCSI devices in the past:

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

Rice. 3. SCSI host adapter from Adaptec

Modern SCSI HDD manufacturers

Currently, the HDD market is undergoing a rapid evolution - new, high-speed Serial ATA standards are replacing Parallel ATA. And, although new SATA devices have already come very close in operating speed to SCSI devices, and in some places they are even ahead of them, SCSI devices remain just as popular in High-End computers - servers and information arrays. This is due, first of all, to the high reliability of SCSI drives - both due to the relative simplicity of SCSI standards and a well-thought-out electrical interface, and due to the traditionally more careful design and manufacturing of devices. SCSI accounts for approximately 30 percent of the entire HDD market, and it is unlikely that it will ever cross this line: equipping a PC with all the necessary cables, adapters, as well as purchasing the host adapter itself will cost approximately $100, while drives will cost several times more their IDE brothers. Modern SCSI drive manufacturers are:

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

Rice. 4. Server equipped with SCSI disks

Rice. 5. SCSI drive bay

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

It is worth noting that very often server manufacturers re-label drives, giving them their own brands. As an example, I will give drives removed from Hewlett Packard and IBM e-Server servers (Fig. 7, 8), on which the real manufacturer of the HDD can be identified only by the model name; The author has also seen disks removed from Dell servers where even this information was missing.

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

SCSI Connector Types

Rice. 9. SCSI connector types currently in use

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

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

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

Rice. 13. Adapters required for connecting SCSI devices

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

How SCSI works

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

All SCSI devices are usually divided into initiators and executors. It should be taken into account that the bus can be standard (8 bits) or extended (16 bits) wide. Taking all this into account, the total number of possible device connection combinations can be reduced to four:

1. Standard initiator - standard executor
2. Extended initiator - extended executor
3. Standard initiator - extended executor
4. Advanced initiator - standard executor

When connecting standard executors to extended initiators, no problems can arise - the extended standard supports all the functions of the standard one, however, when connecting back, difficulties may arise with connecting terminators. In reality, these problems are easily solved by using adapters (see above).

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

After the selection phase, the initiator can time out, for which it can use two methods - perform a hardware reset or go to the “bus free” phase. In any case, the end of the cycle of work on the SCSI bus will be the setting of the “command completed” status or the transmission of a corresponding message with the release of the bus. Similar to the ATA standard, SCSI systems can use two protocols to reset the device - the hardware reset protocol ( hard reset) and according to the soft reset protocol. In both cases, the Reset line will have a one bit set; the differences in types of resets lie in their mechanism and purpose - as a rule, a hardware reset is carried out to reset operations across the entire system of SCSI devices, while a software reset is used to reset only one device, not interfering with the work of others.

Rice. 19. SCSI bus phase sequence block diagram

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

Table 2. Types of information transfer via the SCSI bus

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

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

SCSI Commands

Team

Command code

CHANGE DEFINITION
Compare
Copy (COPY)
Copy and verify (COPY AND VERIFY)
FORMAT UNIT
Request (INQUIRY)
Lock-Unlock Cache
Log selection (LOG SELECT)
Log sensitivity (LOG SENSE)
Mode selection (MODE SELECT)
Mode sensitivity (MODE SENSE)
Preamplification (PRE-FETCH)
Deny permission to change media (PREVENT-ALLOW MEDIUM REMOVAL)
Reading (READ)
Read buffer (READ BUFFER)
Show capacity (READ CAPACITY)
Read defective data (READ DEFECT DATA)
Long reading (READ LONG)
REASSIGN BLOCK
RECEIVE DIAGNOSTIC RESULTS
RELEASE
REQUEST SENSE
Reserve (RESERVE)
Reset the device (REZERO UNIT)
Find identical data (SEARCH DATA EQUAL)
Find the highest data (SEARCH DATA HIGH)
Find low data (SEARCH DATA LOW)
Position (SEEK)
Request for diagnostics (SEND DIAGNOSTIC)
Set limit (SET LIMIT)
Start-stop the device (START STOP UNIT)
Synchronize cache (SYNCHRONIZE CACHE)
Request for device readiness (TEST UNIT READY)
Verification (VERIFY)
Record (WRITE)
WRITE AND VERIFY
Writing to a buffer (WRITE BUFFER)
Long recording (WRITE LONG)
Write the same (WRITE SAME)

