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.
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