13.3 SCSI

We'll devote less space to SCSI than IDE because 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.

13.3.1 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 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 as simply SCSI.

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

13.3.2 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: SCSI when used without qualification to describe a transfer rate 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).
Ultra320 SCSI (Fast-160DT SCSI): Ultra320 SCSI , also called Fast-160DT SCSI, describes the 160 MT/s transfer rate defined in SCSI-3 SPI-4. Fast-160DT uses a double-pumped 80 MHz clock, and transfers 320 MB/s (160 MT/s x 2 bytes/transfer). The fastest current SCSI hard drives transfer less than 80 MB/s, which means that it requires at least two hard drives to saturate Ultra160 SCSI. Accordingly, few desktop systems or workstations require anything faster than Ultra160 SCSI. Ultra320 SCSI is used almost exclusively on midrange or larger servers.

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 next. 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 13-9 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 13-9. 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

13.3.3 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) machines 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 DB25 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 DB25 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.

13.3.3.1 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 13-5 shows a SCSI A cable with an internal 50-pin connector.

Figure 13-5. SCSI A cable (bottom) with IDE cable shown for comparison

Table 13-10 lists the pinouts for SCSI A cables and connectors. A "#" following a signal name indicates that 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 13-10. SCSI cable pinouts
	Connector pin #				Connector pin #
Signal	External	Internal	Cable Conductor #		Internal	External	Signal
Signal return	1	1	1	2	2	26	Signal
Signal return	2	3	3	4	4	27	DB(0)#
Signal return	3	5	5	6	6	28	DB(1)#
Signal return	4	7	7	8	8	29	DB(2)#
Signal return	5	9	9	10	10	30	DB(3)#
Signal return	6	11	11	12	12	31	DB(4)#
Signal return	7	13	13	14	14	32	DB(5)#
Signal return	8	15	15	16	16	33	DB(6)#
Signal return	9	17	17	18	18	34	DB(7)#
Ground	10	19	19	20	20	35	P_CRCA#
Ground	11	21	21	22	22	36	Ground
(reserved)	12	23	23	24	24	37	Ground
(no connection)	13	25	25	26	26	38	(reserved)
(reserved)	14	27	27	28	28	39	TERMPWR
Ground	15	29	29	30	30	40	(reserved)
Signal return	16	31	31	32	32	41	Ground
Ground	17	33	33	34	34	42	ATN#
Signal return	18	35	35	36	36	43	Ground
Signal return	19	37	37	38	38	44	BSY#
Signal return	20	39	39	40	40	45	ACK#
Signal return	21	41	41	42	42	46	RST#
Signal return	22	43	43	44	44	47	MSG#
Signal return	23	45	45	46	46	48	SEL#
Signal return	24	47	47	48	48	49	C/D#
Signal return	25	49	49	50	50	50	REQ#
							I/O#

13.3.3.2 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 the 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 13-6 shows a SCSI P cable with an internal 68-pin high-density connector. Note the twisted pairs in the cable segment at top.

Figure 13-6. 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 13-11 lists the pinouts for SCSI P cables and connectors. A "#" following a signal name indicates that 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.

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

Table 13-11. 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)#

13.3.3.3 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 13-12 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.