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APPENDIX
G
IEEE-488 BUS OVERVIEW
BUS DESCRIPTION
The IEEE-488 bus, which is also frequently
referred to as
the GPIB (General Purpose Interface Bus), was designed
as a parallel transfer medium to optimize data transfer with
a minimum
number of bus lines. In keeping with this goal,
the bus has eight data lines that are used both for data and
many commands. Additionally,
the bus has five manage-
ment lines, which are used to control bus operation,
and
three handshake lines that are used to control the data byte
transfer sequence.
A typical
configuration
for controlled
bus operation
is
shown in Figure G-l. A typical system will have one con-
troller and one or more devices to which commands are
given and, in most cases, from which data is received.
Generally, there are three categories that describe device
operation:
controller,
talker, and listener.
The controller does what its name implies: it controls other
devices on the bus. A talker sends data (usually to the cotl-
troller), and a listener receives data. Depending
on the in-
strument, a particular device may be a talker only, a listener
only, or both a talker and a listener. The Model 194 has
both talker and listener capabilities.
There are two categories of controllers:
system controller
and basic controller. Both are able to control other devices,
but only the system controller
has absolute authority
in
the system. In a system with more than one controller, only
one controller
may be active at any given time. Certain
command protocol allows control to be passed from one
controller
to another.
The bus is limited to 15 devices, including
the controller.
Thus, any number of devices may be present on the bus
at one time. Although
several active listeners may be pre-
sent simultaneously,
only one active talker may be present
on the bus, or communications
would be scrambled.
Figure G-l. IEEE Bus Configuration
G-l
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