40h
39h
18h
3Ah
04h
12h
36h
4Ch
4Dh
15h, 55h
1Ah, 5Ah
34h
1Eh
08h
28h, 3Ch
25h
37h
3Eh
07h
1Ch
17h
03h
16h
01h
31h
30h
32h
0Bh
2Bh, 1Dh
33h
1Bh
35h
00h
2Fh
0Ah
2Ah
2Eh
3Bh
3Fh
41h

The table above lists the main SCSI commands applicable to HDDs. As in the ATA standard, for the SCSI standard there are both mandatory commands, that is, those that must be supported by any SCSI device, and optional, optional commands, the support of which may not be supported by the device. In addition to them, there are so-called vendor commands that are not described in the standard, specific to each manufacturer and often for each specific line of devices - commands that the manufacturer uses for the purpose of repairing or diagnosing the device. These commands are, as a rule, a trade secret of the manufacturer and are not published anywhere.

SE, LVD, HVD

Typically, you will find markings similar to those shown in Figure 1 on a SCSI device. 20. This marking indicates the type of data transmission at the electrical level. The first is SCSI SE (Single Ended), which refers to a type of data transfer where each signal on the bus is provided by one conductor. SCSI LVD (Low Voltage Differential) and SCSI HVD (High Voltage Differential) - low-voltage and high-voltage differential types - are physically organized in the same way: for each signal there are two conductors, one carrying a signal of positive polarity, the other - negative. The differences between HVD and LVD are in the voltage in the conductors; for LVD it is lower than for HVD.

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

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

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

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

Synopsis. Prospects and opportunities

The SCSI interface is very productive and reliable, but it also has a considerable number of disadvantages. First of all, this is the high cost of the devices themselves - both drives and controllers. The next disadvantage is the complexity of configuration and management, which only trained people can handle. Finally, the last drawback of the interface, which makes it even less attractive to the user, is the inability to transfer the media to another PC unless it is equipped with a specialized SCSI adapter...

The use of SCSI devices is not practical for the standard PC market for a very simple reason: high cost. However, manufacturers do not set themselves the goal of winning over the average consumer: it just so happened historically that SCSI drives are mainly a server standard, and an IDE standard for workstations.

Meanwhile, SCSI drives are being closely followed by the latest IDE device standard: SATA. The speed and performance of SATA devices are very high, and their use in servers is becoming increasingly popular. The only disadvantage of SATA is its rather flimsy connector, which is associated with quite frequent failures of these devices. I think that the SCSI interface will undoubtedly win the battle with SATA in the field of server drives.

The development of the SCSI standard promises us in the future faster devices with traditional SCSI reliability; It is not possible to predict the imminent departure of SCSI devices from the market.

Serial Attached SCSI (SAS)

The latest trend in the world of SCSI devices is Serial Attached SCSI, an interface that uses three data transfer protocols (SSP - Serial SCSI Protocol, STP - Serial ATA Tunneled Protocol, SMP - Serial Management Protocol). As can be seen from the names of the protocols, the first two are intended for data transmission itself, the last is intended for interface management. Drives with this interface are currently produced by Seagate, Samsung and Fujitsu.

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

SCSI - Small Computer System Interface

Despite the apparent dominance of devices with the IDE/EIDE interface, SCSI hard drives still account for about 27% of the market in terms of production volume. 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 would argue that recently IDE hard drives have achieved SCSI performance 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 needs to spend at least $100. But there are people who persistently prefer this interface with a difficult-to-read name. By the way, SCSI is read and pronounced as " tell me" I also partially consider myself one of these people and will try to attract at least a few more users to our side, as well as talk a little about SCSI itself.

SCSI vs IDE

The "Which is better: IDE or SCSI" debate 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,” is unsolvable in this formulation. The most common and correct reaction to them is “What for?” Having considered this issue in more detail, you can decide for yourself whether SCSI is necessary for yourself.

Let's tell you in more detail what a simple SCSI controller can provide compared to an IDE and why you should choose it or not choose it.

SCSI offer	EIDE/ATAPI objections	SCSI response
ability to connect 7 devices to one controller (Wide - 15)	it's easy to install 4 IDE controllers and there will be 8 devices in total	Each IDE controller needs 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	Are you sure 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 don’t overclock the PCI bus, you can do it by a meter	few!
you can use caching and RAID technologies to dramatically improve performance and reliability	There used to be caching Tekrams, but now there are RAIDs for IDE	it doesn't work and it's not serious at all

* It is worth noting 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 maximum length connections can be significantly reduced.

