Measurement Computing Extender488 - Manual (Page 21)

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 Extender488 and the Extender488/F conform
to this most recent revision designated IEEE 488-1978.
Prior to the adoption of t his 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 t otal 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. B efore any i nformation transfer can take place, it is first
necessary to specify which will d o 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 r un 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" t o that de vice is called Addressing to Talk. If the committee chairman ca n not atten d 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 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 s tudent is told to rais e their hand if the instructor has exceeded their