13.2 SCSI

We'll devote less space to SCSI than IDE, since IDE drives dominate the PC platform, but we will try to hit the high points of SCSI. SCSI (Small Computer Systems Interface) is a general-purpose I/O bus that is used in PCs primarily for connecting hard disks and other storage devices, and secondarily for connecting a variety of devices, including scanners, printers, and other external peripherals. Although common in the Apple Macintosh world, SCSI has remained a niche product in PCs, limited primarily to network servers, high-performance workstations, and other applications where the higher performance and flexibility of SCSI are enough to offset the lower cost of ATA.

13.2.1 SCSI Standards

SCSI is confusing because of the proliferation of terms, many of which refer to similar things in different ways or to different things in similar ways. There are actually three SCSI standards, each of which refers not to any particular implementation, but to the document that defines that level.

SCSI-1

The Small Computer Systems Interface (SCSI) standard was adopted in 1986 and is now obsolete. Originally called simply SCSI, but now officially SCSI-1, this standard defines a high-level method of communicating between devices, an Initiator (normally a computer) and a Target (normally a disk drive or other peripheral). SCSI-1 permits data to be transferred in asynchronous mode (unclocked mode) or synchronous mode (clocked mode), although commands and messages are always transferred in asynchronous mode. SCSI-1 uses the low-density 50-pin connector for both internal and external connections. The external low-density 50-pin connector is also referred to as the Centronics SCSI connector. SCSI-1 is a single comprehensive document that defines all physical and protocol layers, and is published as ANSI X3.131-1986.

SCSI-2

SCSI-2 was adopted in 1994, and many current SCSI devices are SCSI-2 compliant. SCSI-2 updated the SCSI-1 standard to include faster data rates and to more tightly define message and command structures for improved compatibility between SCSI devices. SCSI-2 devices use various connectors, depending on the width and speed of the implementation. SCSI-2 is a single comprehensive document that defines all physical and protocol layers, and is published as ANSI X3.131-1994.

SCSI-3

The monolithic documents that describe SCSI-1 and SCSI-2 became too unwieldy for the greatly expanded SCSI-3 specification, so beginning with the SCSI-3 specification the document was separated into multiple layered components, each defined by an individual standards document. Together, these individual documents comprise the SCSI-3 standard, which is now officially referred to simply as SCSI.

For more information about SCSI standards, visit the SCSI Trade Association (http://www.scsita.org).

13.2.2 SCSI Implementations

SCSI implementations are characterized by their width (bits transferred per clock cycle), clock rate, and overall throughput, which is the product of those two figures. Bus width determines how much data is transferred per clock cycle, and may be either of the following:

Narrow SCSI

Narrow SCSI transfers one byte per clock cycle, using a one-byte-wide data bus on a 50-pin parallel interface, which is defined by SCSI-1.

Wide SCSI

Wide SCSI transfers two bytes per clock cycle, using a two-byte-wide data bus on a 68-pin parallel interface, which is defined by the SCSI-3 SPI document. Although SCSI-3 allows bus widths greater than two bytes, all current Wide SCSI implementations use two bytes.

The signaling rate (or clock rate), properly denominated in MegaTransfers/Second (MT/s) but more commonly stated in MHz, specifies how frequently transfers occur. Various SCSI implementations use signaling rates of 5 MHz, 10 MHz, 20 MHz, 40 MHz, and 80 MHz, which are given the following names:

SCSI

When used without qualification to describe a transfer rate, SCSI refers to the 5 MT/s transfer rate defined in SCSI-1. Because SCSI-1 supports only narrow (8-bit) transfers, SCSI-1 transfers 5 MB/s (5 MT/s x 1 byte/transfer).

Fast SCSI

Fast SCSI describes the 10 MT/s transfer rate defined in SCSI-2. Used with a narrow interface (called Fast Narrow SCSI or simply Fast SCSI), transfers 10 MB/s (10 MT/s x 1 byte/transfer). Used with a wide interface, called Fast Wide SCSI, transfers 20 MB/s (10 MT/s x 2 bytes/transfer).

