
Section 3 IEEE 488 Primer
3-6
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 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.
3.5.2 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.
3.5.3 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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