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PC PeripheralsPC Peripherals for for TechniciansTechnicians Chapter 2.4 - Chapter 2.4 - Storage: SCSIStorage: SCSI

Systems Manufacturing Training Systems Manufacturing Training and Employee Developmentand Employee Development

Copyright © 1998 Intel Corp.Copyright © 1998 Intel Corp.

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Storage: SCSIStorage: SCSI

OBJECTIVES: At the end of this section, the student will be able to do the following:

Describe the components of a basic SCSI sub-system. Discuss the features of a SCSI-bus system. Explain common terms used with SCSI interfaces. Discuss SCSI IDs & LUNs, Terminations, and ASPI. Discuss trouble-shooting SCSI sub-systems. Discuss advantages of SCSI & EIDE drives.

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The SCSI Sub-system SCSI (pronounced "scuzzy") stands for Small

Computer System Interface.SCSI is a computer bus interface standard that enables

different kinds of peripherals to interface with the host computer by means of a SCSI-bus cable.

» SCSI is used to interface to a number of different device types, such as Hard Drives, Tape Backups, CD-ROM drives, Printers, Scanners, etc.

Most desktop PC's have built-in support for IDE disks. A SCSI disk controller can be used as a replacement or

to provide supplemental storage.SCSI is the standard hard disk interface for servers and

the Apple MAC, but is not as common in desktop PC’s.

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SCSI Sub-system Components

See SCSI Sub-system drawing on next slide A typical PC SCSI sub-system consists of a SCSI Host

Adapter and one or more SCSI devices connected to the host adapter by electrical cables (the SCSI Bus). The SCSI Bus transfers data and commands at high

speed to and from the peripheral device(s).

The SCSI Host Adapter provides a SCSI bus interface to the SCSI devices installed in a computer.The SCSI host adapter is linked to the PC system bus,

so it can transfer data between SCSI devices and the PC's main memory.

The host adapter usually has an onboard BIOS, both internal and external SCSI connectors, and terminators.

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SCSI Sub-system ComponentsSCSI devices are daisy chained from device to device

ID0

ID1

ID7Host Adapter

ID4

ID5

Internal DevicesDevice ID0 & ID1

External DevicesDevice ID4 & ID5

The SCSI Host Adapter may be a PCI, ISA, or EISA

adapter board or a SCSI controller

“chip” integrated on the motherboard in

server systems.

Devices should form a single chain from the device at one end to the device at the other.

(No 'T's are allowed)

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SCSI Sub-system Components SCSI may include both internal & external devices.

SCSI devices located inside the PC are connected to the adapter card by a flat ribbon cable.

External SCSI devices are typically connected via a special "Centronics" like connector.

The SCSI device interface is designed to accommodate intelligent devices attached to the bus.All devices, including the host adapter, are intelligent

devices with their own memory and can exchange data.» SCSI devices interpret & perform high-level commands.

Each SCSI device has a ROM which contains the device parameters (e.g. - Heads, Cyl, SPT, etc.).

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SCSI Sub-system Features A common use for SCSI hard drives in PCs is in

Network File Servers with large capacity hard disks.Large/fast hard disks are available first as SCSI drives.

» Disks with the high capacities & high performance intended for servers aren't as common with an EIDE interface.

SCSI can provide a boost in performance when used on a server with multiple drives attached or when used with a multitasking operating system.

» Supports queuing controller requests: the O/S can request more data while it is waiting for the first data to be returned.

» Several devices can use the bus at the same time.

Note: SCSI is more complex to implement and more expensive to manufacture than EIDE.

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SCSI Sub-system Features PC SCSI adapter cards contain extra logic to convert

the first two SCSI disks into emulated ST506 disks. » The standard PC BIOS allows for only 2 drives.

If system has only SCSI drives, the ROM inserts the first SCSI disk (under ST506 emulation) as the boot device.

» An O/S and its device drivers are loaded using ST506 emulation by the standard BIOS (INT 13h).

> The advanced SCSI feature support is then loaded from the SCSI disk and the system switches over and uses the more advanced support (e.g. higher performance, multitasking, additional device types).

If both IDE & SCSI are present, the SCSI controller defers to the IDE support & boots from the first IDE disk.

If IDE only, then the 1st IDE disk is the boot device.

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SCSI Definitions OverviewNote: Terms defined in more detail on following pages!

The SCSI interface comes in several types» 8-bit (50 wire data cable - SCSI 1)

> The "A" Cable is the "original" or "standard" cable.

» 16-bit (68 wire cable - Wide SCSI) > The 68-wire "P" Cable is now the standard for Wide SCSI

implementations.

