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IEEE-488 Bus Overview

C-1

C.1

Introduction

The IEEE-488 bus is a communication system between two
or more electronic devices. A device can be either an instru-
ment or a computer. When a computer is used on the bus, it
serves as a supervisor of the communication exchange
between all the devices and is known as the controller.
Supervision by the controller consists of determining which
device will talk and which device will listen. As a talker, a
device will output information. 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.

On the bus, only one device can talk at a time and is
addressed to talk by the controller. The device that is talking
is known as the active talker. The devices that need to listen
to the talker are addressed to listen by the controller. Each
listener is then referred to as an active listener. Devices that
do not need to listen are instructed to unlisten. The reason for
the unlisten instruction is to optimize the speed of bus infor-
mation 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 credibil-
ity of the information transfer. The basic handshake
sequence between an active controller (talker) and a listener
is:

1. The listener indicates that it is ready to listen.
2. The talker places the byte of data on the bus and indi-

cates that the data is available to the listener.

3. The listener, aware that the data is available, accepts 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, indi-

cates that it is ready for the next byte of data.

C.2

Bus description

The IEEE-488 bus, which is also referred to as the GPIB
(General Purpose Interface Bus), was designed 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 data and with
most commands. Five bus management lines and three hand-
shake lines round out the complement of bus signal lines

A typical setup for controlled operation is shown in Figure
C-1. Generally, a system will contain one controller 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 controls the
instruments on the bus. The talker sends data while a listener
receives data. Depending on the type of instrument, any par-
ticular device can be a talker only, a listener only, or both a
talker and listener.

There are two categories of controllers: system controller
and basic controller. Both are able to control other instru-
ments, but only the system controller has the absolute
authority in the system. In a system with more than one con-
troller, only one controller may be active at any given time.
Certain protocol is used to pass control from one controller
to another.

The IEEE-488 bus is limited to 15 devices, including the
controller. Thus, any number of talkers and listeners up to
that limit may be present on the bus at one time. Although
several devices may be commanded to listen simultaneously,
the bus can have only one active talker, or communications
would be scrambled.

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