About SCSI Drives

SCSI is an acronym for Small Computer Systems Interface (SCSI). It is a device independent, system level peripheral interface. Each SCSI bus can connect up to seven or up to 15 peripherals. SCSI interfaces not only to hard drives and CD-ROM drives but also to printers and scanners. SCSI devices are designated as either initiators (drivers) or targets (receivers) and the interface to the host computer is called the host adapter.

SCSI uses two electrical alternatives for interfacing: Single-ended and Differential. Single-ended and Differential devices are different and cannot be mixed on the same bus. On a SCSI bus, devices are daisy-chained together using a common cable. Both ends of the cable must be terminated. All signals are common between all SCSI devices.

The interface used in most computers, IDE, is limited to an internal 18-inch cable with two devices per cable. IDE buses execute one command at a time, while SCSI allows queuing to each of the devices on the bus. IDE uses as much as 95% of the CPU during data transfers, but SCSI only uses about 5% of the CPU for buffer management.

SCSI began life in 1979 as a universal interface for future disk drives envisioned by Shugart Associates, a disk drive manufacturing company acquired by Xerox. Shugart wanted to develop an interface that supported logical block addressing instead of head/cylinder/sector parameters, 8-bit parallel data transfer instead of serial, and generic commands instead of control lines. They named this interface SASI, an acronym for Shugart Associates Systems Interface. The SASI specification included some 6-byte commands and a single-ended interface.

In late 1981, Shugart, joined by NCR, submitted SASI to the American National Standards Institute (ANSI) for adoption. ANSI enhanced SASI with the ability for devices to compete for the bus (arbitrate) and to free the bus (disconnect/reselect) temporarily during command execution. In 1986, ANSI adopted the standard, renaming it the Small Computer Systems Interface (SCSI). At the time, devices adhering to this standard had been on the market for two years. The first official version of the standard is known today as SCSI-1. It included 6- and 10-byte commands, single-ended and differential interface options, and synchronous and asynchronous data transfer.

So, whatever happened to Shugart? Founder Alan Shugart left Xerox to start another disk drive manufacturing firm. He chose the anglicized form of his surname for the new company - Seagate!

SCSI Standards

SCSI-1

SCSI-1 supports transfer rates of up to 5 Mbps and up to 7 devices on an 8-bit bus (not including the host adapter). The most common types of connectors for SCSI-1 are the Amphenol 50-pin for external connectivity and 50-pin Dual-Row Socket F (IDC) connectors for internal connections.

SCSI-2

Approved by ANSI in 1994, SCSI-2 raised the bar to 10 Mbps on a 16 bit bus. Using the 32 bit bus from the standard, known as "Wide SCSI", the rate increases to 20 Mbps. SCSI-2 can run as high as 40 Mbps when combining both the Fast and Wide specifications of the SCSI-2 standard. SCSI-2 usually uses a Micro-D subminiature 50-pin connector for external cables. Internally, like SCSI-1, it connects with the same 50-pin IDC connector.

SCSI-3

Primarily implemented in high-end systems, SCSI-3 commonly uses a 68-pin ribbon cable for in-cabinet connections, and a 68-pin shielded twisted-pair for external connections. Unlike SCSI-1 and SCSI-2, the internal and external 68-pin connectors can be interconnected. Although the most common bus width for SCSI-3 is 16-bit with transfer rates of 20 Mbps, SCSI-3 has specifications for 80 Mbps (Ultra2) and 160 Mbps (Ultra160) throughputs. The Ultra2 and Ultra160 use a technology called Low Voltage Differential (LVD) to achieve higher speeds at nominal cable distances.

Termination

SCSI device interconnections (cables) are susceptible to electromagnetic interference (EMI). Left unchecked, EMI causes frequent signaling failures. Termination eliminates reflected signals that can reflect and propagate to adjacent wires like radio waves reaching an antenna.

Both ends of the SCSI bus must be terminated. For a configuration that contains all internal devices, this means the host adapter and the device furthest out on the ribbon cable are terminated. For a configuration consisting entirely of external devices, the host adapter and the last device to be daisy-chained are terminated. For a mixture of internal and external devices, the host adapter is not terminated, but the internal device furthest out on the ribbon cable and the last external device to be daisy-chained are terminated.

Passive termination is the oldest method of termination. Terminating resistors connect to the ends of the bus to minimize reflections at the end of the cable. It uses three components called resistor packs that each contain 12 330-ohm resistors connected in parallel, from ground to the signal line in series with a 220-ohm resistor connected to terminator power. It works best at low data rates (up to 5 Mbps). Passive termination is supported for all single ended buses with the exception of single ended Ultra SCSI. Passive termination simply provides impedance close to the impedance of the cable. Optimum signal transfer occurs when the impedances of the interfacing devices match perfectly. The termination is designated passive because it does nothing to match the impedance or to regulate power for termination; it relies on a SCSI device to provide steady power.

Active termination, also known as regulated or Boulay, uses a circuit - consisting of 18 110-Ohm resistors, three or more capacitors, two voltage reference resistors, and one voltage regulator - to the SCSI signal lines to reduce the variation with terminator power fluctuation and match the impedance of the SCSI bus better. Active termination is required for single ended Ultra SCSI and is recommended for all single ended buses. Active termination supports longer cable lengths and faster data transfer rates than passive termination.

Forced-Perfect termination is the most complex, but the most efficient, termination. Forced-Perfect terminators integrated into a SCSI cable actually changes their impedances to compensate for variations along the bus. They force the impedance of the cable to match each device through diode switching and biasing.

Single-Ended and Differential SCSI

Single-Ended SCSI devices use +5 volts for signal voltages referenced against electrical ground. Single-Ended cables connect up to eight drivers and receivers using either a 50 conductor flat cable or 25 signal twisted-pair cable. The maximum cable length between devices cannot be longer than 6 meters or shorter than 0.1 meters.

Differential SCSI devices use two lines, one with a positive signal voltage and one with a negative signal voltage. These signal voltages are typically +/-3.5 volts. Integrated circuits consisting of Operational Amplifiers (OpAmps) on the SCSI devices take these two signals as inputs.

One property of OpAmps is that if the identical signal is applied to both inputs, the output is an amplified version of any differences (noise) between the signals (hence the name differential). By applying a signal with opposite polarities to the inputs, the noise is cancelled out, yielding a very clean output signal. The result is reliability, both at much higher speed and at greater distances.

The maximum cable length between devices cannot be longer than 25 meters or shorter than 0.2 meters.

There are two Differential SCSI technologies: High Voltage Differential (HVD) and Low Voltage Differential (LVD). High Voltage Differential (HVD) operates at +/-5 volts and Low Voltage Differential (LVD) operates at +/- 3.3 volts. Because of their very different properties, Single-Ended and HVD devices cannot be mixed on the same SCSI cable. However, LVD devices are designed to function as Single-Ended devices when sharing a bus with true Single-Ended devices.