Measurement Computing DAC488 v.1 - Manual (Page 43)

DAC488 User’s Manual IEEE 488 Background 37
Handshake Lines
The IEEE 488 interface 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.
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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.
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Not Ready for Data ( NRFD ): The NRFD 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.
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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.
Data Transfer Lines
To transfer bit-parallel/byte-serial information between devices on the bus, the IEEE 488 interface provides
eight data lines (
DIO1 through DIO8 ). These data lines are bidirectional and are active low.