Measurement Computing Analyzer488 - Manual (Page 270)

Appendix G IEEE 488 Primer
G.2
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 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.
The GPIB transfers information in a similar way. This method of data transfer is
called handshaking. More on this later.
For data transfer on the IEEE 488, the Active Controller must…
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.