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6.5 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.
6.5.1 D
ATA
V
ALID
(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.
6.5.2 N
OT
R
EADY
F
OR
D
ATA
(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.
6.5.3 N
OT
D
ATA
A
CCEPTED
(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.
CHAPTER 6: IEEE 488 Primer