ANSI SCSI Standards

The SCSI standard defines the physical and electrical parameters of a parallel I/O bus used to connect computers and peripheral devices in daisy-chain fashion. The standard supports devices such as disk drives, tape drives, and CD-ROM drives. The original SCSI standard (ANSI X3.131-1986) was approved in 1986, SCSI-2 was approved in 1994, and the first portions of SCSI-3 were approved in 1995. Note that SCSI-3 has evolved into an enormous standard with numerous sections and is an evolving, growing standard still very much under development. Because it has been broken down into multiple standards, there really is no single SCSI-3 standard.

The SCSI bus is defined as a standard by ANSI (the American National Standards Institute), specifically by a committee currently known as T10. T10 is a technical committee of the InterNational Committee on Information Technology Standards (INCITS, pronounced "insights"). INCITS is accredited by ANSI and operates under rules approved by ANSI. These rules are designed to ensure that voluntary standards are developed by the consensus of industry groups. INCITS develops information-processing system standards, whereas ANSI approves the processes under which they are developed and publishes them. Working draft copies of all SCSI-related standards can be downloaded from the T10 Technical Committee site (www.t10.org).

Most companies indicate that their host adapters follow both the ANSI X3.131-1986 (SCSI-1) and the X3.131-1994 (SCSI-2) standards. Note that because virtually all parts of SCSI-1 are supported in SCSI-2, most SCSI-1 devices are also considered SCSI-2 by default. Many manufacturers advertise that their devices are SCSI-2, but this does not mean they support any of the additional optional features that were incorporated in the SCSI-2 revision. For example, an optional part of the SCSI-2 specification includes a fast synchronous mode that doubles the standard synchronous transfer rate from 5MBps to 10MBps. This Fast SCSI transfer mode can be combined with 16-bit Wide SCSI for transfer rates of up to 20MBps. An optional 32-bit version was defined in SCSI-2, but component manufacturers have shunned it as too expensive. In essence, 32-bit SCSI was a stillborn specification, as it was withdrawn from the SCSI-3 standard. Most SCSI implementations are 8-bit standard SCSI or 16-bit Fast/Wide SCSI. Even devices that support none of the Fast or Wide modes can still be considered SCSI-2.

SCSI-3 is broken down into a number of standards. The SCSI Parallel Interface (SPI) standard controls the parallel interconnection between SCSI devices, which is mostly what we are talking about here. So far, five versions of SPI have been created: SPI, SPI-2, SPI-3, SPI-4, and SPI-5.

What can be confusing is that several terms can be used to describe each of the newer SPI standards, as shown in Table 7.1.

Table 7.1. SPI Standards

SCSI-3 Standard	Also Known As	Speed	Throughput
SPI	Ultra SCSI	Fast-20	20/40MBps
SPI-2	Ultra 2 SCSI	Fast-40	40/80MBps
SPI-3	Ultra 3 SCSI or Ultra160(+)	Fast-80DT	160MBps
SPI-4	Ultra 4 SCSI or Ultra320	Fast-160DT	320MBps
SPI-5	Ultra 5 SCSI or Ultra640	Fast-320DT	640MBps

To add to the confusion, SPI-3 or Ultra 3 SCSI is also called Ultra160 or Ultra160+, SPI-4 or Ultra 4 SCSI is also called Ultra320, and SPI-5 or Ultra 5 SCSI is also called Ultra640. The Ultra160 designation refers to any device that includes the first three of the five main features from the Ultra 3 SCSI specification. Ultra160+ refers to any device that supports all five main features of Ultra 3 SCSI. Ultra320 and Ultra640 include all the features of Ultra160+ as well as several additional features.

Table 7.2 shows the maximum transfer rates for the SCSI bus at various speeds and widths and the cable types required for the specific transfer widths.

