Measurement Computing Serial488/4 - Manual (Page 77)

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 w aits until NRFD is unasserted (high) w hich 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) w hich they will do at their ow n 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 ow n rate, but the
NDAC will not go high until the slowest Listener has accepted the data byte.