Ultra SCSI (Fast-20 SCSI)

Ultra SCSI, also called Fast-20 SCSI, describes the 20 MT/s transfer rate defined in an extension to the SCSI-3 SPI document (ANSI standard X3T10/1071D revision 6). Used with a narrow interface (called Narrow Ultra SCSI or simply Ultra SCSI), transfers 20 MB/s (20 MT/s x 1 byte/transfer). Used with a wide interface (called Wide Ultra SCSI), transfers 40 MB/s (20 MT/s x 2 bytes/transfer).

Ultra2 SCSI (Fast-40 SCSI)

Ultra2 SCSI, also called Fast-40 SCSI, describes the 40 MT/s transfer rate defined in SCSI-3 SPI-2. Used with a narrow interface (called Narrow Ultra2 SCSI or simply Ultra2 SCSI), transfers 40 MB/s (40 MT/s x 1 byte/transfer). Used with a wide interface (called Wide Ultra2 SCSI or U2W SCSI), transfers 80 MB/s (40 MT/s x 2 bytes/transfer).

Ultra3 SCSI (Fast-80DT SCSI)

Ultra3 SCSI, also called Fast-80DT SCSI or Ultra160 SCSI, describes the 80 MT/s transfer rate defined in SCSI-3 SPI-3. Fast-80DT actually uses a 40 MHz clock, but is double-pumped, which is to say that it makes two transfers during each clock cycle. Only wide interfaces are defined for speeds higher than Ultra2 SCSI, which means that Ultra3 SCSI transfers 160 MB/s (80 MT/s x 2 bytes/transfer).

In addition to being differentiated by bus width and signaling speed, SCSI devices may be one of two general types, which are incompatible with each other:

Single-ended

Single-ended SCSI (SE SCSI) devices use unbalanced transmission (one wire per signal), which minimizes the number of wires required in the connecting cable, but also limits maximum bus length and maximum data rates. Until recently, all PC-class SCSI devices were SE, but SE SCSI devices are now obsolescent.

Differential

Differential SCSI devices use balanced transmission (two wires per signal, plus and minus), which reduces the effects of noise on the SCSI channel. This requires a more expensive cable with additional wires, but extends the maximum allowable bus length and allows increased data rates. Originally, differential SCSI was used only on large computers, where the greater bus length of differential SCSI allows connecting mainframes and minicomputers to external disk farms. In modified form, differential SCSI is now commonplace on PCs. Two forms of differential SCSI exist.

High-Voltage Differential

High-Voltage Differential SCSI (HVD SCSI) was originally called simply Differential SCSI before the advent of Low-Voltage Differential SCSI, described below. HVD SCSI is very seldom used in the PC environment.

Low-Voltage Differential

Low-Voltage Differential SCSI (LVD SCSI) devices use differential transmission, but at lower voltage than HVD SCSI devices. LVD is where the action is in high-performance PC SCSI drives now, and where it is likely to remain for the foreseeable future. Although they are technically unrelated, LVD and U2W were often used as synonyms because most U2W hard drives use LVD transmission. However, Ultra160 devices have become common, and they also use LVD.

Table 13-7 summarizes implementations of SCSI you may encounter. For Narrow SCSI implementations, the word "Narrow" in the name is optional, and is assumed unless Wide is specified. The Clock column lists the signaling rate in MT/s. The DTR column lists the total Data Transfer Rate, which is the product of the signaling rate and the bus width in bytes. The Devices column lists the maximum number of SCSI devices that may be connected to the SCSI bus, including the host adapter. The maximum number of devices supported on any Narrow SCSI bus is 8, and on a Wide SCSI bus is 16. Because a longer bus results in signal degradation, the number of devices supported is sometimes determined by the length of the bus. For example, Wide Ultra SCSI supports up to eight devices on a 1.5 meter (~ 4.9 foot) bus, but only four devices (host adapter plus three drives) on a bus twice that length.

