Measurement Computing Parallel488 - Manual (Page 38)

Section 5 IEEE 488 Primer
5.6
5.5 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.
5.5.1 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.
5.5.2 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.
5.5.3 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.