National Instruments 653 Series User Manual page 87

Chapter 3 Timing Diagrams
Using the Long-Pulse Protocol
ACK
REQ
Reference
Point
Initial State ACK is deasserted. The NI 653X waits for an active REQ to indicate that the
peripheral device is ready. The peripheral device may optionally drive the first
data at this time. The transfer cannot begin until the peripheral asserts REQ: the
peripheral may either pulse REQ or hold REQ high until the first ACK occurs.
If the peripheral pulses REQ, start the transfer on the NI 653X before the pulse
occurs to avoid missing the pulse.
1 The NI 653X asserts an ACK signal when it is ready to receive data, assuming
the peripheral device has deasserted the REQ signal. Otherwise, the ACK signal
remains asserted until the REQ signal deasserts.
2 To slow down the data transfer, you can insert a programmable delay before
deasserting the ACK signal. Unlike in the leading-edge protocol, the pulse width
is programmable.
3 After receiving the leading edge of the ACK pulse, the peripheral device can
strobe data into the NI 653X by asserting REQ.
4 The same programmable delay that controls the minimum ACK pulse width
further slows down the transfer by delaying next occurrence of the next ACK
pulse.
NI 653X User Manual
1
2
3
Initial State
ACK and REQ are shown as active high.
Steps 1-4 are repeated for each transfer.
Figure 3-32. Long-Pulse Input Handshaking Sequence
4
Action Steps
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