Chapter 3
Timing Diagrams
©
National Instruments Corporation
3-29
Using the Leading-Edge Protocol
Figure 3-26.
Leading Edge Input Handshaking Sequence
Reference
Point
Action Steps
Initial State
ACK is deasserted. The 653
X
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 653
X
device
before the pulse occurs, to avoid missing the pulse.
1
The 653
X
device 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 653
X
device by asserting REQ.
3
To slow down the data transfer, you can insert a programmable delay before the
ACK signal is asserted.
4
The 653
X
device sends another ACK when it is ready for another input.
ACK
REQ
1
2
4
3
ACK and REQ are shown as active high.
Steps 1-3 are repeated for each transfer.
Initial State