Table 13-7. SCSI implementations

				Bus Length (meters)
Name	Clock	Width	DTR	SE	LVD	HVD	Devices
(Narrow) SCSI-1	5 MHz	8 bit	5 MB/s	6	-	25	8
Fast (Narrow) SCSI	10 MHz	8 bit	10 MB/s	3	-	25	8
Fast Wide SCSI	10 MHz	16 bit	20 MB/s	3	-	25	16
(Narrow) Ultra SCSI	20 MHz	8 bit	20 MB/s	1.5	-	25	8
(Narrow) Ultra SCSI	20 MHz	8 bit	20 MB/s	3	-	-	4
Wide Ultra SCSI	20 MHz	16 bit	40 MB/s	-	-	25	16
Wide Ultra SCSI	20 MHz	16 bit	40 MB/s	1.5	-	-	8
Wide Ultra SCSI	20 MHz	16 bit	40 MB/s	3	-	-	4
(Narrow) Ultra2 SCSI	40 MHz	8 bit	40 MB/s	-	12	25	8
Wide Ultra2 SCSI	40 MHz	16 bit	80 MB/s	-	12	25	16
Ultra3 SCSI (Ultra160)	80 MHz	16 bit	160 MB/s	-	12	-	16
Ultra320 SCSI	160 MHz	16 bit	320 MB/s	-	12	-	16

13.2.3 SCSI Cables and Connectors

SCSI devices use a variety of connectors. Until recently, there was little standardization, and no way to judge the SCSI standard of a device by looking at its connector. For example, current U2W devices use the 68-pin high-density connector, but that connector has also been used by old Digital Equipment Corporation (DEC) for single-ended devices. By convention, all SCSI devices have female connectors and all SCSI cables have male connectors. This rule is generally followed by modern SCSI devices intended for use on PCs, although it is frequently violated by very old PC devices and by devices intended for use outside the PC environment. Mainstream SCSI devices use the following cables and connectors:

DB25 SCSI connector

Some scanners, external Zip drives, and other Narrow SCSI devices use the DB25 SCSI connector, also called the Apple-Style SCSI connector. Unfortunately, this is the same connector used on PCs for parallel ports, which makes it easy to confuse the purpose of the connector on the PC. Devices are linked using a straight-through DB25M-to-DB25M cable.

Avoid using DB-25 SCSI connectors if possible. Connecting a SCSI device to a parallel port or a parallel device to a SCSI port may damage the device and/or the interface. If you must use DB-25 SCSI, make sure all ports are clearly labeled.

50-pin Centronics SCSI connector

The 50-pin Centronics SCSI connector is also called the Low-density 50-pin SCSI connector or the SCSI-1 connector and resembles a standard Centronics printer connector. Male SCSI-1 connectors are used on external cables for SCSI-1 devices, and by internal ribbon cables for both SCSI-1 and SCSI-2 devices.

Micro DB50 SCSI connector

The Micro DB50 SCSI connector is also called the Mini DB50 SCSI connector, the 50-pin High-density SCSI connector, or the SCSI-2 connector. Male SCSI-2 connectors are used on external cables for SCSI-2 devices.

Micro DB68 SCSI connector

The Micro DB68 SCSI connector is also called the Mini DB68 SCSI connector, the 68-pin High-density SCSI connector, or the SCSI-3 connector. Male SCSI-3 connectors are used on external cables and internal ribbon cables for SCSI-3 devices.

Ultra Micro DB68 SCSI connector

The Ultra Micro DB68 SCSI connector is also called the Very high-density condensed 68-pin SCSI connector or the VHDCI SCSI connector, and is also often incorrectly called the SCSI-4 connector or the SCSI-5 connector. The VHDCI SCSI connector is used by Ultra160 SCSI devices.

