Keithley 6482 Reference Manual page 311

Appendix C: 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 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 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 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 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 lis-
tener.
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.
C-2
Model 6482 Dual-Channel Picoammeter / Voltage Source Reference Manual
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