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