Input / Output Organization 259
To conclude the discussions on USB, it may be indicated that it has become a widely popular inter-
face for slow-speed and medium-speed device interfacing because of its minimum number of pins, plug
and play characteristics and classical simplicity in data communication technique.
8.10.3 Small Computer System Interface (SCSI)
Unlike USB, which depends completely on serial communication, small computer system interface
(SCSI) depends on parallel communication. This bus standard was de ned by American National Stan-
dards Institute (ANSI) and has undergone many revisions and versions. To have a preliminary idea
about the variations of SCSI, we may consider its following factors:
R SCSI data bus may be 8-bit or 16-bit wide, capable of communicating either one or two bytes of
information at any time, depending upon its version.
R SCSI allows two voltage levels, 5 volts (known as high voltage differential or HVD) or 3.3 volts
(known as low voltage differential or LVD), depending upon its version.
R The number of pins of SCSI connectors may be 50, 68 or 80, depending upon its version.
R The maximum length of SCSI cable may vary from 1.6 m to 25 m, depending upon its
version.
R The data transfer rate for SCSI may vary from 5 megabytes/second to 640 megabytes/second,
depending upon its version.
R Name of some of the versions of SCSI are: Fast SCSI, Ultra SCSI, Wide Ultra2 SCSI, Ultra4
SCSI and so on.
Just like USB, the devices interfaced through the SCSI bus do not directly communicate with
the processor of the system (Figure 8.13 ). To communicate with these devices, the system must
have a SCSI controller, which communicates with the processor as well as with the attached
devices. Therefore, the devices attached through the SCSI bus cannot be directly addressed by
the processor. As a matter of fact, the most interesting characteristics about the SCSI bus is that
it does not offer any address line at all (just like USB). Then, how is it that any device is targeted
among several devices attached with the SCSI bus? Well, this duty is accomplished by the data
lines present in the SCSI bus. If there are only 8 data lines offered by the SCSI bus, then a maxi-
mum of eight devices may be serviced through its SCSI controller, using one data line to target a
particular device. If a wider data bus with 16 data lines is available depending on the SCSI version
being used, then 16 devices may be serviced as a maximum. In SCSI one data line is assigned for
one device. This is a major difference with USB, which may communicate with a maximum of
127 devices.
Apart from the data lines, SCSI bus offers a few handshaking and status signals. All signals of
SCSI bus are active low, including the data signals. That means that the logic level 1 is indicated by
0 volt. The bus supports four major types of operations—arbitration, selection, information transfer and
reselection. To understand why the rst and the last provisions have been made available, we need to
look at the background of evolution of SCSI bus.
Originally, SCSI bus was conceptualized for interfacing high-speed devices, like disc drives. In
these cases, after selecting a drive for data reading (or writing) operation, some delay (in milliseconds)
is expected as the read/write head has to seek the correct track and nd the proper sector. When the
drive is ready, it generally sends a large chunk of data, which is handled by DMA. However, during the
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260 Computer Architecture and Organization
seek-like operations, the device remains unproductive and need not use the bus, although it was selected
to do some work. It is always economical to let some other active device to use the bus during these
temporarily-free time slots. That is the reason of reselection coming into the picture. The arbitration
comes into the picture as the multiple devices may request at the same time as they are identi ed by
one-each data line.
8.10.4 GPIB/HPIB/IEEE488
To start with, GPIB is the acronym for General Purpose Interface Bus and HPIB stands for Hewlett
Packard Interface Bus. As a matter of fact, all these three acronyms stand for the same interface, which
was originally designed by Hewlett Packard, the well-known manufacturer of electronic instruments.
The bus was primarily developed to interface various electronic instruments with a host and was des-
ignated as HPIB. It became so popular in those days that the interface was adopted by many others and
was renamed as GPIB. Finally, IEEE accepted it and thereafter it was designated as IEEE488. The only
minor difference between HPIB and the other two is the mechanical dimensions of the connector, which
is adopted to interface various devices around the bus. Hereafter, in this section we shall refer this bus
as HPIB, as it was its original designation.
Figure 8.18 Schematic representation of HPIB
HPIB adopts an 8-bit parallel asynchronous half-duplex communication technique. Devices using
this bus must fall in one of the three possible categories, that is: talker , listener and controller (refer
Figure 8.18 ). A talker is a device which can only talk or keep on sending data but never receives any data
(never listens). Example of talker may be any keyboard or mouse. Listener is a device which only listens
or keeps on receiving data but never sends data out (never talks). Example of a good listener is not a
student of my class but any printer or plotter. A device like a tape-drive may be taken as an example of
talker-cum-listener as it may either send or receive data. However, for any given time it must be desig-
nated as either a talker or a listener. No device in HPIB can simultaneously talk and listen as the bus is
a half-duplex one, which we have already noted.
Apart from talker and listener, there is a third category, designated as controller, which is the proces-
sor of the system by default (ex-of cio). Note that HPIB allows only one controller within its bus and
never a multiple one. By default, the controller is also empowered to be a talker or a listener, depending
upon the need of the hour.
Another important aspect of HPIB is that at any time there would be only one talker. HPIB does not
allow more than one talker at any given time. However, the number of listener may be one or more,
depending upon the situation. During any communication, who would be the talker and who would be
the listener, is decided by the controller.
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