Measurement-computing Digital HS Manuel d'utilisateur télécharger pdf (Page 45)

4.5 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.

4.5.1 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 waits until

NRFD

unasserted (high) which indicates that all Addressed Listeners are ready to accept the

information. When

NRFD

and

NDAC

are in the proper state, the

Talker

asserts

DAV

( active low) to indicate that the data on the bus is valid.

4.5.2 Not Ready for Data (NRFD)

This line is used by the

Listeners

to inform the

Talker

when they are ready to accept

new data. The

Talker

must wait for each

Listener

to unassert this line (high) which

they will do at their own rate when they are ready for more data. This assures that all

devices that are to accept the information are ready to receive it.

4.5.3 Not Data Accepted (NDAC)

The

NDAC

line is also controlled by the

Listeners

. This line indicates to the

Talker

that each device addressed to listen has accepted the information. Each device

releases

NDAC

(high) at its own rate, but the

NDAC

will not go high until the

slowest Listener has accepted the data byte.

1 2 ... 40 41 42 43 44 45 46 47 48 49 50 ... 55 56

Commentaires sur ces manuels

Pas de commentaire

Measurement-computing Digital HS Manuel d'utilisateur Page 45

Commentaires sur ces manuels

Produits connexes et manuels pour Matériel Measurement-computing Digital HS