National Instruments 653 Series User Manual page 85

Chapter 3 Timing Diagrams
Reference
Point
Initial State ACK is deasserted.
1 The NI 653X sends the ACK pulse after driving output data to indicate that it has
new, valid output data. The ACK pulse width is fixed, assuming the peripheral
device has deasserted the REQ signal. Otherwise, the ACK signal remains
asserted until the peripheral device deasserts the REQ signal.
Once the data is latched, the peripheral device must respond with an active-going
2
REQ signal edge to request additional data.
To slow down the data transfer, you can insert a programmable delay before the
3
ACK signal is asserted.
When REQ
Asserted
Wait
For
REQ
Clear
ACK
Pulse
When REQ
Unasserted
NI 653X User Manual
Initial State
1
ACK
REQ
ACK and REQ are shown as active high.
Steps 1-3 are repeated for each transfer.
Figure 3-29. Leading-Edge Output Handshaking Sequence
Programmable
Delay
Wait
For
REQ
Figure 3-30. Leading-Edge Output State Machine
3
2
Action Steps
Initial State: ACK Cleared
Wait
For
Data
Programmable
Delay
Send
ACK
Pulse
* With REQ-edge latching enabled, the data output is
delayed until the next inactive-going REQ edge.
3-30
When 653 X Device has
data to output, output data.*
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