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CHAPTER 6: IEEE 488 Primer
6.1 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 interface converter conforms to this most recent revision, designated
IEEE 488-1978.
Before this standard was adopted most instrumentation manufacturers
offered their own versions of computer interfaces. This placed the burden
of system hardware design on the end user. If your application required the
products of several different manufacturers, then you 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.
6.2 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 the Active Controller at power-on.
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 that 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 cannot 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.
6. IEEE 488 Primer