What is a SCSI hard drive? High-speed interfaces: SCSI What does SCSI mean?

leading specialist of the company "EPOS"

IN Lately A large number of different devices have appeared on our market that significantly expand the capabilities of a computer. These are primarily Zip, Jaz drives and magneto-optics, this various types magnetic tape drives, as well as write-once and write-once devices to CDs. Scanners have become very popular. Prices for hard drives have dropped to such an extent that a computer with two or three drives is no longer uncommon, and the server necessarily contains a fault-tolerant disk array. In this regard, quite often the task of connecting new devices to a computer arises. Most simply this task can be solved if a SCSI controller is installed in the computer.

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

What is a SCSI interface?

Basic SCSI (Small Computer System Interface, sometimes called SCSI-1) is a universal interface for connecting various devices. In the basic standard, up to eight devices, including the controller, could be connected to one bus. The interface contains advanced 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 acknowledgments" 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.

In the process of development, the SCSI-2 standard was adopted - 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).

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

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

Each Ultra-2 Wide bus signal is transmitted over two wires in antiphase (differential). This is the so-called LVD (Low Voltage Differential), low-voltage differential signal transmission. Thanks to differential signal transmission, the permissible length of the connecting cable was increased to 12 m.

A comparison of various SCSI interfaces is shown in the table:


Standard	Length cable, m	Speed, Mb/s	Quantity devices
SCSI-1	6	5	8
SCSI-2	6	5...10	8 or 16
Fast SCSI-2	3	10...20	8
Wide SCSI-2	3	20	16
Fast Wide SCSI-2	3	20	16
Ultra SCSI-3, 8-bit	1,5	20	8
Ultra SCSI-3, 16-bit	1,5	40	16
Ultra-2 SCSI	12	40	8
Wide Ultra-2 SCSI	12	80	16

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

Further development of technology led to the emergence of the Ultra160/m SCSI standard. The transfer rate is increased from 80 to 160 MB per second by using both edges of the request/acknowledge signal to synchronize data. The Ultra160/m SCSI standard uses a low-level differential (LVD) interface and allows cable lengths of up to 12 meters. New component Ultra160/m SCSI interface – environmental control. This intelligent technology examines the storage subsystem, including interconnect cables, backplanes, terminators, etc. If there is a risk of data loss, transmission occurs at lower speeds - a method widely used by modems and fax machines.

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

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

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

The transfer speed for a single drive was the same in both Ultra and Ultra-2 modes (in Chart 1 SE). Software RAID level 0 in Ultra mode increased disk system performance by approximately 2 times (4 SE). The same drives switched to Ultra-2 mode increased performance by more than 3 times (4 LVD).

To compare the efficiency of simultaneous operation of several devices with a SCSI interface and an IDE interface, a software RAID level 0 was also assembled on four IDE drives. Despite the fact that the performance of a single IDE disk was comparable to the performance of SCSI disks (1 IDE), the use of RAID on four IDE disks practically did not increase the performance of the disk system (4 IDE).

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

How to properly connect SCSI devices

All SCSI types are (at least in theory) compatible with each other. Devices independently establish an acceptable communication protocol. Therefore, installing devices comes down to setting the correct value for the device number (SCSI ID), physical connection device to the bus and switching on the terminators. However, quite often computer owners who independently connect SCSI devices to their computer complain about their unstable operation. In most cases this is due to correct connection devices and, most often, terminators (sometimes for some reason these terminators are completely forgotten).

What is a terminator?

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

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

Rice. 3. Passive terminator

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

Rice. 4. Active Ultra Wide SCSI Terminator

Terminators for Ultra-2 Wide SCSI buses must have the abbreviation LVD in their designation (Fig. 5). Currently, universal SE/LVD terminators are also available that automatically detect the interface type and perform negotiations for this interface type (Fig. 6).

Rice. 5. Terminator markings for Ultra2 Wide SCSI

How to connect terminators correctly?

