Measurement Computing DAC488 v.1 - Manual (Page 41)

DAC488 User’s Manual IEEE 488 Background 35
IEEE 488 Background 5
History…… 35
General Bus Structure…… 35
Bus Lines & Bus Commands…… 36
Bus Management Lines…… 36
Handshake Lines…… 37
Data Transfer Lines…… 37
Bus Command Groups…… 38
More On Service Requests…… 39
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. 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 the design of the
interface, to the design of the high level software that is specific to the measurement application.
General Bus Structure
The main purpose of the IEEE 488 interface 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 cannot attend the meeting, or
if other matters require his attention, he can appoint an acting chairman to take control of the proceedings.
For the IEEE 488 interface, this device becomes the Active Controller.
At a committee meeting, everyone present usually listens. This is not the case with the IEEE 488 interface.
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 a lot of 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.