Measurement Computing Extender 488HS Rev.2.0 - Manual (Page 19)

Section 3 IEEE 488 Primer
3.1
IEEE 488 Primer
3.1 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 General Purpose Interface
Bus 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.
3.2 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.