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In the previous discussion, the terms

Addressed to Talk

and

Addressed to Listen

were used. These terms require some clarification.

The

IEEE 488.1

standard permits up to 15 devices to be configured within one

system. Each of these devices must have a unique address to avoid confusion. In a

similar fashion, every building in town has a unique address to prevent one home

from receiving another home's mail. Exactly how each device's address is set is

specific to the product's manufacturer. Some are set by DIP switches in hardware,

others by software. Consult the manufacturer's instructions to determine how to set

the address.

Addresses are sent with

universal

(

multiline

) commands from the

Active

Controller

. These commands include

My Listen Address

(MLA),

My Talk

Address

(MTA),

Talk Address Group

(TAG), and

Listen Address Group

(LAG).

4.4 BUS MANAGEMENT LINES

Five hardware lines on the

GPIB

are used for bus management. Signals on these

lines are often referred to as

uniline

(single line) commands. The signals are active

low, i.e. a low voltage represents a logic "1" (asserted), and a high voltage represents

a logic "0" (unasserted).

4.4.1 Attention (ATN)

ATN

is one of the most important lines for bus management. If Attention is asserted,

then the information contained on the data lines is to be interpreted as a multiline

command. If it is not, then that information is to be interpreted as data for the

Active

Listener

s. The

Active Controller

is the only bus device that has control of this line.

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Measurement-computing Digital HS Manuale Utente Pagina 43

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