F-2 IEEE-488 Bus Overview
Introduction
The IEEE-488 bus is a communication system between two or more electronic devices. A
device can be either an instrument 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 con-
troller. 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 device will
receive information. To simplify the task of keeping track of the devices, a unique address num-
ber 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 instruc-
tion 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 indicates 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, indicates that it is ready for the next
byte of data.
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