SCSI Buses - An introduction to SCSI technology


Credit for this information should go to Dinah McNutt.

SCSI is an 8-bit parallel I/O bus.

It comes in 3 flavours, SCSI-1, SCSI-2 and SCSI-3. SCSI-1 is the only one which is an official standard, as of the middle of 1994. The current SCSI-1 standard was finalized in 1986.

Incompatibility between SCSI host adapters and SCSI devices is still a monumental problem. I speak from experience. Many manufacturers who produce allegedly SCSI-2 devices actually produce incompatible SCSI-2 devices.

Worse, SCSI hard disk prices are 2 or 3 times the price is same-size IDE hard disks.


Up to 8 devices can be attached to a single SCSI bus. One of these is the host adapter, leaving room for 7 'real' devices.

No matter how many devices are actually connected, only one pair of devices can communicate with each other at any one time.


SCSI uses a 3-bit addressing scheme, where each device is assigned an address ranging from 0 to 7.

Device 7 has the highest priority, so the host computer is usually assigned to be device 7.

Initiator/Target = Client/Server

Communication occurs when the initiator, which is typically the host computer, originates a request, and the target (eg a device controller) performs the request.

The SCSI lets all devices communicate with each other, but some devices are implemented in such a way that the cannot initiate communications.

Electrical Specifications

There are 2 electrical specifications for SCSI: single-ended and differential.

Single-ended is limited to a maximum cable length of 6m.

Differential supports cables up to 25 m in length. Differential is much more expensive to design and manufacture.


SCSI devices can use either asynchronous or synchronous communication protocols.

In the original SCSI specification, synchronous communication allowed speeds of up to 5 Mb/sec. Note that if your SCSI bus is short, asychronous communication can be fast.


SCSI-2 extends the original Common Command Set (CCS) to support CD-ROMs, scanners, communications devices and optical memory drives (eg WORM and erasable media).

SCSI-2 supports 8-bit, 16-bit and 32-bit buses; also it supports up to 10 Mb/sec on 8-bit buses, and thus up to 40 Mb/sec on 32-bit buses.

Fast-Wide SCSI-2

               5 Mb/sec          10 Mb/sec
     8-bit                       Fast
     16-bit    Wide              Fast-Wide
     32-bit    Wide              Fast-Wide

The SCSI-2 specification allows fast SCSI only on differential SCSI buses, but in practice vendors sell single-ended fast SCSI devices.


SCSI-3 is planned to support optical fibre, longer cables, and more that 8 targets per bus.

Also, it will run at up to 20 Mb/sec.

SCSI-3 will include a specification for serial SCSI over fibre-optic or high-speed copper cabling. Speeds aimed for are 51 Mb/sec up to 1 Gb/sec. These serial proposals are called IEEE P1394 (nicknamed FireWire), Fibre Channel and Serial Storage Architecture (SSA).

Another advantage of serial is that fewer wires are required, simplifying cabling. Frankly though, mixing serial SCSI and parallel SCSI looks like a recipe for disaster.

Related Articles

See my articles ``Serial Ports'' and ``Parallel Ports''.


'SCSI and Beyond', Byte, August 1994, p 111, by Dinah McNutt.


This article was written by Ron Savage in 1997. Email: