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Section 4 IEEE 488 Primer

4.1

IEEE 488 Primer

4.1 HISTORY

The

IEEE 488

bus is an instrumentation communication bus adopted by the Institute of Electrical and

Electronic Engineers in 1975 and revised in 1978. The

Digital488

conforms to this most recent revision

designated

IEEE 488-1978

Prior to the adoption of this standard, most instrumentation manufacturers offered their own versions of

computer interfaces. This placed the burden of system hardware design on the end user. If his application required

the products of several different manufacturers, then he might need to design several different hardware and

software interfaces. The popularity of the

IEEE 488

interface (sometimes called the

eneral

urpose

nterface

us or

GPIB

) is due to the total specification of the electrical and mechanical interface as well as the data transfer

and control protocols. The use of the

IEEE 488

standard has moved the responsibility of the user from design of

the interface to design of the high level software that is specific to the measurement application.

4.2 GENERAL STRUCTURE

The main purpose of the

GPIB

is to transfer information between two or more devices. A device can

either be an instrument or a computer. Before any information transfer can take place, it is first necessary to

specify which will do the talking (send data) and which devices will be allowed to listen (receive data). The

decision of who will talk and who will listen usually falls on the

System Controller

which is, at power on, the

Active Controller

The

System Controller

is similar to a committee chairman. On a well run committee, only one person may

speak at a time and the chairman is responsible for recognizing members and allowing them to have their say. On

the bus, the device which is recognized to speak is the

Active Talker

. There can only be one Talker at a time if the

information transferred is to be clearly understood by all. The act of "giving the floor" to that device is called

Addressing to Talk

. If the committee chairman can not attend the meeting, or if other matters require his attention,

he can appoint an acting chairman to take control of the proceedings. For the

GPIB

, this device becomes the

Active Controller

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Measurement-computing Digital488 Manuale Utente Pagina 61

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