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

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

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

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

Connecting terminators for devices with different interfaces

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

The most common mistake is to connect several hard drives with the Wide SCSI-2 (or Ultra Wide SCSI-2) interface to the Wide SCSI-2 bus, and connect to the last connector via a CD-ROM adapter with a SCSI-2 interface. Even though a terminator will be enabled on the CD-ROM, this terminator will terminate only 8 lines of the bus, while the remaining 8 lines used in the Wide SCSI interface will be “floating in the air.”

More the right decision devices with an 8-bit SCSI interface will be connected to the intermediate bus connectors (terminators for 8-bit devices are turned off). Connect a Wide SCSI device with an enabled terminator (or an active external terminator) to the last connector. Of course, the presence of an adapter still worsens the system's performance. This option should be avoided if possible (as well as generally using high-speed and slow devices on the same bus). However, in this situation this is still the correct connection option. Ultra-2 SCSI controllers have a built-in interface converter, which allows you to connect all Ultra-2 devices to a separate bus, without mixing them with lower-speed devices.

Features of controllers with two connectors

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

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

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

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

Power supply for active terminators

Active terminators currently in use require supply voltage to operate. This voltage can be supplied to the active terminator either from any SCSI device or from the controller. Modern SCSI devices have a special switch for selecting the supply voltage source for the active terminator built into these devices. Typically, the factory sets the terminator power supply mode from the device itself (“Power from Drive”). If only one or several internal SCSI devices with the same interface are connected to the controller, then no problems arise.

If, under the conditions of normal bus termination, it is necessary to use an active external terminator, then care must be taken to supply the supply voltage to it. To do this, one of the devices connected to this bus must have the “Power to SCSI Bus” mode enabled. If this is not done, the external terminator simply will not work normally.

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

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

Specialized SCSI controllers

Often scanners and some 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, it often works without interruptions (polling mode), sometimes it only supports one device (not 7). Basically, such a controller can only be used with your own device. Other devices most often will not work on such a controller. Moreover, many devices (most often scanners) will not be able to work with a standard controller. Therefore, it is better not to count on compatibility, but to connect standard SCSI devices to a separate standard controller.

General concepts

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

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

These devices are connected to the computer via a special SCSI adapter, and the operating system gains access to them through the appropriate drivers. The presence of a proprietary processor adapter on the SCSI board significantly reduces the load on the central processor when performing I/O operations. This is a great advantage when working on a network, as well as in multi-user and multi-tasking environments, due to the fact that the time required to obtain client access to the device is reduced. On desktop computers, download central processor is not so critical for most user programs and applications, however, when working with graphics (especially when working with computer animation), the use of the SCSI subsystem can increase system performance, since in this case most of the load on I/O operations will be transferred to the SCSI adapter.

SCSI Specifications

Today there are several SCSI specifications:

