A+ (2006-07) / Lessons

SCSI

Overview

SCSI ("scuzzy") — Small Computer Systems Interface — was designed as a general purpose interface for a range of devices but it is most commonly used for high-speed hard drives.

The SCSI data bus connects to the PC via a controller card (called the host adaptor).  The bus is also fed to SCSI-compatible devices (such as disk or tape drives and scanners) via internal and external connections.

The host adaptor does not know how to control the attached devices; it merely feeds control and data signals to them via the bus.  The devices themselves have their control electronics built in.  (This is similar to IDE hard drives)

The original SCSI design allowed for an 8-bit (i.e. parallel) data bus running at 5 MBps.  Later versions of SCSI are capable of transferring up to 320 MBps.

Power is fed to internal devices from the case PSU; external devices require their own power.

Device IDs

The SCSI bus supports 8 or 16 devices, depending on design.  One of these devices will be the host adaptor.  Therefore we speak of SCSI allowing up to 7 or 15 devices to be connected.

To distinguish devices each is given an ID number: if two devices share the same ID number the devices will fail to function correctly.  This ID number is usually selected by a pattern of jumpers.

The host adaptor is typically given ID number 7, which has the lowest priority.The first internal hard drive is often given ID number 0.

Later SCSI designs allow automatic ID assignment as part of PnP (plug'n'play).

Termination

When electrical signals travel along a wire the signal can 'bounce' back at the end of the wire.  This phenomenon is most pronounced in long-distance transmission cables or when using high frequency signalling over shorter distances.  Therefore the bus must be fitted with a dummy electrical load to avoid signal reflections.  This is called bus termination.

There are two types of SCSI terminator: passive (consisting of a number of electrical resistors) and active (containing a small circuit).  Some types of SCSI require active termination.

One way to terminate a SCSI bus is to connect a special terminator adaptor to the end of the bus.  Many modern SCSI devices have self-termination: by setting a jumper or switch, the bus is terminated at the device.

Poor termination is a major cause of problems, causing failed system start-up, hard drive crashes and random system failures.

Drivers

SCSI devices are not recognised by some BIOSes.  When installing an operating system, it may be necessary to install specific drivers to allow the device to be recognised.

Signals

Single-Ended (SE)

This term describes signals that are transmitted by placing a voltage (5V or 0V) on a signal wire, which is measured relative to signal ground (GND, i.e. 0V).

Differential

This is a method by which the same signal is applied to two wires, one positive and the other a negative mirror image.  The signals are compared at the other end.  This substantially reduces the effect of electrical noise picked up in transit.

Cables and connectors

50-pin (A-cables)

Early SCSI used a 50-pin ribbon cable inside PCs, often be formed from 25 twisted pairs.

At either end, and at points along the middle, the cable was tapped with 50-pin BERG style connectors.  Thus an early SCSI ribbon cable looks like those used for IDE, but is wider and has more connection headers.

Later, a new 50-pin connector was introduced for internal connections.  This design is known as a high-density micro D connector ("Alternative 1").

External devices used a 50-pin Centronics-style connector ("Alternative 2") with specially shielded round cables.  The external device would sport two connections to allow daisy-chaining.

Apple

Unfortunately, Apple preferred to use a 25-pin D connector with many SCSI peripherals.  This was achieved by removing a number of signal ground wires, giving reduced performance.  The use of a 25-pin connector also ensured confusion with parallel printer interfaces; attempting to connect SCSI to a parallel printer bus would undoubtedly damage equipment!

68-pin (B-cable)

For a short while an extra 68-pin cable was used alongside the A-cable to allow 16-bit and 32-bit parallel transmissions.  It was an unpopular design and this combination of A- and B-cables was soon replaced by the P-cable.

68-pin (P-cable & Q-cable)

To support 16-bit parallel transfers a revised 68-pin cable (P-cable) was introduced.  This used a wider high-density micro D connector for both internal and external devices.  By using a duplicate cable in parallel (Q-cable), 32-bit transfers are possible.

Other connectors

In drive arrays, drive caddies are often fitted with wide, Centronics-like 80-pin connectors.  These allow fast connection of data and power.

Types of SCSI

SCSI

The original SCSI design gave 8-bit data transmission at 5 MBps, using an A-cable.

SCSI-2

Improvements to the signalling system in 1989 led to faster (10 MBps) 8-bit transmission, known as Fast SCSI.  This used the conventional A-cable.

Wide SCSI used a P-cable to form 16-bit connections.  It also supported up to 15 devices.  By combining this with fast signalling, Wide Fast SCSI gave speeds of up to 20 MBps.

SCSI-3 (Ultra SCSI)

Changes in 1992 improved speeds to 20 MBps (Ultra SCSI, using an A-cable) and 40 MBps (Ultra Wide SCSI, using a P-cable).

Ultra-2 SCSI

In 1997 Ultra 2 SCSI introduced the Low Voltage Differential (LVD) bus.  This doubled speeds to 40 MBps (Ultra-2 SCSI, A-cable) and 80 MBps (Ultra-2 Wide SCSI, P-cable).

Ultra-160 SCSI (Ultra-3 SCSI)

By 1999 the speed had doubled again, giving 160 MBps (Ultra-160 SCSI, P-cable).  Note that this is available in 16-bit (P-cable) form only and is also known as Ultra-3 Wide SCSI.

Ultra-320 SCSI

Nearly all current SCSI drives are Ultra-320, operating at 320 MBps data transfer.

Text by John King, illustrations pinched from the Computer Desktop Encyclopaedia and Scott Mueller's Upgrading & Repairing PCs (15th Edition)