To avoid the impression that the IDE is very bad and you should be ashamed of using it, let us also note the positive qualities of the IDE interface, partly in light of the table above:

Price. It's undeniable sometimes Very important.
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.
It is difficult to use a cable longer than 80cm in a minitower case :)
IDE HD is much easier to install, there is only one jumper, and not 4-16 as on SCSI :)
Most people already have an IDE controller. motherboards
IDE devices always have a 16-bit bus, and for models of comparable price, IDE wins in speed.

Now about the price. The simplest SCSI on the ISA bus costs about $20, but now no one needs such things, so you can find them cheaper. The next option is a controller on PCI bus. The simplest version of FastSCSI costs about $40. However, now there are many motherboards on which the Adaptec 7880 UltraWideSCSI can be installed for just +$70. Even the famous ASUS P55T2P4 and P2L97 have SCSI options. For UWSCSI cards, the price varies from $100 to $600. There are also dual-channel (like IDE on Intel Triton HX/VX/TX) controllers. Their price is naturally higher. Note that in the case of SCSI, unlike IDE, where it is difficult to come up with something new, for additional money the controllers can be expanded with the functions of a cache controller, RAID-0..5, hotswap, etc., so we are talking about the upper the cost limit of the controller is not entirely correct.

And finally about speed. As you know, today the maximum information transfer speed over the IDE bus is 33 Mb/s. For UWSCSI, the same parameter reaches 40 Mb/s. The main advantages of SCSI appear when working in multitasking environments (well, a little in Windows95:). Many tests given under WindowsNT show the undoubted advantage of SCSI. This is perhaps 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) for which it is simply impossible to use an IDE. We will not talk about differences in internal architectures, which also affect performance, in this article, since there are too many special terms there. Let us only note that as we watch the development of the IDE, we are surprised to notice that it is acquiring many SCSI features, but, hopefully, they will not merge completely.

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, the connectors take up the most space. The largest (and oldest) is the 8-bit internal device 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 suggests, external SCSI devices can and should be connected to it. The picture shows a 50-pin mini-sub D connector.

For Wide devices, a similar one is used, but with 68 pins; the fastening is also used not in the form of latches, but with screws - like COM mice and printers. It is even smaller than narrow due to the higher contact density. (By the way, despite the name, the wide train is also narrower than the narrow train). Sometimes you can find old version

external connector - just centronix. You can find the same one (externally, but not functionally:) on your printer. Some devices, such as the IOmega ZIP Plus, and those designed for Mac, use a regular 25-pin Cannon (D-SUB), like a modem. Mini-centronics are also used for external high-speed connections. Here's the full table:

(sizes are almost original)
Domestic	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
connecting internal wide devices, mainly HDDs
External	DB-25
Domestic	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
connection of internal narrow devices - HDD, CD-ROM, CD-R, MO, ZIP. (like IDE, only for 50 pins)	or Micro DB68, Mini DB68. standard external wide connector
connection of internal narrow devices - HDD, CD-ROM, CD-R, MO, ZIP. (like IDE, only for 50 pins)	or Micro Centronics. according to some sources it is used for external connection SCSI devices

As you know, any device requires software support to operate. For most IDE devices, the minimum is built into Motherboard BIOS boards; the rest require drivers for various operating systems. For SCSI devices, things are a little more complicated. For initial boot from SCSI hard drive and working in DOS, you need your own SCSI BIOS. There are 3 options here.

The SCSI BIOS chip is on the controller itself (like on VGA cards). When the computer boots, it is activated and allows you to boot from a SCSI hard drive 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 drive devices to run under DOS. The SCSI BIOS image is flashed into the flash BIOS of the motherboard. Further according to point 1. Usually in
Board BIOS

add SCSI BIOS for a controller based on the NCR 810 chip, Symbios Logic SYM53C810 (it’s the one in the first picture) or Adaptec 78xx. If desired, you can manage this process and 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.

The next remark follows from the first. As you know, motherboards usually have CMOS. The BIOS stores board settings in it, including the configuration of hard drives. For SCSI BIOS it is often necessary to also 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. In this SCSI chip, the BIOS can save both SCSI device parameters and its own. 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.

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

The last picture (I couldn't find it quickly:) shows a two-channel Ultra Wide SCSI controller.

Its specification includes the following items: RAID levels 0,1,3,5; Failure Drive Rebuilding; Hot Swap and on-line Rebuilding; cache memory 2, 4, 8, 16, 32 Mb; Flash EEPROM for SCSI BIOS. The 486 processor is very clearly visible, which apparently is trying to manage all this stuff.

