D
IEEE-488 Bus Overview
D-1
Introduction
Basically, the IEEE-488 bus is simply a communication
system between two or more electronic devices. A de-
vice can be either an instrument or a computer. When
a computer is used on the bus, it serves to supervise the
communication exchange between all the devices and
is known as the controller. Supervision by the control-
ler consists of determining which device will talk and
which device will listen. As a talker, a device will out-
put information and as a listener, a device will receive
information. To simplify the task of keeping track of
the devices, a unique address number is assigned to
each one.
On the bus, only one device can talk at a time and is ad-
dressed to talk by the controller. The device that is talk-
ing is known as the active talker. The devices that need
to listen to the talker are addressed to listen by the con-
troller. Each listener is then referred to as an active lis-
tener. Devices that do not need to listen are instructed
to unlisten. The reason for the unlisten instruction is to
optimize the speed of bus information transfer since
the task of listening takes up bus time.
Through the use of control lines, a handshake sequence
takes place in the transfer process of information from
a talker to a listener. This handshake sequence helps
ensure the credibility of the information transfer. The
basic handshake sequence between an active controller
(talker) and a listener is as follows:
1. The listener indicates that it is ready to listen.
2. The talker places the byte of data on the bus and in-
dicates that the data is available to the listener.
3. The listener, aware that the data is available, ac-
cepts the data and then indicates that the data has
been accepted.
4. The talker, aware that the data has been accepted,
stops sending data and indicates that data is not
being sent.
5. The listener, aware that there is no data on the bus,
indicates that it is ready for the next byte of data.
Bus Description
The IEEE-488 bus, which is also frequently referred to
a the GPIB (General Purpose Interface Bus), was de-
signed as a parallel transfer medium to optimize data
transfer without using an excessive number of bus
lines. In keeping with this goal, the bus has only eight
data lines that are used for both data and with most
commands. Five bus management lines and three
handshake lines round out the complement of bus sig-
nal lines
A typical set up for controlled operation is shown in
Figure D-1. Generally, a system will contain one con-
troller and a number of other instruments to which the
commands are given. Device operation is categorized
into three operators: controller, talker and listener. The
controller does what its name implies; it controls the in-
struments on the bus. The talker sends data while a lis-
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