The SCSI Clock can be 5 , 10, 20, or 40 MHz

SCSI bus clock 8-bit (50 pin) Wide 16-bit (68 pin)

5 MHz (SCSI 1) 2-5 MBytes/s N/A

10 MHz (Fast SCSI) 10 MByte/s 20 MByte/s

20 MHz (Fast-20, Ultra SCSI) 20 MByte/s 40 MByte/s

40 MHz (Fast-40, Ultra-2) 40 MByte/s 80 MByte/s

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SCSI DefinitionsAsynchronous or Synchronous protocols can be used

by SCSI devices for communication.» Asynchronous is a classic Req/Ack handshake.

> The time between successive data bytes is NOT fixed> Maximum transfer rate ~2MB/sec

» Synchronous mode boosts performance for long cables.> Allows several REQs before receiving an ACK> An “offset” determines the max number of outstanding

REQs with no corresponding ACK.> Can increase the transfer speed by a factor of three.

Note: SCSI-2 and SCSI-3 devices must start as follows to ensure that initial communications will work.

» 8-bit (not wide) mode; 5 MByte/S transfer (not fast) mode; Asynchronous (not synchronous) mode

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SCSI DefinitionsSCSI-1 (1986):

» The original standard that defined the SCSI interface, a command set, & the electrical characteristics of SCSI bus.

» Transmits either command or data bytes on 8 wires running in parallel at a 2-5 MB/sec transfer rate.

> 1986 SCSI-1 (asynchronous) 2 MB/sec > 1990 SCSI-1 (synchronous) 5 MB/sec

Fast SCSI (1993): Doubles synchronous data transfer rate (to 10 MB/sec) on the 8 bit bus by increasing the data bus clock speed from 5MHZ to 10MHZ.

» Synonymous with "10 MByte SCSI"

» Both controller and device must be able to accommodate this mode (often uses differential SCSI buses).

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SCSI DefinitionsWide SCSI (1993): A SCSI-2 enhancement that allows

data to be transferred 16 bits at a time » Can have up to 16 devices & be coupled with Fast SCSI.

» 32 bit data supported but not common due to requirements for special cable & connector.

SCSI-2 (1993): Adds features to the SCSI-1 standard. » Fast SCSI; Wide SCSI; More Devices per Bus; Active

Termination; Command Queuing; Additional Command Sets; Command Set Enhancements

> Support for new SCSI devices: (tape drives, CD-ROM, etc.) > Note: 32-bit SCSI not common due to connector requirements

» SCSI-2 supports both standard SCSI devices & SCSI-2 devices attached to the same bus.

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SCSI DefinitionsDifferential SCSI: An enhancement to make SCSI less

susceptible to noise (max length up to 25 meters).» Uses 2 wires (normal & inverted signals) and the voltage

difference determines whether the signals are asserted or de-asserted--Noise on lines cancels at differential receiver.

> Note: Do NOT mix Differential devices & Single-Ended devices or Differential & Single-Ended terminators on the same bus.

SCSI-3 (1997): Adds features to SCSI-2. » Ultra SCSI is the SCSI-3 parallel solution (20 MHz & 16 bit).

> "Fast-20 Wide" SCSI, also referred to as "Ultra" SCSI,> Formally defines 16-bit wide 68-pin (.025 pitch) P-Connector.

» Adds support for a serial channel interface: Fiber channel & Twisted pair will provide faster transfer rates, more devices per bus, longer cables (~1km), and simplified connectors.

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SCSI Addresses: SCSI ID’s Each device on the

SCSI bus must have a unique 2-part address.

The SCSI address consists of an ID & a Logical Unit Number.

A SCSI ID is an identifying label (from 0 to 7) assigned to each device on the SCSI bus, including the host adapter itself.

ID0

ID1

ID7

ID4

ID5

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SCSI Addresses: SCSI ID’s Each device on the SCSI bus must have a unique 2-

part address consisting of an ID and a LUN.Each SCSI device must be uniquely identified when it is

trying to use the SCSI bus or when another device is sending a command to it.

A SCSI ID is a unique identifying number (SCSI address), ranging from 0 to 7 (or 0 to 15 for Wide SCSI)

SCSI ID’s are commonly set by DIP switches or jumpers » The ID is transferred on the data bus and formed by bytes

where LSB bit 0 corresponds to ID0 and MSB bit 7 to ID7.

» ID's are logical assignments and have nothing to do with where they go on the bus or if they should be terminated.

> Note: If two or more devices on the SCSI bus are set to the same ID, you must change one of them.

