National Instruments 653 Series User Manual page 82

Using the Leading-Edge 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 can either pulse REQ or hold REQ high until the first ACK occurs. If
the peripheral pulses REQ, make sure to start the transfer on the NI 653X before
the pulse occurs, to avoid missing the pulse.
1 The NI 653X sends an ACK pulse when it is ready to receive data. The ACK
pulse width is fixed, assuming the peripheral device has deasserted the REQ
signal. Otherwise the ACK signal remains asserted until the REQ signal
deasserts.
2 After receiving at least the leading edge of the ACK pulse, the peripheral device
can strobe data into the NI 653X by asserting REQ.
3 To slow down the data transfer, you can insert a programmable delay before the
ACK signal is asserted.
4 The NI 653X sends another ACK when it is ready for another input.
© National Instruments Corporation
1
2
Initial State
ACK and REQ are shown as active high.
Steps 1-3 are repeated for each transfer.
Action Steps
Figure 3-26. Leading-Edge Input Handshaking Sequence
3-27
Chapter 3
4
3
Timing Diagrams
NI 653X User Manual