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RS-232↔IEEE 488 INTERFACE CONVERTER
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 that 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. The students are told to raise
their hands 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.