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SCSI Addresses: SCSI ID’s The SCSI ID decides which device has priority when

two or more are trying to use the bus at the same time. The higher the SCSI ID number, the higher the priority

» The sequence is 7, 6, 5, 4, 3, 2, 1 , 0, 15, 14, 13, 12, 11, 10, 9, 8.

Most host adapters are set by default to SCSI ID 7.Host adapter has highest priority for using the SCSI bus.

Recommended SCSI ID Assignments Leave the host adapter SCSI ID at the default of ID 7.Set the first two SCSI hard drives in your system to ID0

and ID1Assign SCSI IDs 2 through 6 to CD-ROM drives, tape

drives, and other SCSI peripherals in your system.

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SCSI Addresses: SCSI LUNs

Host Adapter

Host Adapter

Initiator

SCSI Bus

Target (e.g. Disk)

Logical Unit(LUN 0)

CONTROLLER

Initiator

SCSI Bus

Target (e.g. Dual CD-ROM)

Logical Unit(LUN 0)

Logical Unit(LUN 1)

CONTROLLER

Most SCSI devices have one Logical Unit

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SCSI Addresses: SCSI LUNs Logical Unit Number (LUN)

Each device on the SCSI bus must have a unique 2-part address consisting of an ID and a LUN.

» SCSI devices require a unique LUN from 0-7 (up to eight LUNs) which must be set for each device on the bus.

» Most SCSI devices have a single embedded controller with a LUN of 0.

A SCSI initiator (usually a host adapter) can address each LUN separately through the target.

» An example of a LUN is a hard disk.

» An example of a device with multiple LUNs is a “juke-box” CD-ROM drive.

> The CD changer takes up only one SCSI ID and each of the "platters" uses a separate LUN.

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SCSI Characteristics: Terminations

Terminators are a set of electrical resistors installed, or enabled, at each end of a SCSI bus.

When terminators are installed in a host adapter or other device, the device is said to be terminated.

The SCSI bus needs exactly two terminators

NotTerminated

Terminated

Not terminated

NotTerminated

Terminated

If you have only internal or external devices, the host adapter is terminated; otherwise it is not.

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SCSI Characteristics: Terminations Terminators are electrical components that provide

impedance matching which results in clean signals without reflections or ringing.The significance of termination becomes greater as cable

lengths and data transfer rates increase. IF an incorrectly terminated system happens to work

now, don't count on it continuing to do so.

The device at each end of the SCSI bus must be terminated, all other devices must be unterminated.Terminators must be installed, or enabled, on the devices

at each end of the SCSI bus.Terminators must be removed, or disabled, on devices in

the middle of the SCSI bus.

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SCSI Characteristics: Terminations There are two types terminations which are electrically

the same. Passive termination is a 220-Ohm pull-up resistor to a

~5-V supply (TERMination power) and a 330-Ohm resistor to ground, for each signal line.

Active termination has a voltage regulator that provides a 2.85-V output and a 110- Ohm resistor from each signal line to this 2.85-V source.

» Active termination (with termination power provided by the host adapter) maintains a constant 2.85 V and provides less line noise and more reliable data-transfers for long buses.

SCSI-2 defined the active terminator (it is optional)SCSI-3 requires active termination.

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SCSI Characteristics: Terminations Each device can provide it’s own terminating power.

Note: Terminator power is often fused.External devices must be switched on.Newer designs may use auto-restoring fuses which

‘reset’ themselves after a period of time.

There are several ways to change the termination, depending on what kind of device you have.Physically remove terminating resistors from their

sockets or insert them in the sockets.Change a switch setting on the device's switch block.Switched electronically (by running a configuration

program supplied with the SCSI adapter)

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Advanced SCSI Programming Interface The Advanced SCSI Programming Interface (ASPI) is

a standardized software protocol developed by Adaptec and adopted by other vendors.ASPI has become a de facto standard that enables

programmers to develop applications and device drivers that work with all ASPI-compatible SCSI adapters.

» Each host adapter-specific device driver and each hardware-specific device driver is written according to the same ASPI protocol standards.

» Note: A Device Driver is a hardware-specific software program that acts as an interface between a computer's O/S and the hardware devices attached to the PC.

An alternative interface protocol is called Common Access Method (CAM) is not as widely used as ASPI.

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Troubleshooting SCSI Check these potential problems if SCSI devices do

not work properly after setting up the sub-system:Are all SCSI bus and power cables properly connected?

» Is pin-1 orientation correct for the internal SCSI cable?

Are all SCSI devices powered? » The host adapter cannot detect a device on the SCSI bus at

boot up unless the device is powered up.

Do all devices on the SCSI bus have unique IDs?» Note: If two or more devices on the SCSI bus are set to the

same ID, you must change one of them.

