Measurement Computing Isolator488 - Manual (Page 23)

Section 3 IEEE 488 Primer
3-2
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.