SCSI-1: 8-bit data bus and synchronous data transfer rate of 5 MB/s. Connector 25- or 50-pin;
SCSI-2 or Fast SCSI: increased speed up to 10 MB/s over an 8-bit bus. Connector 50 pin;
Wide SCSI (Wide SCSI): increase in bus width to 16. Data transfer speed has increased from 10 MB/s to 20 MB/s. 68- or 80-pin connector (Single Connector), combining power and signal circuits;
Ultra SCSI (Fast-20) / Ultra Wide SCSI or SCSI-3: data transfer speed has increased to 20 MB/s on an 8-bit bus and up to 40 MB/s on a 16-bit bus. SCSI-3 provides support for a larger number of devices (up to 15 per channel). 50/68- or 80-pin connector (Single Connector), combining power and signal circuits;
Ultra2 SCSI (LVD): To further increase SCSI speed, it was necessary to use a Low Voltage Differential (LVD) bus, in which signals are transmitted simultaneously on two wires, but in different polarities. Thanks to this, the noise immunity of the bus sharply increases, it becomes possible to increase the data transfer speed on a 16-bit bus to 80 MB/s and increase the length of the interface cable to 12 m! For full implementation, an Ultra2 SCSI adapter, an Ultra2 SCSI cable with an Ultra2 SCSI active terminator and disk drives that support Ultra2 SCSI are required. If any of these components are missing, the Ultra2 SCSI standard is automatically disabled and the system operates in one of the previous SCSI specifications. 68- or 80-pin connector (Single Connector), combining power and signal circuits;
Ultra3 SCSI (Ultra160 SCSI): Data transfer rates can reach up to 160 MB per second thanks to double data synchronization (data is transferred twice as fast without increasing the clock frequency), an improved mechanism for optimizing data transfer rates across devices, and the use of CRC instead of parity for increasing the reliability of data transmission. The Ultra160 SCSI specification is fully compatible with Ultra2 SCSI across cables, connectors and terminators. The Ultra160 SCSI controller can simultaneously support Ultra160 SCSI and Ultra2 SCSI devices on the same bus, each operating at maximum speed. 68- or 80-pin connector (Single Connector), combining power and signal circuits;
Ultra160+ SCSI: modification of Ultra160 SCSI, which implements Packetized SCSI - batch method information transfer (commands, data and status registers are transferred in one block at the same speed) and Quick Arbitration Select (QAS) a method of quickly transferring bus control from one SCSI device to another. As a result, delays are reduced and the integral data transfer rate is increased.

Basic requirements for SCSI interface implementation

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

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

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

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

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

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

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

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

Characteristic impedance

Propagation Delay

Cumulative length

Allowable length of branches

Interval between devices

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

What is the acceptable length of a SCSI cable?

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

Specification	Data transfer rate	Max. length of cable	Max. number of devices
Fast SCSI	10 MByte/sec	3 meters	8
Wide SCSI	20 MByte/sec	3 meters	16
Ultra SCSI (8 bit, Narrow)	20 MByte/sec	3 meters	5
Ultra SCSI (16 bit, Wide)	40 MByte/sec	3 meters	5
Ultra SCSI (8 bit, Narrow)	20 MByte/sec	1.5 meters	6-8
Ultra SCSI (16 bit, Wide)	40 MByte/sec	1.5 meters	6-8
Ultra2 SCSI	80 MB/sec	1.5 meters	16

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

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

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

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

Unfortunately, based on the appearance of a SCSI device, one can only tell whether the SCSI interface is “Narrow” or “Wide”. Below is appearance from the connector side of some SCSI devices:

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

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

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

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

?"> What does it mean?

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

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

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

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

Try the following:

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

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

· Make sure that all SCSI devices have different SCSI IDs. The SCSI ID for SCSI devices can be anything except 7th, which is usually reserved for the SCSI controller.

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

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

What is necessary for the computer to boot from a SCSI drive.

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

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

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

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

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

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

· SCSI controller must support LVD interface

· there must be active LVD terminators at both ends of the SCSI chain

· all SCSI devices on the bus must support the LVD interface

Failure to meet any of these requirements will result in the SCSI system being able to function only on higher SCSI specifications.

How compatible are LVD devices with SCSI devices of previous specifications?

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

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

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 might argue that recently IDE hard drives have achieved SCSI performance and are much cheaper. AND IDE controller, which is already the fastest, is usually located on the motherboard and does not require additional material costs, while you need to spend at least $100 on a good SCSI controller. 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 regular 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 when using an Ultra or Ultra Wide SCSI interface, additional restrictions are imposed on the quality of connecting cables and their length, as a result of which the maximum connection length may 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 positive traits IDE interface, partially in light of the above table:

