Week 34 Lesson Plan Addendum


SCSI – Small Computer Systems Interface

SCSI is another technology used to connect various devices to the motherboard. Many devices such as scanners, printers, disk drives, cd drives, JAZ and ZIP drives can be configured to talk to the motherboard through SCSI.

As a technician, you will be required to be familiar and competent in SCSI device installation and maintenance. You will not probably see a lot of it in the field but there are a lot of legacy pieces out there that need to be maintained.

SCSI devices are just a little harder than IDE devices. The only difference is the setting of a SCSI ID address on the device. This address ID can be set using jumpers, DIP switches, rocker switches, or just the software (like Plug and Play).

SCSI subsystem

SCSI devices actually constitute a separate subsystem within the PC. The SCSI subsystem consists of a Host Controller (adapter), the SCSI devices, cabling, and terminators. Also required is a device driver package.

The SCSI devices are daisy changed. The order of the devices does not matter and is usually a physical requirement base only. SCSI cables employ a system of terminators to stop signal reflection and bounce back. The terminators may be a separate device on the cable ends, may be a resister pack on the device at the end of the cable, or it may be jumpers that apply the resister packs across the cabling wires.

Each device including the host adapter must be given a unique ID address. This can be accomplished using thumb wheels on the devices, DIP switches, jumpers, or software (PNP). There are three types of SCSI – I, II, III. The SCSI one components use an 8 bit address bus and thus can only support 0-7 for its ID numbers. The SCSI II and III devices each support 16 devices (0-15) because they use 16 bit addressing. The SCSI III and newer standards can support up to 32 and 64 bit architectures. Those devices that use 8 bit processing are called narrow SCSI and those that use 16 bit are called wide SCSI. (If no name is given, then it implies the narrow).

The host adapter is responsible for controlling all the devices in the SCSI subsystem. Usually the host adapter is given the highest ID number but it is not a necessity. The SCSI controller can allow communication between two SCSI devices without going through the CPU. For example, to back up from one SCSI device to another, would require no CPU time.

Signaling methods

SE – single ended – one wire carries the voltage and the other carries the ground. 50 pin cable

DC – differential cables – both wires carry voltage and signal is difference between the voltage levels. Comes as two types – low level and high level voltages. 68 pin connector

HVD – required a large amount of voltage and power – is obsolete

LVD – low voltage – good yum yum

LVD/SE – use both LVD and SE types of signaling.

Three types of SCSI

SCSI-1 (Regular SCSI)

No common command set

8-bit data bus

7 possible devices

SCSI-2 (Fast or wide SCSI)

Common command set

16-bit data bus

15 possible devices

Mandatory parity checking

SCSI-3 (Ultra SCSI)

Supports both parallel and serial data transmission

Supports FireWire connections

Data transfer of 320 MB/sec

Includes SPI standard

SCSI Symbols


Two Main types but other connectors can be found. Typical connectors are:

A typical internal cable is shown below. It contains multiple connectors (usually 3 or 4) but could contain as many as seven) – note the terminator:

Termination is the hardest to understand about SCSI – Here is a typical terminator for the 50 pin Centronics type connector.

There are passive and active terminators. SE uses all types, differential uses wither HVD or LVD terminators.

Cabling is associated with type of SCSI according to chart below.

Connectors on devices – as shown below