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5. IEEE 488 Primer
5.1 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 IEEE 488 Bus Extender and the IEEE 488 Fiber Bus Extender conform
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.
5.2 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’s 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 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.
At a committee meeting, everyone present usually listens. This is not the case with
the GPIB. The Active Controller selects which devices will listen and commands all
other devices to ignore what is being transmitted. A device is instructed to listen by
CHAPTER 5: IEEE 488 Primer