Table 7.2. SCSI Types, Data-Transfer Rates, and Cables^[1]

SCSI Standard	SCSI Technology	Marketing Term	Clock Speed (MHz)	Transfer Width	ST/DT	Transfer Speed (MB/s)	Max. No. Devices[2]	Cable Type	Max. Length (SE)	Max. Length (HVD)	Max. Length (LVD)
SCSI-1	Async	Asynchronous	5	8-bit	ST	4	7	A (50-pin)	6m	25m
SCSI-1	Fast-5	Synchronous	5	8-bit	ST	5	7	A (50-pin)	6m	25m
SCSI-2	Fast-5/Wide	Wide	5	16-bit	ST	10	15	P (68-pin)	6m	25m
SCSI-2	Fast-10	Fast	10	8-bit	ST	10	7	A (50-pin)	3m	25m
SCSI-2	Fast-10/Wide	Fast/Wide	10	16-bit	ST	20	15	P (68-pin)	3m	25m
SCSI-3/SPI	Fast-20	Ultra	20	8-bit	ST	20	7	A (50-pin)	3/1.5m[3]	25m
SCSI-3/SPI	Fast-20/Wide	Ultra/Wide	20	16-bit	ST	40	7	P (68-pin)	3/1.5m[3]	25m
SCSI-3/SPI-2	Fast-40	Ultra 2	40	8-bit	ST	40	7	A (50-pin)			12m[4]
SCSI-3/SPI-2	Fast-40/Wide	Ultra 2/Wide	40	16-bit	ST	80	15	P (68-pin)			12m[4]
SCSI-3/SPI-3	Fast-80DT	Ultra 3 (Ultra160)	40[5]	16-bit	DT	160	15	P (68-pin)			12m[4]
SCSI-3/SPI-4	Fast-160DT	Ultra 4 (Ultra320)	80[5]	16-bit	DT	320	15	P (68-pin)			12m[4]
SCSI-3/SPI-5[6]	Fast-320DT	Ultra 5 (Ultra640)	160[5]	16-bit	DT	640	15	P (68-pin)			10m[7]

^[1] Cable lengths are in meters: 25m = 80ft., 12m = 40ft., 6m = 20ft., 3m = 10ft., 1.5m = 5ft; SE = single-ended signaling, HVD = high-voltage differential signaling (obsolete), LVD = low-voltage differential signaling, SPI = SCSI Parallel Interface (part of SCSI-3), ST = single transition (one transfer per clock cycle), and DT = double transition (two transfers per clock cycle; 16-bit only).

^[2] Not including the host adapter.

^[3] Ultra SCSI cable total length is restricted to 1.5m if more than three devices exist on the bus (not including the host adapter). A maximum of seven devices is allowed.

^[4] A 25m cable can be used if only one device exists (point-to-point interconnect).

^[5] Ultra 3 (Ultra160), Ultra 4 (Ultra320), and Ultra 5 (Ultra640) SCSI transfer twice per clock cycle and are 16-bit only.

^[6] No Ultra640 products have been produced to date.

^[7] For Ultra 5 (Ultra640) only, cable length is restricted to 2m using ribbon cable, and a 20m cable can be used if only one device exists (point-to-point interconnect).

Note

The A cable listed in Table 7.2 is the standard 50-pin SCSI cable, whereas the P cable is a 68-pin cable designed for 16-bit transfers. High-voltage differential (HVD) signaling was never popular and is now considered obsolete. Low-voltage differential (LVD) signaling is used in the Ultra 2 and Ultra 3 modes to increase performance and cable lengths. Pinouts for the cable connections are listed in this chapter in Tables 7.47.7 (starting on p. 468), and you can see the connectors in Figures 7.77.9 (starting on p. 476).

SCSI is both forward and backward compatible, meaning you can run faster devices on buses with slower host adapters or vice versa. In each case, the entire bus runs at the lowest-common-denominator speed. Thus, although you can mix and match different speeds of devices on the same bus, you should place fast devices on a separate SCSI adapter (or connector, if you use dual-bus adapters) from slower devices. In fact, as stated earlier, virtually any SCSI-1 device can also legitimately be called SCSI-2 (or even SCSI-3) because most of the improvements in the later versions are optional. Of course, you can't take advantage of the faster modes on an older, slower host adapter. By the same token, you can purchase an Ultra 3capable SCSI host adapter and still run older standard SCSI devices. You can even mix standard 8-bit and 16-bit (Wide) devices on the same bus, using cable adapters.

Modern SCSI hard disks and tape backups suitable for servers correspond to SCSI-3 (SPI-3, SPI-4, and SPI-5) standards. However, to understand SCSI, it is also helpful to review the original SCSI-1 and follow-on SCSI-2 implementations. The following sections provide more details.