National Instruments 653 Series User Manual page 80

Initial State
REQ
Reference
Point
Initial State ACK is deasserted.
1 The NI 653X sends an ACK pulse of programmable width. This pulse indicates
new, valid output data.
2 The peripheral device responds with a REQ pulse. The trailing edge of the REQ
pulse deasserts the ACK signal if it has not previously been deasserted and
requests additional data.
When REQ
Unasserted
Wait
For
REQ
When REQ
Asserted
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ACK
Figure 3-23. Trailing-Edge Output Handshaking Sequence
Programmable
Delay
Wait
For
REQ
Figure 3-24. Trailing-Edge Output State Machine
1
2
ACK and REQ are shown as active high.
Steps 1-2 are repeated for each transfer.
Action Steps
Initial State: ACK Cleared
Wait
For
Data
Send
ACK
Programmable
Delay
Clear
§
ACK
* With REQ-edge latching enabled, the data output is
delayed until the next inactive-going REQ edge.
§
The programmable delay determines which ACK is
cleared, ACK does not need to be cleared before
REQ can be acknowledged.
3-25
Chapter 3 Timing Diagrams
When NI 6533 has data
to output, generate data.*
NI 653X User Manual