Is the SCSI bus correctly terminated?» IF an incorrectly terminated system happens to work now,

don't count on it continuing to do so. Fix the termination.

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SCSI vs. EIDE Enhanced IDE represents a good low-cost solution,

while SCSI retains a significant performance advantage. Advantages of SCSI:

Can link multiple (up to 7 or 15) SCSI devices together (e.g. scanner and a drive, or multiple drives).

» Enhanced IDE can support a maximum of four.

SCSI devices can be daisy-chained together, whereas all IDE devices must be directly connected to the controller.

» This can save IRQ's, DMA channels, and/or ISA/PCI slots

SCSI will coexist with any other type of controller (e.g. IDE) or host adapter (other SCSI cards).

» SCSI host adapters do not take up one of the two available hard drive controller port addresses (1FX,3FX,17X,37X).

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SCSI vs. EIDE Advantages of SCSI: (Cont.)

SCSI allows devices to be physically external. » SCSI cables can be up to 6 meters long.

» IDE allows a maximum cable length of 46cm (18”) - insufficient to support external devices.

SCSI supports different types of devices simultaneously with the same host adapter.

» For multitasking operating systems, SCSI allows multitasking and command queuing: multiple I/O requests can be outstanding and be completed out of order.

> With EIDE, only one of the two devices on each channel (cable) can be active at a given time.

» Support for almost any type of peripheral--Scanners and CD-writers aren't commonly available with EIDE (1997).

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SCSI vs. EIDE Advantages of SCSI: (Cont.)

Automatically configures device type, geometry (size), speed and manufacturer/model number.

» Set up as "not installed" in the CMOS.

Superior bus-mastering DMA support.» SCSI supports device to device transfers on the bus

without host computer interruption.

» Bus-mastering DMA decreases the amount of CPU time required to do I/O, leaving more time to work on other tasks.

Software portability - drivers are written for the host adapter, not the specific device.

» The CD-ROM driver written for your host adapter will work with any brand or speed SCSI CD-ROM drive.

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SCSI vs. EIDE Advantages of EIDE:

EIDE controllers are cheaper than SCSI host adapters. Less overhead per command than SCSI.

» In general, DOS/Windows users will experience as good or better disk performance with Enhanced IDE in single user, non-multitasking evnvironments.

SCSI configuration can be more complex:» Loading drivers, setting IDs, installing terminations.

SCSI requires device drivers to take advantage of advanced and high performance SCSI features.

High price of SCSI justified for high-end servers and power users running multitasking operating systems!

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REVIEW & SUMMARYWE HAVE DISCUSSED THE FOLLOWING:

The components of a basic SCSI sub-system.A typical PC SCSI sub-system consists of a SCSI Host

Adapter and one or more SCSI devices connected to the host adapter by electrical cables (the SCSI Bus).

All devices, including the host adapter, are intelligent devices with their own memory and can exchange data.

The features of a SCSI-bus system. Large/fast hard disks are available first as SCSI drives. SCSI boosts performance on servers with multiple drives

or when used with a multitasking operating system.PC SCSI adapter cards contain extra logic to convert the

first two SCSI disks into emulated ST506 disks.

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REVIEW & SUMMARY Common terms used with SCSI interfaces.

SCSI bus clock 8-bit (50 pin) Wide 16-bit (68 pin)

5 MHz (SCSI 1) 2-5 MBytes/s N/A

10 MHz (Fast SCSI) 10 MByte/s 20 MByte/s

20 MHz (Fast-20, Ultra SCSI) 20 MByte/s 40 MByte/s

40 MHz (Fast-40, Ultra-2) 40 MByte/s 80 MByte/s

SCSI IDs & LUNs, Terminations, and ASPI. Each device on the SCSI bus must have a unique 2-part

address consisting of an ID and a LUN.The device at each end of the SCSI bus must be

terminated to provide impedance matching which results in clean signals without reflections or ringing.

ASPI enables programmers to develop apps and drivers that work with all ASPI-compatible SCSI adapters.

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REVIEW & SUMMARY Trouble-shooting SCSI sub-systems.

Bus & power cables properly connected; All SCSI devices powered; Unique Ids; Correctly terminated.

Advantages & disadvantages of SCSI & EIDE drives.SCSI links multiple SCSI devices (up to 7 or 15) together

and allows devices to be physically external. SCSI supports different types of devices simultaneously

with the same host adapter (Printers, Scanners, etc.) SCSI auto configures device type, geometry (size), etc,EIDE controllers are cheaper than SCSI host adapters

and have less overhead per command than SCSI.High price of SCSI justified for servers and power users

running multitasking operating systems!End of Chapter 2-4