Measurement Computing Extender 488HS Rev.2.0 - Manual (Page 24)

Section 3 IEEE 488 Primer
3.6
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.