Introduction; Eieee-488 Bus Overview - Keithley 2000 User Manual

E-2 IEEE-488 Bus Overview

Introduction

Basically, the IEEE-488 bus is simply a communication system between two or more elec-
tronic devices. A device 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 controller consists of determining which device will talk
and which device will listen. As a talker, a device will output information and as a listener, a de-
vice will receive information. To simplify the task of keeping track of the devices, a unique ad-
dress number is assigned to each one.
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 instruc-
tion is to optimize the speed of bus information transfer since the task of listening takes up bus
time.