You can also find on the SCSI controller board
SCSI bus activity LED and/or connector for its connection
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

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

SCSI Specifications

its width is 8 or 16 bits. Or, in other words, "narrow" or "wide".
speed (roughly - the frequency at which the bus is clocked)
physical type of interface (unipolar, differential, optics...). sometimes this can be called a connector type for connection

Speed is mainly affected by the first two parameters. They are usually written as prefixes to the word SCSI.

The maximum transmission speed 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 know only in the Wide version and only with the LVD interface.

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

DC	80-pin Differential
F.C.	Fiber Channel
N	50-pin SCSI connector
ND	50-pin Differential SCSI connector
W	68-pin Wide SCSI connector
W.C.	80-pin Single connector SCSI
W.D.	68-pin Wide Differential SCSI connector
LW	68-pin Wide SCSI connector, low-voltage Differential
L.C.	80-pin Single connector SCSI connector, low-voltage Differential

In everyday life, you mainly encounter interfaces designated N and W. Their “Differential” versions provide increased noise immunity and an increased permissible length of the SCSI bus. "Low-voltage" is used with the new Ultra2 protocol. “Single connector” is used mainly in hot-swap configurations, because combines SCSI power and ground signals into one connector. "Fiber Channel" is more like an interface local network than on SCSI, because it is serial interface. A speed of 100Mb/s is quite normal for it. Used in Hi-End configurations.

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 it develops physical level SCSI also changed its software interface. One of the most common today is ASPI. On top of this interface you can apply scanner drivers, CD-ROMs, MO. For example, the correct CD-ROM driver can work with any device on any controller, as long as the controller has an ASPI driver.

By the way, Windows95 emulates ASPI even for IDE/ATAPI devices. This can be seen, for example, in programs such as EZ-SCSI and Corel SCSI. Each device on the SCSI bus has its own number. This number is called SCSI ID. For devices on a narrow SCSI bus, it can be from 0 to 7, on a wide bus, 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, 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,

To install a SCSI controller and device, the minimum requirement is to have them and a SCSI cable :). You may also need 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 connected:) for driver media. More complex configurations may include external SCSI cables, external terminators (see below), Wide-Narrow adapters, etc. Questions often arise about the ability to connect Fast/Ultra/Narrow/Wide devices in various combinations. For the most common devices general rule in this case it is this: if the connectors match, then you can connect. In other words, in this case it is important to distinguish between Narrow/Wide and not pay attention to Fast/Ultra.

(Ultra2 remains aside, since it only exists in the LVD connector/interface version). However, speed and reliability may drop significantly. See the SCSI Characteristics/Interfaces section above for more details. In addition, there are various narrow-wide adapters, but their use is not recommended.

Controller

As already mentioned, usually the controller 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 disks, support for removable as a disk during boot, etc.

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

Terminator options:
internal. usually found 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 this is exactly the case. are turned off by removing all assemblies from the panels. external. as in point 3, but more beautiful. for example on the HP T4e streamer. The device (usually external) has two SCSI connectors. one connects the cable to the controller, the other connects the terminator or cable to next device

in a 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 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.

Option with several 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 internal and external devices.

There are internally and several external devices.

Terminators 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:

Using Narrow and Wide devices on the same SCSI bus

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

Homework: The Wide controller has 3 connectors: external and internal wide and internal narrow. You can connect three cables with devices to them. Question: On which devices should terminators be enabled?

Using a Narrow device on a Wide controller (bus) This option is quite workable. 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 a wide external connector. If you still use adapters, pay attention to the 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).

Connecting hard drives is very simple, you just 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 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 is nothing terrible there :).

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 handle most

operating systems

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 600dpi 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, because drivers for this standard card are usually 16-bit (for 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 FastSCSI PCI controller - Tekram DC-390. This controller is based on the 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.

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.

Materials used in preparing this article
companies IBM, Seagate, ASUSTeK, Tekram

What is SCSI?

A: The [SCSI Basics] section is devoted to answering this question.
What is SAS, what is better than SCSI or SAS and how do they differ?
A: The [SAS or SCSI] section is devoted to answering this question.
What is eSATA?
A: eSATA is a SATA interface designed to connect external SATA devices. It provides a 3 Gbps channel, eliminating the bandwidth lag associated with today's external storage devices.

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

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

Is it possible to use SATA drives with SAS controllers?

A: Yes, you can, and you can simultaneously use both SAS and SATA drives on one controller. This allows you to begin the transition to SAS technology right now at a reasonable cost.