Single Connector Attachment (SCA)

The SCA interface, originally used primarily in large IBM computers, uses a standard 80-pin connector that provides power, configuration settings (such as SCSI ID), and termination of the SCSI bus. SCA was designed to allow hot-swappable drives to connect directly to the SCSI bus via an SCA backplane connector, without requiring separate power or interface cables. SCA interface drives can be connected to a standard 50- or 68-pin connector on a PC SCSI host adapter by using an SCA-to-SCSI adapter, which is readily available from most computer stores and mail-order sources. SCA devices are seldom used in PC-class hardware except in servers with hot-swappable drives.

13.2.3.1 Narrow Single-Ended SCSI Cables, Connectors, and Signals

Narrow (8-bit) SCSI transfer modes use narrow (50-pin) cables. Officially, a narrow cable is called a SCSI A cable, but it may also be called a SCSI-1 cable or a 50-pin SCSI cable. An A cable may use any of several connectors, including standard-density 50-pin internal, high-density 50-pin internal, DD-50 50-pin external, Centronics 50-pin external, and high-density 50-pin external. Narrow SCSI uses 50 signals, each carried on one of the 50 wires in the SCSI A cable, with the 50 wires organized into 25 pairs. For SE SCSI, each pair includes a signal wire and a signal return (ground) wire. Figure 13-3 shows a SCSI A cable with an internal 50-pin connector.

Figure 13-3. SCSI A cable (bottom) with IDE cable shown for comparison

Table 13-8 lists the pinouts for SCSI A cables and connectors. A # following a signal name indicates that the signal is active-low. In an A cable SCSI bus, (reserved) lines should be left open in SCSI devices, may be grounded at any point, and are grounded in the terminator. All A cables use the same signals on the same conductor in the cable, but the pinouts to the connectors vary by connector type. In the table, "External" refers to a SCSI A cable that uses an external shielded connector. "Internal" refers to an unshielded internal header-pin connector.

Table 13-8. SCSI A cable pinouts

	Connector Pin #				Connector Pin #
Signal	External	Internal	Cable Conductor #		Internal	External	Signal
Signal return	1	1	1	2	2	26	DB(0)#
Signal return	2	3	3	4	4	27	DB(1)#
Signal return	3	5	5	6	6	28	DB(2)#
Signal return	4	7	7	8	8	29	DB(3)#
Signal return	5	9	9	10	10	30	DB(4)#
Signal return	6	11	11	12	12	31	DB(5)#
Signal return	7	13	13	14	14	32	DB(6)#
Signal return	8	15	15	16	16	33	DB(7)#
Signal return	9	17	17	18	18	34	P_CRCA#
Ground	10	19	19	20	20	35	Ground
Ground	11	21	21	22	22	36	Ground
(reserved)	12	23	23	24	24	37	(reserved)
(no connection)	13	25	25	26	26	38	TERMPWR
(reserved)	14	27	27	28	28	39	(reserved)
Ground	15	29	29	30	30	40	Ground
Signal return	16	31	31	32	32	41	ATN#
Ground	17	33	33	34	34	42	Ground
Signal return	18	35	35	36	36	43	BSY#
Signal return	19	37	37	38	38	44	ACK#
Signal return	20	39	39	40	40	45	RST#
Signal return	21	41	41	42	42	46	MSG#
Signal return	22	43	43	44	44	47	SEL#
Signal return	23	45	45	46	46	48	C/D#
Signal return	24	47	47	48	48	49	REQ#
Signal return	25	49	49	50	50	50	I/O#

13.2.3.2 Wide Single-Ended SCSI Cables, Connectors, and Signals

Wide (16-bit) SCSI transfer modes use wide (68-pin) cables. Officially, a wide cable is called a SCSI P cable, but it may also be called a SCSI-2 cable or a 68-pin SCSI cable. A P cable may use any of several connectors, most commonly high-density 68-pin internal, high-density 68-pin external, and VHDCI 68-pin external. Wide SCSI uses 68 signals, each carried on one of 68 wires in the SCSI P cable, with the 68 wires organized into 34 pairs. For SE SCSI, each pair includes a signal wire and a signal return (ground) wire. Figure 13-4 shows a SCSI P cable with an internal 68-pin high-density connector. Note the twisted pairs in the cable segment at top.

