SCSI Untangled.

Over the last few years, there has been an explosion in the variation of SCSI devices that are available. This has led to an array of terms being used to describe these variations. As a result, there have been recent articles published that may leave the reader with the false impression that SCSI is a loosely defined interface with incompatible devices being deployed on the market. This article is intended to explain the earlier naming conventions, the naming evolution and to clarify the current state of SCSI names and the devices that carry those names.

The Players

To understand SCSI terminology, the first thing one needs to understand is that there are two organizations that define what SCSI is and what it is called. The first, formed in 1981, is the T10 SCSI committee, which is a technical committee of the National Committee on Information Technology Standards (NCITS, pronounced "insights"). NCITS is accredited by and operates under rules that are approved by the American National Standards Institute (ANSI). T10 is responsible for writing the documents that become the SCSI standards used by developers in designing SCSI products.

The other, formed in 1996, is the SCSI Trade Association (STA). STA's mission is to help educate the industry, including OEMs, resellers, and information technology professionals about SCSI. STA also helps shape the future of SCSI to ensure performance and compatibility. STA does this by promoting the use of SCSI and by defining marketing names that represent a minimum set of SCSI features, making it easier for companies to tell customers what SCSI products they are selling.

These two organizations work closely together. The T10 SCSI committee develops standards with many options and features. STA decides on marketing names and also selects the options and features to be associated with each name.

Name Evolution

In the early days there was only SCSI, which was replaced by SCSI-2. SCSI-2 was a single standard and it contained everything you needed to know about how to develop a SCSI device, but when it came time to start work on SCSI-3, the T10 committee realized that, if the work was not split among several working groups, SCSI-3 would never be completed. The main reason was that different interface proponents wanted to use the SCSI commands, but not the parallel interconnect. In response, the T10 committee broke apart the SCSI-2 standard and created several SCSI-3 standards.

What was originally one standard became several, including SCSI-3 command standards, a SCSI-3 architecture model standard (SAM), a SCSI-3 parallel protocol standard (SIP), and a SCSI-3 parallel interface standard (SPI). Along with those standards, several new standards were added to the SCSI family, including two SCSI-3 command standards and three SCSI-3 protocol standards. Today, there are approximately 28 different SCSI standards, some approved, some in the process of being approved, some in various stages of development and a few in the beginning stages.

In spite of the fact that SCSI is now a family of standards, when most people refer to SCSI, they are really referring to the SPI standard. In many cases, when an article is written or a paper presented, the term SCSI does not refer to all the SCSI standards, but only to the SCSI parallel interface standards.

Enter SPI-2

When T10 split SCSI-2, it made an error by splitting the protocol and physical parts into two standards (i.e., SIP and SPI). This split caused the definition of an artificial interface that was only used to link those standards together. The link was not useful and was confusing to developers. This resulted in SIP and SPI being ignored with developers using SCSI-2 for everything except electrical characteristics defined in SPI. To make matters worse, another SCSI standard, Fast 20, was developed that contained only the electrical characteristics for what became known as Ultra SCSI.

It took some time, but T10 finally realized that forcing developers to have to look at several standards to develop the protocol and physical parts of SCSI was a bad idea. As a result, they combined all the physical and protocol parts of parallel SCSI into a single standard that was called SCSI Parallel Interface-2 (SPI-2). In addition to this combination, SPI-2 also defined Low Voltage Differential (LVD).

One result of SPI-2 specifying multiple variations of SCSI was that there was no longer an easy way for manufacturers to indicate which part of SPI-2 their product complied to. As a result, STA defined Ultra names for specific funtions defined in SPI-2.

For example:

* Narrow Ultra2 SCSI was defined by STA to use the LVD, 8-bit bus and 40megatransfers/sec data rate options of SPI-2.

* Wide Ultra2 SCSI was defined by STA to use the LVD, 16-bit bus and 40megatransfers/sec data rate options of SPI-2.

