Measurement Computing Micro 488/EX rev.2.1 - Manual (Page 184)

Section 8 IEEE 488 Primer
8.2
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.