Measurement Computing Extender488 - Manual (Page 22)

Section 3 IEEE 488 Primer
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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 u p. I n this way, the instructor is certain that each and every student
receives all the informatio n he is trying to present. Since ther e 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 i nstructor 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. Thi s information transfer can then happen much quicker,
because it need not wait for the slowest student.
The GPIB transfers information in a sim ilar way. This method of data transfer is called
handshaking. More on this later.
For data transfer on the IEEE 488, the Active Controller mustÉ
a) Unlisten all devices to protect against eavesdroppers.
b) Designate who will talk by addressing a device to talk.
c) Designate all the devices who are to listen by addressing those
devices to listen.
d) Indicate to all devices that the data transfer can take place.