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Section 4 IEEE 488 Primer

4.6

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

is 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

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.

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