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

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

What is the difference between SCSI-1, SCSI-2, Fast, Wide,Ultra Wide and Ultra2 SCSI?
A: The main difference is the set of SCSI commands and the bus width (respectively, the speed).
SCSI-1 5MB/Sec 8 bit SCSI bus
SCSI-2 5MB/Sec 8 bit SCSI bus
SCSI-2 Fast 10MB/Sec 8 bit SCSI bus
SCSI-2 Fast Wide 20MB/Sec 16 bit SCSI bus
SCSI Ultra 20MB/Sec 8 bit SCSI bus
SCSI Ultra Wide 40MB/Sec 16 bit SCSI bus
Ultra2 Wide 80MB/sec 16 bit SCSI bus
Ultra160 160MB/sec 16 bit SCSI bus
Ultra320 320MB/sec 16 bit SCSI bus

When should you use a Low Voltage Differential (LVD) controller?
A: In case:
High data transfer speed required - 80 - 320 MB/s
The surrounding environment has a very high level of electromagnetic noise that affects data transmission. LVD mode provides much greater noise immunity than Single Ended (SE) SCSI
It is necessary to ensure significant removal of SCSI devices from the computer. LVD devices can be removed from the SCSI controller at a distance of up to 12 meters (this is the maximum allowable length of an LVD SCSI cable.

What is a SCSI terminator and why is it needed?
A: SCSI Terminator is small electronic device, which should be located at both ends of the SCSI bus and there should be exactly two of them (terminators) for each SCSI bus. Most often, the first SCSI Terminator is the SCSI controller (as a rule, this function can be “turned off” in the controller’s BIOS, but by default it is enabled), and the second is the terminator connected to the last (from the SCSI controller) connector of the SCSI cable.

Some SCSI devices (legacy disks, floppy drives, tape drives) have a built-in terminator, which can be enabled by using the appropriate jumper on the device. In this case, you need to ensure that the device with the terminator enabled is located at the very end of the SCSI bus.

But everything works for me even without a SCSI terminator, maybe this will do?
A: For the time being, it may be fine, especially if you have only one disk and it is not used very intensively. But as you increase the number of devices on the SCSI bus, or as the load on it increases, you eventually risk losing data, so you shouldn't skimp on it.

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

The SCSI ID is assigned either automatically (for example, if hotswap drive cages that support such a function are used), or by manually setting the appropriate jumpers on the SCSI devices. The SCSI ID is in no way related to the physical order of devices on the SCSI bus (for example, a SCSI controller, as a rule, has a default SCSI ID value of 7, although most often, but not always, it is located at the beginning of the SCSI bus), it is only important so that there are no devices with the same SCSI ID on the same SCSI bus.

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

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

It should be taken into account that the vast majority of controllers do not report the occurrence of such an error, so when connecting new devices to the SCSI bus, you must pay attention to maintaining the uniqueness of the SCSI ID.

Please note that the SCSI controller itself also has a SCSI ID (as a rule, it is equal to 7, and can be changed in the controller’s BIOS), so you should not assign the same SCSI ID to the disks.

What is SAF-TE?
A: SAF-TE - SCSI Accessed Fault-Tolerant Enclosure is an "open" specification designed to provide a comprehensive and standardized method for monitoring and reporting the status of disk drives, power supplies, and cooling systems used in applications. high reliability servers and data storage subsystems. Technical requirements independent of hardware input-output, operating systems and server platform, because the case itself appears as just another device on the SCSI bus. SAF-TE specifications have been adopted by many leading manufacturers of servers, storage devices and RAID controllers. Products that meet the SAF-TE specification reduce the cost of monitoring the status of enclosures, simplify the work of the network administrator, and provide emergency notifications and information about the status of equipment.

External PC interfaces - SCSI bus

SCSI (Small Computer System Interface), pronounced “skazi”, is a system-level interface, standardized by ANSI, in contrast to 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 - 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 interface and a minimum of 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.

Maximum throughput depends on the frequency and bit width of the bus and for combinations of these extensions is given in table. 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 that define individual aspects of the interface, and in many ways overlaps with 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	connecting internal wide devices, mainly HDDs
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(posted on printed circuit board devices) 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 only be enabled on the end 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 new 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.

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).

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.

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. Description possible options it’s too big, 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).

Usually it is installed on top of the ASPI driver.

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

operating systems

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 through 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)

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
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.

UltraWideSCSI PCI

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) - here everything 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)

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