Figure 13-4. SCSI P cable with 68-pin high-density connector

Early wide SCSI implementations used an awkward combination of two cables: a standard 50-pin A cable and a special 68-pin B cable. The B cable was never popular, and the combination A+B cabling was quickly replaced by the single 68-pin P cable.

Table 13-9 lists the pinouts for SCSI P cables and connectors. A # following a signal name indicates that the signal is active-low. In a P cable, (reserved) lines are left open in SCSI devices and terminators. Although conductor numbers do not map directly to pin numbers, all P cable connectors use the same pinouts.

Table 13-9. SCSI P cable pinouts

Signal	Pin #	Cable Conductor #		Pin #	Signal
Signal return	1	1	2	35	DB(12)#
Signal return	2	3	4	36	DB(13)#
Signal return	3	5	6	37	DB(14)#
Signal return	4	7	8	38	DB(15)#
Signal return	5	9	10	39	DB(Parity1)#
Signal return	6	11	12	40	DB(0)#
Signal return	7	13	14	41	DB(1)#
Signal return	8	15	16	42	DB(2)#
Signal return	9	17	18	43	DB(3)#
Signal return	10	19	20	44	DB(4)#
Signal return	11	21	22	45	DB(5)#
Signal return	12	23	24	46	DB(6)#
Signal return	13	25	26	47	DB(7)#
Signal return	14	27	28	48	P_CRCA#
Ground	15	29	30	49	Ground
Ground	16	31	32	50	Ground
TERMPWR	17	33	34	51	TERMPWR
TERMPWR	18	35	36	52	TERMPWR
(reserved)	19	37	38	53	(reserved)
Ground	20	39	40	54	Ground
Signal return	21	41	42	55	ATN#
Ground	22	43	44	56	Ground
Signal return	23	45	46	57	BSY#
Signal return	24	47	48	58	ACK#
Signal return	25	49	50	59	RST#
Signal return	26	51	52	60	MSG#
Signal return	27	53	54	61	SEL#
Signal return	28	55	56	62	C/D#
Signal return	29	57	58	63	REQ#
Signal return	30	59	60	64	I/O#
Signal return	31	61	62	65	DB(8)#
Signal return	32	63	64	66	DB(9)#
Signal return	33	65	66	67	DB(10)#
Signal return	34	67	68	68	DB(11)#

A 32-bit SCSI Q cable was defined, but that cable was never implemented, and so was dropped from the SCSI-3 specification.

13.2.3.3 Low-Voltage Differential (LVD) SCSI Cables, Connectors, and Signals

LVD SCSI transfer modes use a wide (68-pin) cable of special design and construction, which is labeled and referred to as a SCSI LVD cable. An LVD cable uses the same high-density 68-pin external and VHDCI 68-pin external connectors as a P cable. However, all LVD connectors, internal or external, must be shielded, so the high-density 68-pin internal connector is not supported for LVD.

Although a narrow (50-pin) LVD cable is defined by the SCSI standard, all actual LVD implementations are wide, so you will never encounter a narrow LVD cable.

Table 13-10 lists the pinouts for SCSI LVD cables and connectors. Because LVD uses differential signaling rather than the signal/ground method used by SE implementations, each LVD signal is actually a plus and minus signal pair, carried on a twisted pair within the cable. So, for example, whereas in SE SCSI conductors 2 and 1 carry the DB(12)# (active-low) signal and its "signal return" (ground), in LVD SCSI those same conductors carry the DB(12)- (negative) and DB(12)+ (positive) signal pair, respectively. LVD adds one signal not used by earlier variants. The DIFFSENS signal (conductor 31 in LVD Wide, and conductor 21 on LVD Narrow) is used to control differential signaling.

