Measurement-computing Extender 488HS Rev.2.0 Instrukcja Użytkownika Pobierz pdf (Strona 23)

Section 3 IEEE 488 Primer

3.5

3.4.3 Remote Enable (REN)

When the

System

Controller

sends the

REN

command, bus

devices will respond to remote operation. Generally, the

REN

command

should be issued before any bus programming is attempted. Only the

System

Controller

has control of the

Remote

Enable

line.

3.4.4 End or Identify (EOI)

The

EOI

line is used to signal the last byte of a multibyte data

transfer. The device that is sending the data asserts

EOI

during the transfer

of the last data byte. The

EOI

signal is not always necessary as the end of

the data may be indicated by some special character such as carriage return.

The

Active

Controller

also uses

EOI

to perform a

Parallel

Poll

by simultaneously asserting

EOI

and

ATN

3.4.5 Service Request (SRQ)

When a device desires the immediate attention of the

Active

Controller

it asserts

SRQ

. It is then the Controller's responsibility to

determine which device requested service. This is accomplished with a

Serial Poll

or a

Parallel Poll

3.5 HANDSHAKE LINES

The

GPIB

uses three handshake lines in an "I'm ready - Here's the data - I've

got it" sequence. This handshake protocol assures reliable data transfer, at the rate

determined by the slowest Listener. One line is controlled by the Talker, while the

other two are shared by all Active Listeners. The handshake lines, like the other

IEEE 488

lines, are active low.

3.5.1 Data Valid (DAV)

The

DAV

line is controlled by the

Talker

. The

Talker

verifies that

NDAC

is asserted (active low) which indicates that all Listeners have accepted

the previous data byte transferred. The

Talker

then outputs data on the bus and

1 2 ... 18 19 20 21 22 23 24 25 26 27 28 ... 32 33

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