Measurement Computing Extender488 - Manual (Page 26)

Section 3 IEEE 488 Primer
3-6
3.5 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.
3.5.1 Data Valid (DAV)
The DAV line is controlled by the Talker. The Talker verifies that NDAC is asserted (active
low) which indicates that all Lis teners have accepted the previous data byte tra nsferred. The Talker
then outputs data on the bus and waits until NRFD is unasserted (high) which i ndicates 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 u sed by the Listeners to inform the Talker when they a re ready to acce pt 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 lis ten 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.