Does this mean that the SPI-2 standard only defines those features? No, there are many other features defined in SPI-2 [e.g., fairness, HVD (High Voltage Differential), SE (Signal Ended), MSE (Multimode Single Ended), and messaging].

Does this mean that Ultra2 devices can only have the functions defined by STA? No, an Ultra2 drive can have any number of SPI-2 features and still be called an Ultra2 drive.

Does this mean that an Ultra2 device need only support a 20megatransfers/sec data rate because SPI-2 allows this? No, even though SPI-2 allows devices to be slower, those devices are not Ultra2 devices.

The Ultra3/Ultra160/Ultra160m/Ultra160+ Name Game

The industry understood SCSI-2 and SCSI-3, but when the T10 committee went to Fast-20 and then to SPI-2, everyone was at a loss as to what to call SCSI devices that supported those standards. Realizing that not having clear names would be detrimental, SCSI vendors formed STA. Naming was not the only reason for STA's formation, but it is also one of the major issues STA has dealt with over the years. The resulting new names were Ultra SCSI, which was to be used for devices that followed the Fast 20 standard, Ultra2 SCSI, which was to be used for devices that followed the SPI-2 standard, and Ultra3 SCSI, which was to be used for devices that followed the SPI-3 standard. See Table 1 for a cross-reference between the STA names, the SCSI standards, and the minimum feature set in each.

On completion of SPI-2, the T10 committee started working on the next generation of parallel SCSI, which was called SPI-3. SPI-3, in addition to increasing the data rate, added many new options, i.e., packetization, Quick Arbitration and Selection (QAS), more advanced error checking (CRC), and domain validation.

As a result of these options, there were differences between STA members as to which of the new options should be included under the Ultra3 name. In addition, some STA members wanted to change to Ultral60 to reflect the top speed the new SCSI devices could achieve. A compromise was worked out, whereby a member could label their SCSI devices Ultra 3 if they supported at least one of a list of five options, i.e., Double Transition clocking (DT), CRC, domain validation, packetization, or QAS.

The first "160" name was Ultral60m SCSI, which included three options; DT; CRC, and domain validation. Then came the shortened version of Ultra 160m called Ultra 160. There is essentially no difference between Ultra 160m and Ultra 160. The final "160" name was Ultra 160+ that includes all five options and a version of Ultra3 that does not contain packetization or QAS.

There is not much that can be done to fix the Ultra3/Ultra 160 problem. STA has done what it can by endorsing the Ultral6O SCSI label and stating that SCSI devices using it must contain at least DT, CRC, and domain validation. STA has defined the next generation of SCSI to be Ultra 320. For a company to use that label, it must, at a minimum, support the 320MB/sec data rate, packetization, and domain validation.

SCSI Compatibility (Why It AU Works)

With all the confusion over names, one might think there would be a problem when Ultra3, Ultra160m, Ultra160, and Ultra160+ devices are connected together, but there is no problem. This is because, no matter what they are called, all those SCSI devices are designed using the SPI-3 standard and they are all compatible.

When two SCSI devices transfer data between them, that data will be transferred using protocols and speeds that both devices understand. Those protocols and speeds are agreed to before any data transfers take place. For example, if SCSI device A that does not support packetization is transferring data with SCSI device B that does support packetization, then SCSI device B will know not to use packetized operations.

In general, the SCSI standards exist to allow SCSI devices with different options to interoperate on a single bus. The fact is, SCSI is more tolerant than other interfaces in allowing other devices with different feature sets to co-exist on the same bus. Not only can SCSI devices co-exist on the same bus, but they can also use their full set of features even if other SCSI devices have smaller feature sets, (e.g., an Ultra2 initiator can talk to an Ultra target at 40MB/sec and then talk to an Ultra2 target on the same bus at 80MB/sec).

