National Instruments 653 Series User Manual page 75

Chapter 3 Timing Diagrams
Initial State
ACK
REQ
Reference
Point
Initial State ACK is deasserted.
1 When the NI 653X has data to output, it drives the data onto the data lines, and
then asserts ACK. ACK stays asserted, indicating the NI 653X is ready, until the
active-going REQ edge occurs.
2 The peripheral device responds with an active-going REQ signal edge. ACK
stays asserted, indicating the NI 653X is ready, until the active-going REQ
occurs. Since the REQ is already asserted, the NI 653X waits until REQ deasserts
and reasserts to deassert the ACK signal and request additional data.
3 The asserted REQ signal deasserts the ACK signal.
4 To slow down the data transfer, you can insert a programmable delay before the
ACK signal is asserted.
When REQ
Asserted
Wait
For
REQ
Clear ACK
When REQ
Unasserted
NI 653X User Manual
1 3
2
ACK and REQ are shown as active high.
Steps 1-4 are repeated for each transfer.
Figure 3-17. Level-ACK Output Handshaking Sequence
Programmable
Delay
Send
ACK
Wait
For
* With REQ-edge latching enabled, the data output is
REQ
Figure 3-18. Level-ACK Output State Machine
3-20
4
Action Steps
Initial State: ACK Cleared
Wait
For
Data
When 6533 Device has data
to output, output data.*
Programmable
Delay
delayed until the next inactive-going REQ edge.
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