1

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.
2

The SCSI Sub-system

SCSI (pronounced "scuzzy") stands for S mall
C omputer S ystem I nterface.
 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.
3

SCSI Sub-system Components

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.
4

SCSI Sub-system Components
SCSI devices are daisy chained from device to device
Internal Devices
Device ID0 & ID1
ID0
ID1
Devices should form a single chain from the device at one end to the device at the other.
(No 'T's are allowed)
ID4
ID7
Host Adapter
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.
ID5
External Devices
Device ID4 & ID5
5

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.).
6

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.
7

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.
8

SCSI Definitions Overview
Note: 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
5 MHz (SCSI 1)
10 MHz (Fast SCSI)
8-bit (50 pin)
2-5 MBytes/s
Wide 16-bit (68 pin)
N/A
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
9

SCSI Definitions
 Asynchronous 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
10

SCSI Definitions
 SCSI-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).
11

SCSI Definitions
 Wide 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.
12

SCSI Definitions
 Differential 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.
13

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
14

SCSI Addresses: SCSI ID’s

Each device on the SCSI bus must have a unique 2part 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.
15

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 ID1
 Assign SCSI IDs 2 through 6 to CD-ROM drives, tape drives, and other SCSI peripherals in your system.
16

SCSI Addresses: SCSI LUNs
Most SCSI devices have one Logical Unit
Initiator
Host Adapter
SCSI Bus
Target (e.g. Disk)
C
O
N
T
R
O
L
L
E
R
Logical Unit
(LUN 0)
Initiator
Host Adapter
SCSI Bus
Target (e.g. Dual CD-ROM)
C
O
N
T
R
O
L
L
E
R
Logical Unit
(LUN 0)
Logical Unit
(LUN 1)
17

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.
18

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
Terminated
Not
Terminated
Not
Terminated
Not terminated
Terminated
If you have only internal or external devices, the host adapter is terminated; otherwise it is not.
19

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.
20

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.
21

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)
22


The A dvanced S CSI P rogramming I nterface (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 C ommon
A ccess M ethod (CAM) is not as widely used as ASPI.
23

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.
24

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).
25

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).
26

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.
27

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!
28

REVIEW & SUMMARY

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.
29

REVIEW & SUMMARY

Common terms used with SCSI interfaces.
SCSI bus clock
5 MHz (SCSI 1)
8-bit (50 pin) Wide 16-bit (68 pin)
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.
30

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
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