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 motherboards already have an IDE controller.
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 ISA bus costs about $20, but now no one simply needs these, so you can find them cheaper. The next option is a controller on PCI bus. The simplest option 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 the old version of the 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	connection of internal wide devices, mainly HDD
External
DB-25	connecting external slow devices, mainly scanners, IOmega Zip Plus. most common on Mac. (like a modem)
Low-Density 50-pin	or Centronics 50-pin. external connection of scanners, streamers. usually SCSI-1
High-Density 50-pin	or Micro DB50, Mini DB50. standard external narrow connector
High-Density 68-pin	or Micro DB68, Mini DB68. standard external wide connector
High-Density 68-pin	or Micro Centronics. according to some sources it is used for external connection 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. 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 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 necessary for devices other than hard drives to run under DOS.
The SCSI BIOS image is flashed into the flash BIOS of the motherboard. Further according to point 1. Usually, SCSI BIOS is added to the BIOS of boards 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.
There is no SCSI BIOS at all. The operation of all SCSI devices is provided only by operating system drivers.

In addition to supporting booting from SCSI devices, the BIOS usually has several more functions: setting the adapter configuration, checking the disk surface, low-level formatting, setting the initialization parameters of SCSI devices, setting the boot device number, etc.

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

The main characteristics of the SCSI bus are

its width is 8 or 16 bits. Or, in other words, "narrow" or "wide".
speed (roughly - the frequency 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 a 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 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 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,

Installation

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, the general rule in this case 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. For each device on the SCSI bus you can configure: parity check, startup delay (so that all 7 disks do not turn on at the same time), maximum device speed. For non-PnP controllers on the ISA bus, do not forget to set the interrupt they use in BIOS SETUP to “Legal ISA”. For the PCI controller, check that it also gets an interrupt and does not share it with anyone, although for the latest models this is often not important.

Terminators

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 the next device in the chain.

In addition, terminators can be passive or active. Today, most are active, which provide greater noise immunity and reliability at high speeds. You can usually determine which 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. Here we will look at the most common variants of devices on one SCSI bus (wide or narrow)

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

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 internal 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).

Hard disks

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 schemes sector translations (can be compared with LBA, CHS, LARGE for IDE disks) 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 tape drives, they can work with most operating systems with standard drivers. It’s very fortunate that you can, for example, under WindowsNT, use the standard backup program, 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 600dpi is a picture of about 90 Mb and transferring this amount of information through the 8-bit ISA bus not only takes a lot of time, but also 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 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 based on Symbios Logic 53C875 chip. Many people note its speed and reliability.

Devices

HDD - well, of course Seagate Cheetah - with an RPM of 10,000 it’s hard to argue. But without additional cooling fans, this drive will not last long :(. Other Seagate drive series - Barracuda and Hawk - are also distinguished by their reliability.

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

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 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.

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 additional commands, Plug and Play support. 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(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 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 specialized processor, large capacity 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 disks

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. 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

Streamers

Similar to CD-ROM SCSI tape 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 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)

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)

Since the world has witnessed the rapid evolution of the personal computer, the computer has evolved from a very expensive and large computer, used by rare companies and corporations, into an item of everyday use for hundreds of millions of people, there has been a change in more than a dozen technologies. Including technologies related to the use of certain buses, connectors, and peripheral devices. Connection standards used to connect to a computer, such as SCSI, SATA and IDE, were no exception.

SCSI

Story
Around the 70s, the need arose for physical and logical interfaces between peripheral devices and computers. A man named Alan F. Shugart, by the way, after whom the interface was later named, (Shugart Computer Systems Interface) came up with the idea to use a device that acts as a bridge between hard drive and a computer. A 50-pin flat connector was developed, known and sold commercially as SCSI-I. This is what the standard looks like.

This standard was supported by many manufacturers and industry leaders of the time. Since then, several versions of this interface have been released, and although it is considered more or less obsolete these days, some older PCs still use it.
The very first version used a 50-pin flat connector. While the first SCSI connectors used parallel interfaces, more modern SCSI connectors operate over a serial interface. Serial interface SCSI, compared to parallel, provides more high speed data transmission.
SCSI can either be installed physically on the motherboard or can be implemented using adapters.
Storage
SCSI allows you to use up to 7 - 15 (depending on the bus width) connected devices. This allows you to connect all your devices to one board, rather than buying different boards for different devices, which will inevitably increase costs.
Speed
Modern versions can transfer data up to 80 megabytes/sec. Modern SCSI devices have backwards compatible, i.e. If an older device is connected, the SCSI bus will still support it, although the data transfer speed may be reduced.