Table 13-10. SCSI LVD cable pinouts

Signal	Pin #	Cable Conductor #		Pin #	Signal
DB(12)+	1	1	2	35	DB(12)-
DB(13)+	2	3	4	36	DB(13)-
DB(14)+	3	5	6	37	DB(14)-
DB(15)+	4	7	8	38	DB(15)-
DB(Parity1)+	5	9	10	39	DB(Parity1)-
DB(0)+	6	11	12	40	DB(0)-
DB(1)+	7	13	14	41	DB(1)-
DB(2)+	8	15	16	42	DB(2)-
DB(3)+	9	17	18	43	DB(3)-
DB(4)+	10	19	20	44	DB(4)-
DB(5)+	11	21	22	45	DB(5)-
DB(6)+	12	23	24	46	DB(6)-
DB(7)+	13	25	26	47	DB(7)-
P_CRCA+	14	27	28	48	P_CRCA-
Ground	15	29	30	49	Ground
DIFFSENS	16	31	32	50	Ground
TERMPWR	17	33	34	51	TERMPWR
TERMPWR	18	35	36	52	TERMPWR
(reserved)	19	37	38	53	(reserved)
Ground	20	39	40	54	Ground
ATN+	21	41	42	55	ATN-
Ground	22	43	44	56	Ground
BSY+	23	45	46	57	BSY-
ACK+	24	47	48	58	ACK-
RST+	25	49	50	59	RST-
MSG+	26	51	52	60	MSG-
SEL+	27	53	54	61	SEL-
C/D+	28	55	56	62	C/D-
REQ+	29	57	58	63	REQ-
I/O+	30	59	60	64	I/O-
DB(8)+	31	61	62	65	DB(8)-
DB(9)+	32	63	64	66	DB(9)-
DB(10)+	33	65	66	67	DB(10)-
DB(11)+	34	67	68	68	DB(11)-

You might think that because a wire neither knows nor cares what signal it carries, it would be possible to use a standard SCSI P cable with the appropriate connectors to link LVD devices. Physically, such a cable will fit, and electrically all the connections are correct, but the SCSI P cable will not work, or, if it works, it will not work reliably. LVD implementations, Ultra2 Wide and Ultra160, use higher signaling rates than a standard SCSI P cable is designed to support. LVD cables are of much higher quality than standard P cables, use twisted pairs rather than individual conductors, are always clearly marked as being LVD cables, and are required for LVD applications.

13.2.4 SCSI IDs and Termination

SCSI uses a logical bus topology, which means that all SCSI devices on a single SCSI bus connect to and share that bus. The logical bus is implemented with a daisy chain, whereby the first device connects to the second device, which connects to the third device, and so on. The physical cabling used to implement this daisy chain varies with the type of SCSI device, as follows.

• Many external SCSI devices and some older internal SCSI devices have two narrow SCSI connectors. To build the daisy chain, you use a cable to connect the "out" SCSI connector on the first device to the "in" SCSI connector on the second device, the "out" SCSI connector on the second device to the "in" SCSI connector on the third device, and so on.

• Some external SCSI devices and most recent internal SCSI devices have only one SCSI connector. These devices connect to a cable that contains multiple device connectors, similar to a standard IDE cable. You can connect as many devices to these cables are there are positions. In effect, the daisy-chaining is done within the cable itself.

Each SCSI device on a bus is identified by a unique SCSI ID. On a Narrow SCSI bus, the SCSI ID must be in the range of 0 through 7, inclusive. By convention, the SCSI host adapter is assigned SCSI ID7, the primary hard disk (if present) is assigned SCSI ID0, and the secondary hard disk (if present) SCSI ID1. A Wide SCSI bus doubles the number of supported devices from 8 to 16, using SCSI IDs 0 through 15, with the same default assignments.

