Digital488 User’s Manual 11-20-01 IEEE488 Primer, 4-1
IEEE 488 Primer 4
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.
General Structure
The main purpose of the GPIB is to transfer information between two or more devices. A device can
be either 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 roceedings.
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 being Addressed to Listen. This device is then referred
to as an Active Listener. Devices which are to ignore the data message are instructed to Unlisten.
The reason some devices are instructed to Unlisten is quite simple. Suppose a college instructor
is presenting the day's lesson. Each student is told to raise their hand if the instructor has exceeded their
ability to keep up while taking notes. If a hand is raised, the instructor stops his discussion to allow the
slower students the time to catch up. In this way, the instructor is certain that each and every student
receives all the information he is trying to present. Since there are many students in the classroom, this
exchange of information can be very slow. In fact, the rate of information transfer is no faster than the rate
at which the slowest note-taker can keep up. The instructor, though, may have a message for one particular
student. The instructor tells the rest of the class to ignore this message (Unlisten) and tells it to that one
student at a rate which he can understand. This information transfer can then happen much quicker,
because it need not wait for the slowest student.
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