Tektronix 2714, Руководство программиста

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Appendix B: GPIB System Concepts
B–12
2714 & 2715 Programmer Manual
ATN
Controller
DAV
Talker
NRFD
Listener
DCAC
Listener
Data
Bus
Last Interface MSG Byte
From Controller
First Data Byte From Talker
Device-Dependent MSG
Byte Capture Time
Figure B–4: Handshake Timing Sequence, Idealized
DAV (Data Valid).
The DAV signal line is asserted by the talker after the talker
places a data byte on the data bus. When asserted (low), DAV tells each assigned
listener that a new data byte is on the bus. The talker is inhibited from asserting
DAV as long as any listener holds the NRFD signal line asserted.
NRFD (Not Ready For Data).
An asserted NRFD signal line indicates one or more
of the assigned listeners are not ready to receive the next data byte from the
talker. When all of the assigned listeners for a particular data byte transfer have
released NRFD, the NRFD line becomes unasserted (high). When NRFD goes
high, the RFD message (Ready For Data) tells the talker it may place the next
data byte on the data bus.
NDAC (Not Data Accepted).
Each assigned listener holds the NDAC signal line
asserted until the listener accepts the data byte currently on the bus. When all
assigned listeners have accepted the current data byte, the NDAC signal line
becomes unasserted (high) telling the talker to remove the data byte from the
bus. The DAC message (Data Accepted) tells the talker that all assigned listeners
have accepted the current data byte.