A SCSI bus must be terminated on both ends to prevent Standing Wave Reflection (SWR). When a SCSI signal on an unterminated bus reaches the end of the cable, it is reflected back toward the source, which causes errors because SCSI devices cannot differentiate between the reflected wave and the original signal. Two types of terminators exist:

Passive SCSI terminator

A passive SCSI terminator is simply a resistor pack that roughly matches the impedance of the SCSI bus. It is connected to the end of the bus, where it absorbs signals before they can be reflected, preventing SWR. A passive terminator relies on the SCSI host adapter to provide consistent voltage to the bus. If that voltage fluctuates, an impedance mismatch occurs between the cable and the terminator, which allows SWR and may cause errors on the bus. Passive terminators are used by SCSI-1 and some SCSI-2 devices.

Active SCSI terminator

An active SCSI terminator uses a live electronic circuit (a voltage regulator and associated circuitry) to maintain constant impedance at the end of the SCSI bus. Because active termination can regulate impedance much more accurately than a simple resistor, voltage fluctuations from the host adapter cannot cause the wide impedance swings that may occur with a passive terminator. More tightly controlled impedance translates into a more stable SCSI bus that allows higher speeds without errors. Many SCSI-2 and all SCSI-3 devices use active termination.

The method used to terminate the SCSI bus depends on the type of cable and devices used on the bus, as follows:

Standalone termination

Some external SCSI devices and a few internal SCSI devices have two SCSI connectors, which allows those devices to be physically daisy-chained by using separate cables to connect to the previous and next devices in the SCSI chain. Although some of these devices can be terminated by setting a switch or jumper to activate an internal terminator, many require instead using a separate SCSI terminator pack, which is connected to the unused SCSI connector on the last physical device in the chain.

Device-based termination

Most SCSI devices other than LVD/U2W drives contain internal SCSI terminators, which are activated by setting a switch or jumper. When connecting such devices, activate termination for the last physical devices on each end of the chain, and make sure that all intermediate devices have termination disabled. On most drives, disable termination by connecting the jumper labeled Terminator Disable (or similar) or disconnecting the jumper labeled Terminator Enable (or similar). On some older drives, the terminator is a resistor pack that you physically install or remove to enable or disable termination.

Cable-based termination

LVD/U2W drives make no provision for manual termination. If those devices are used as the last device on an SE SCSI bus, termination must be supplied by external means. For this reason, special cables are available that have built-in terminators. Figure 13-5 shows an LVD SCSI cable with a built-in active terminator.

Figure 13-5. LVD SCSI cable with a built-in active terminator

Automatic termination

Some SCSI devices, particularly host adapters, sense whether or not they are the last device on the bus, and enable or disable termination automatically as appropriate.

Technically, in addition to terminating the last physical device on the bus, you should also terminate the cable itself if unused positions exist beyond the last device. In practice, we have never bothered to do so and have never experienced problems attributable to not doing so. Usually, we just connect the last device to the last cable position, which sidesteps the problem.

When configuring SCSI devices, do not confuse termination with termination power. The former specifies which is the last device on the bus. The latter specifies the power source for termination, which may be the device or the SCSI bus. Configuring termination power incorrectly may cause various symptoms, including the system failing to boot or locking up immediately after boot.

A special case exists when you have both internal and external devices connected to a single SCSI bus. In this case, the host adapter, which is ordinarily on the end of the bus and therefore terminated, is instead in the middle of the bus and must not be terminated. In this situation, turn off termination for the host adapter, and terminate the last physical device on the internal chain and the last physical device on the external chain.

