Measurement-computing Analyzer488 Bedienungsanleitung PDF herunterladen (Seite 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…

Unlisten

all devices to protect against eavesdroppers.

b) Designate who will

talk

addressing

a device to

talk

c) Designate all the devices who are to

listen

addressing

those

devices to

listen

d) Indicate to all devices that the data transfer can take place.

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