IOtech Isolator488, Руководство пользователя

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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 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.
3.5.2 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.
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 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.