National Instruments DAQCard-6533 User Manual page 70

Chapter 3 Timing Diagrams
ACK
REQ
Reference
Point
1 When the NI 653X has data to output, it asserts the ACK signal, then waits for
the peripheral device to assert REQ to indicate it is ready to accept data.
2
The peripheral device asserts a REQ signal to accept the data.
The peripheral device can receive the data on the falling or rising edge of the ACK
3
signal or any time in between before the next rising edge on REQ.
4
The REQ signal edge in step 2 causes the ACK signal to return to deassert.
The rising REQ signal edge enables a new transfer to occur. The peripheral device
5
should wait until it has received data before deasserting the REQ signal. The
peripheral device can also wait for the ACK signal to deassert before deasserting
the REQ line.
The NI 653X reasserts the ACK signal when it has data and is ready for another
6
output. A programmable delay can be inserted here.
Note: The DIO-32HS drops the ACK line to indicate that the NI 653X is ready to
receive data regardless of whether or not "count" has been reached. The output
device controls the timing of the transfer by dropping the REQ line when it is
ready to transfer data. The timing is not controlled by the software.
NI 653X User Manual
4
1
3
2
ACK and REQ are shown as active low.
Steps 1-6 are repeated for each transfer.
Action Steps
Figure 3-11. 8255-Emulation Output Handshaking Sequence
3-14
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