Measurement Computing Serial488/4 - Manual (Page 72)

Section 4 IEEE 488 Primer
4.1
IEEE 488 Primer
4.1 HISTORY
The IEEE 488 bus is an instrumentation communication bus adopted by the
Institute of Electrical and E lectronic Engineers in 1975 and revised in 1978. T he
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. T he 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.
4.2 GENERAL STRUCTURE
The main purpose of the GPIB is to transfer information betw een 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 allow ing them to have their say. O n 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.