National Instruments 653X User Manual page 86

Using the Long-Pulse Protocol
ACK
REQ
Reference
Point
Initial State ACK is deasserted. The 653X device 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, make sure to start the transfer on the 653X device
before the pulse occurs, to avoid missing the pulse.
1 The 653X device 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 653X device 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.
© National Instruments Corporation
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
3-35
Chapter 3 Timing Diagrams
653X User Manual