Measurement Computing Isolator488 - Manual (Page 22)

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 Isolator488 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.