Incorrect termination is one of the three most common causes of problems when installing SCSI devices (the others being assigning duplicate SCSI IDs and using poor-quality cables). Failing to terminate one or both ends of the SCSI bus may cause various symptoms, including one or more devices not being accessible, frequent errors and retries, slow throughput, or a complete failure of the SCSI bus. Another common error is terminating the bus at both ends and in the middle. This usually occurs when someone adds a terminated device to an existing bus and forgets to disable termination on one of the existing devices. If the new device is added to the end of the chain without disabling termination on the device that formerly ended the chain, the new device is not recognized. If the new device is added to the middle of the chain, the new device is recognized, but all existing devices downstream of the new device disappear.

On older SCSI devices, SCSI IDs and device termination are usually assigned manually by setting jumpers or switches on the devices, or, on external devices, by turning a small dial. Most newer SCSI devices support SCSI Configured Auto-Magically (SCAM), which is essentially Plug-N-Play for SCSI.

SCAM-compliant devices

SCAM-compliant SCSI devices automatically report their current SCSI ID and termination status to the host adapter, and allow the host adapter to change those settings dynamically. In a system with a SCAM-compliant host adapter and all SCAM-compliant devices, you need never set SCSI ID or termination manually.

SCAM-compatible devices

SCAM-compatible SCSI devices automatically report their current SCSI ID and termination status to the host adapter, but do not allow the host adapter to change those settings. In a system with a SCAM-compliant host adapter and a mix of SCAM-compliant and SCAM-compatible devices, you ordinarily do not need to set SCSI IDs manually, because the host adapter works around the IDs in use by SCAM-compatible devices by assigning unused IDs to the SCAM-compliant devices. You may, however, need to set termination manually, because SCAM cannot reset an improperly terminated SCAM-compatible device.

Non-SCAM devices

Non-SCAM SCSI devices neither report their current SCSI ID and termination status to the host adapter, nor allow the host adapter to change those settings. In a system with all non-SCAM devices, you must set SCSI ID and termination manually for each device. In a system with a SCAM-compliant host adapter and one or more non-SCAM devices, you must disable SCAM on the host adapter and configure all devices manually to avoid conflicts that may occur if SCAM unwittingly assigns the same SCSI ID to a SCAM-compliant device that is already being used by a non-SCAM device.

13.2.5 SCSI Interoperability

SCSI host adapters and drives used in PCs are in theory interoperable whatever their age and level of standards compliance. That is, if you have the proper cable, you can connect a new Ultra Wide SCSI hard drive to an old SCSI-1 host adapter and it will work, albeit at only the 5 MB/s transfer rate supported by the old host adapter. Similarly, you can connect an elderly SCSI-1 CD-ROM drive to a U2W host adapter and expect it to work. But just because you can do something doesn't mean you want to. Keep the following in mind if you mix SCSI device types:

• All devices on a SCSI bus communicate at the speed of the slowest device. For example, if you connect a U2W (80 MB/s) hard disk and a Fast SCSI (20 MB/s) CD-ROM drive to the same bus, the hard disk operates at 20 MB/s, which may significantly degrade hard disk performance. In general, assuming that your hard disks are all of the same type, the best practice is to place all hard drives on one host adapter or channel and put other SCSI devices (like CD-ROM drives, tape drive, and scanners) on another, slower channel or host adapter.

• Although you can connect both wide and narrow devices to the same channel on a wide host adapter, you must install the wide devices physically closest to the host adapter, and use a cable converter that terminates the wide portion of the cable between the last wide device and the first narrow device.

• The presence of one SE device on the SCSI bus forces all other devices on the bus to operate in SE mode.

• Most LVD drives make no provision for setting termination on the drive, and hardcode termination power to Drive Supplies the Bus. Both of these are standard practice for LVD host adapters, but may be incompatible with earlier host adapters. If you need to mix SE and LVD devices on one channel, construct the daisy chain such that the final device is an SE device, which allows you to use its built-in terminator to terminate the channel. If for some reason the only choice is to put an LVD device as the final device on the cable, the only option is to use a cable with built-in termination.