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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.

• 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.

• 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.

• 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.

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Measurement-computing DAC488 v.1 Manuale Utente Pagina 43