Speed with SCSI

SCSI (Small Computers’ System

Interface), pronounced as skuzzy, is an ANSI standard for connecting peripherals to your computer using a hardware interface that uses standard commands. It had its beginning way back in 1979, when Alan Shugart, head of Shugart Associates created the SASI (Shugart Associates, Systems Interface).

Incidentally, he later went on to create Seagate Technologies. SASI was a revolutionary new interface for its time, and could reach signaling speeds of 1.5 MB/sec. Though this might seem slow for now, remember we are talking about a time when 8” floppy drives were still in use! Alan Shugart wanted SASI to be made into an ANSI standard, since that would make it more widely acceptable. A result of this acceptance by ANSI was the renaming of SASI to SCSI, since having Shugart Associates’ name on the interface would’ve suggested that it was proprietary. Several other changes were also later made to widen the command set, and improve performance. Hence, the first true SCSI interface standard was actually published in 1986 and is today referred to as SCSI-1.

Even though the term SCSI is generally associated with hard drives, it is actually a system-level bus. All SCSI devices have in-built controllers that manage the flow of data through the cable connecting them. Thus, not only hard drives, but several other devices also, like CD-writers, scanners, and tape drives, can be connected using SCSI. All these devices exist as peers on the bus. 

SCSI was designed to be a high-level, expandable, and high-performance interface. Unfortunately, such features do not come cheap, a reason why most computers sold today do not provide native SCSI support. In computers which do not have in-built SCSI controllers, a PCI controller card is usually added, which acts as just another SCSI device on the bus. At the same time it connects the SCSI chain to the computer system for information interchange. This controller card is usually called the host adapter. 

SCSI standards

The number of SCSI standards has swelled up quite a lot over the past decade or so. SCSI today has become a broad term, encompassing several protocols and methods. The latest standard, SCSI-3 has only added to the confusion, as it is actually a collection of several different standards.Let’s look at some of the major standards.

SCSI-1

The first SCSI standard defines the basics of the first SCSI buses, like cable length, signaling characteristics, and transfer modes.

SCSI-1 devices supported only an 8-bit wide bus, and 5 MB/sec transfer rate. Only single-ended (SE) transmission with passive termination was supported. Single-ended transmission means that this standard used the age-old convention of representing a binary ‘one’ with positive voltage, and ‘zero’ with zero voltage. SE transmission limits performance because of issues like bouncing signals, degradation over distance, and cross-talk between adjacent wires.

Termination is a concept in which signals are killed at the end of the SCSI cable to prevent them from bouncing back onto the wires. Passive termination is the oldest and least reliable of all known methods, and works by simply inserting resistors where the signal is to be terminated. SCSI-1 has become so obsolete that the standard has in fact been withdrawn by ANSI.

SCSI-2

Work on SCSI-2 began in the year 1985. The most important objective of the development of this SCSI standard was the standardization and formalization of the SCSI command set, which is a set of commands that makes SCSI devices perform actual tasks. This led to the creation of the CCS (common command set) for SCSI hard drives. This eventually formed the basis of SCSI-2, and SCSI-2 was approved by ANSI in 1994. SCSI-2 doubled the speed of the bus to 10 MHz, thus resulting in a 2x increase in the transfer rate as well to 10 MB/sec. This transfer rate was further enhanced by using 16 or 32-bit wide buses, which came to be called Wide SCSI. Wide SCSI also had the advantage of supporting up to 16 devices on the same bus. Passive Termination was phased out, and the use of Active Termination, in which voltage regulators were added to the resistors used in Passive Termination, was encouraged. SCSI-2 also made an important contribution called HVD (High Voltage Differential) signaling. HVD signaling used a very different concept from the earlier SE signaling. Instead of simply passing a high or a zero voltage down a wire, it uses two wires for every bit. The first wire is a mirror image of the second. For a logical ‘zero’, zero voltage is sent on both wires. For a logical ‘one’, one of the wires carries a high voltage, and the other a low voltage. The circuitry at the receiving end takes the difference between the two signals sent, and thus sees a relatively high voltage for a ‘one’, and zero voltage for a ‘zero’. This allows for usage of relatively longer cables and have higher speed devices compared to SE signaling.

Ultra2 SCSI

Not to be confused with SCSI-2, Ultra2 SCSI is a marketing term for the SCSI-2 Parallel Interface-2 (SPI-2) standard. It was originally defined for an 8-bit bus, using 40 MHz bus speed that gave it a net throughput of 40 MB/sec. However, almost all modern-day implementations of Ultra2 SCSI are done as Wide Ultra2 SCSI in which a 16-bit bus gives it a maximum transfer rate of 80 MB/sec. Again, a change in the signaling technology was made. LVD (Low Voltage Differential) signaling was introduced which was the same as HVD, except that it ran on a lower voltage. This took care of HVD’s two primary disadvantages: high cost, and electrical incompatibility with SE signaling devices. SE devices can run on LVD buses, and some LVD devices can run on SE buses. As far as termination was concerned, both LVD and HVD required their own terminators.

Ultra3 SCSI

This term refers to devices that support some or all of the features mentioned in SCSI-2 Parallel Interface (SPI-3) standard. The main feature was support for transfer speeds of 160 MB/sec. Ultra3 SCSI is by default 16-bit wide, and 8-bit support was dropped during the specification of the standard. Unfortunately, all the five key features of Ultra3 SCSI are optional. Thus, a manufacturer could claim his device to be Ultra3 SCSI, even if it didn’t support 160 MB/s transfer rates, but satisfied one of the other conditions. Hardware makers wanted a more concrete standard, and this led to the creation of the Ultra160 SCSI feature sets. All Ultra160 devices had to support three features: Fast-80 bus (double pumped 40 MHz bus), CRC (Cyclic Redundancy Check) checking, and domain validation in which the host adapter keeps decreasing its speed until it knows that communication will be reliable. Later, Ultra160+ SCSI made all the five key features of SCSI-3 mandatory.

Future of SCSI

Development work has already started on Ultra320 SCSI, which will boost the maximum transfer rates to a whopping 320 MB/sec. SCSI is the most well known and reliable method for high-speed communication between different devices in high-performance computers like servers and workstations.