Chapter 3
Timing Diagrams
©
National Instruments Corporation
3-25
Figure 3-23.
Trailing-Edge Output Handshaking Sequence
Figure 3-24.
Trailing-Edge Output State Machine
Reference
Point
Action Steps
Initial State
ACK is deasserted.
1
The NI 653
X
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.
ACK
REQ
2
1
ACK and REQ are shown as active high.
Steps 1-2 are repeated for each transfer.
Initial State
Wait
For
Data
Wait
For
REQ
Programmable
Delay
Programmable
Delay
Wait
For
REQ
When REQ
Unasserted
When NI 6533 has data
to output, generate data.*
Clear
ACK
§
When REQ
Asserted
Initial State: ACK Cleared
Send
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.