Measurement Computing Parallel488 - Manual (Page 34)

Section 5 IEEE 488 Primer
5.2
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 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.