Price
SCSI has always been an expensive solution. New versions have not made it lower. Considering that there are at least 10 different (3 new generations) types, there are no plans to completely withdraw this type of interface from the market anytime soon. The advantage of SCSI is the support of various devices, from dot matrix printers, scanners, plotters, to modern keyboard and mice and performance.

IDE

Story
The IDE (Integrated Drive Electronics) interface was developed by Western Digital Electronics in collaboration with Control Data Corporation and Compaq Computers, and was launched in 1986. By the mid-90s, IDE-ATA technology was already supported everywhere and almost completely replaced the SCSI bus. The abbreviation PATA (Parallel ATA) is now widely used to denote IDE, which emphasizes that a parallel interface is used for data transfer. Unlike SCSI, in IDE, the controller is located in the device itself, and not as a separate board.
IDE initially had a 40-wire cable, which was later replaced by an 80-wire cable. Here is an example of an IDE hard drive.

Connection
PATA allows you to connect two devices per channel.
Speed
The most recent versions may support data transfer rates of up to 133 MB/s.
Price
PATA, the successor to SCSI, was extremely successful due to its low price and best value for money. PATA interfaces are still used in large industrial installations, but in consumer systems they have almost been replaced by SATA technology.

SATA

Story
Serial ATA technology was created at the turn of the century and replaced PATA (IDE). In 2003, SATA was launched with great fanfare, and within just ten years, it had captured 98% of the market share. personal computers. SATA was originally launched with an interface supporting speeds of 1.5 Gbps, the modern version (SATA Revision 3.0) can transfer data at speeds up to 6 Gbps.

An example of connecting a hard drive to .

Connection
SATA uses a serial port and supports hot-plug technology. With Plug and Play technology, computer components can be replaced without shutting down the system.
The data cable has 9 pins and is no more than a meter long. A SATA cable has much fewer cores than a PATA cable and, as a result, is significantly narrower. Thanks to this, systems with such connectors ensure better cooling. It is much easier and more convenient to connect devices to the connector itself. In addition, with the advent of SATA, you can forget about distinguishing devices into Master and Slave. A separate cable is connected to each device. SATA comes in several varieties, including the mini-SATA connector for small drives and the E-SATA connector, which is used to connect external devices.
Speed
The first SATA supported speeds of 1.5 Gbit/s. Modern versions support data transfer rates of 3 Gbit/s and up to 6 Gbit/s.

Price
SATA devices are the cheapest compared to other similar interfaces.
Comparing the three interfaces above gives us an idea of why most modern personal computers use SATA. IDE turned out to be less convenient and expensive and therefore was successfully replaced by SATA. The SCSI interface is almost obsolete and is currently used only on some servers. So far there are no worthy alternatives to the SATA interface that would be faster, cheaper and more convenient. Most likely, the SATA interface will dominate the PC market in the coming years.

Popular

How to number pages in OpenOffice

« From: Belgorod Registered: 2015.03.18 Posts: 550 Likes: 81 Topic: Open Document How to number pages Page numbers in a text... »

The largest public pages on VKontakte Cook it!

« 1. Collections of movies, Public films with those “save them on your wall so you don’t forget to watch them.” For convenience, in publications along with lists... »

Samsung Galaxy A9 Star Pro: the first smartphone with a quad camera Information about the type of speakers and audio technologies supported by the device

« The Koreans presented the world's first smartphone with four cameras - Samsung Galaxy A9 (2018). The price at the start of sales is 39,990 rubles. Someday this... »