National Instruments 653X Manuale Utente

© National Instruments Corporation

Figure 3-32.  Long Pulse Input Handshaking Sequence

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.

ACK

REQ

1

3

4

ACK and REQ are shown as active high.
Steps 1-4 are repeated for each transfer.

Initial State

2