APPENDIX F
IEEE-488 Bus Overview
INTRODUCTION
Basically, the IEEE-488 bus is simply a communication
system between two or more electronic devices. A device
can be either an instrument or a computer. When a com-
puter is used on the bus, it serves to supervise the com-
munication exchange between all the devices and is known
as the controller. Supervision by the controller consists of
determining which device will talk and which devices will
listen. As a talker, a device will output 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 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 m&ten. 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 hand-
shake 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, 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, in-
dicates that it is ready for the next byte of data.
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
without using an excessive number of bus lines. In keep-
ing 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 signal lines.
A typical set up for controlled operation is shown in Figure
F-l. 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, tier and listener. The controller does what its
name implies; it controls the instruments on the bus. The
talker sends data while a listener receives data. Depending
on the type of instrument, any particular device can be
a talker only, a listener only or both a talker and a listener.
There are two catagories of controllers: system controller,
and basic controller. Both are able to control other in-
struments, but only the system controller has the absolute
authority in the system. In a system with more than one
controller, only one controller may be active at any given
time. Certain protocol is used to pass control from one con-
troller 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.
A device is placed in the talk or listen state by sending an
appropriate talk or listen command. These talk and listen
commands are derived from an instrument’s primary ad-
F-l
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