Measurement Computing Parallel488 - Manual (Page 33)

Section 5 IEEE 488 Primer
5.1
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 Parallel488 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.
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 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.