Keithley 6517B, Справочное руководство

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In this appendix:
Introduction .............................................................................. D-1
Bus description ........................................................................ D-2
Bus lines .................................................................................. D-3
Bus commands ........................................................................ D-6
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 talks and which device listens. As a talker, a
device outputs information and as a listener, a device receives 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
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.
Appendix D
IEEE-488 bus overview