This means that a system can be upgraded with Ultra3/Ultra160 controller cards without having to replace the Ultra2 disk drives and everything will work as before--including the performance. Then at a later time, Ultra3/Ultra160 disk drives can be installed for additional capacity or to replace the Ultra2 disk drives. Ultra320 will allow the same type of upgrading. The system would, then, have a mix of Ultra2, Ultra3/Ultra160, and Ultra320 disk drives that would all transfer data at their optimum rates.

The mixing of SCSI-2, SCSI-3, or Ultra SCSI devices with Ultra2, Ultra3/Ultra160 (or higher) SCSI devices is allowed and it will work if the Ultra2 and higher SCSI devices have MSE (Multi-mode Single Ended) drivers. However, with that kind of mix, all the SCSI devices are limited to the SE (Single Ended) data transfer rates because the LVD drivers must be turned off. The drivers are turned off for any SCSI device that does not have MSE drivers.

Last Word

SCSI lives on and has a bright future. There is no other interface that has lasted longer with six generations already shipped and the seventh (Table 2) to be shipped in 2001. Regardless of what the different generations have been called by organizations and manufacturers, they are compatible with each other.

George Penokie is a senior engineer at IBM Corporation (Rochester, MN). He has been involved in SCSI standards since 1988, is vice chairman of the TJO SCSI committee, and the technical editor for the SCC-2, SPI-2, SPI-3, and SPI-4 standards. [TABULAR DATA NOT REPRODUCIBLE IN ASCII]

                 STA/T10 SCSI TERMINOLOGY CROSS-REFERENCE
Marketing           Standards Original          Max Bus  Bus
Terms               Terms     Standard          Speed    Width
                                                (MB/Sec) (Bits)
SCSI 1, SCSI-2      Fast-5    SCSI, SCSI-2      5        8
Fast SCSI           Fast-10   SCSI-2            10       8
Fast SCSI           Fast-lO   SCSI-3 (SPI, SIP) 10       8
Fast Wide SCSI      Fast-lO   SCSI-3 (SPI, SIP) 20       16
Ultra SCSI          Fast-20   F-20              20       8
Ultra SCSI          Fast-20   F-20              20       8
Wide Ultra SCSI     Fast-20   F-20              40       16
Wide Ultra SCSI     Fast-20   F-20              40       16
Wide Ultra SCSI     Fast-20   F-20              40       16
Ultra2 SCSI         Fast-40   SPI-2             40       8
Wide Ultra2 SCSI    Fast-40   SPI-2             80       16
Wide Ultra3 SCSI    Fast-80   SPI-3             160      16
Ultra 160 SCSI [*]  Fast-80   SPI-3             160      16
Ultra 320 SCSI [**] Fast-160  SPI-4 [***]       320      16
Marketing           Max. Bus Length (Meters)         Max.
Terms               Single-                  HVD LVD Device
                    ended                            Support
SCSI 1, SCSI-2      6                        25  N/A 8
Fast SCSI           N/A                      25  N/A 8
Fast SCSI           3                        25  N/A 8
Fast Wide SCSI      3                        25  N/A 16
Ultra SCSI          1.5                      25  N/A 8
Ultra SCSI          3                        25  N/A 4
Wide Ultra SCSI     N/A                      25  N/A 16
Wide Ultra SCSI     1.5                      25  N/A 8
Wide Ultra SCSI     3                        N/A N/A 4
Ultra2 SCSI         N/A                      25  12  8
Wide Ultra2 SCSI    N/A                      25  12  16
Wide Ultra3 SCSI    N/A                      N/A 12  16
Ultra 160 SCSI [*]  N/A                      N/A 12  16
Ultra 320 SCSI [**] N/A                      N/A 12  16
(*.)Ultra 160 SCSI is a fixed subset of Wide
Ultra3 SCSI (i.e., DT, CRC, and Domain Validation)
(**.)Ultra 320 SCSI minimum requirements is support
for 320 MB/sec, Domain Validation, and Packetization
(***.)SPI-4 is currently under development in
the SCSI T10 committee