SCSI - Computer Busses - page 167

7.9.11 Discuss how the MSG , C/D and I/O signals are used to set up different transfer

phases.

7.9.12 Explain how SCSI uses the SCSI-ID address to set up a device priority system.

7.9.13 Discuss the usage of the message phase in SCSI and cite typical examples of its

usage.

7.9.14 Discuss the usage of the command phase in SCSI and cite typical examples of its

usage.

7.9.15 Discuss the usage of the status phase in SCSI and cite typical examples.

7.10 Notes from the author

Well I did it. I covered SCSI in a single chapter. It wasn't easy, but its here. SCSI is a mas-
sive area, and one which could fill this topic three or four times over. So, as I do not have
enough space for the full specification, I've tried to give a flavour of the bus.
SCSI's full grown-up name is the small computer systems interface. It is difficult to define
exactly what a small computer system is 1 , but SCSI has outgrown its original application of
interfacing to 'small' systems and to external disk drives. It now has the potential of being
able to interface virtually any external peripheral to a system. It can also be used to connect
devices internally within a system. Typically, it takes a bit longer to initially boot the system,
but once it has, it should be as reliable as any non-SCSI device.
An important concept in SCSI is the prioritisation of devices using SCSI IDs. Few busses
allow the system to prioritise peripherals. Thus, in a properly configured system, fast devices
which require to be quickly serviced will always get access onto the bus before slow devices
which do not require fast servicing. Unfortunately, the method SCSI uses limits the number
of devices to one less than the number of bits on the data bus (seven for an 8-bit data bus and
15 for a 16-bit data bus). In most cases, this is not a major problem. For example, two hard
disks, two CD-ROM drives, a tape backup system, a zip drive and a midi keyboard could all
be attached to a standard SCSI-I bus.
In most PCs the IDE drive is still used in the majority of systems, as it is relatively easy to
set up and its cheap. It is also dedicated to interfacing to the disk drives; thus, no other pe-
ripheral can hog the disk drive bus. However, for most general-purpose applications, SCSI is
best. New standards for SCSI give a 16-bit data bus, at a transfer rate of 20 MHz, giving a
maximum data throughput of 40 MB/s, which is much faster than IDE. It is also much easier
to configure a SCSI system than it is connecting peripherals internally in a PC. A SCSI sys-
tem only requires a single interrupt line, for all the devices that are connected.
Ask someone who has set up a Unix network, or who has configured an Apple computer,
and they will tell you that there is little to beat a well set up SCSI bus. It's reliable, and it is
easy-to-upgrade.

1 Probably, 'small computer' means 'not a mainframe computer' or 'a less powerful com-
puter'. One must remember that SCSI was developed at a time when mainframe com-
puters were kings and PCs were seen as glorified typewriters.