Measurement Computing Digital488 - Manual (Page 66)

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 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.