National Instruments 653X Manuale Utente

With REQ edge latching enabled (default) REQ edge determines when data will be 

latched. Input data valid has to be held before active going REQ edge a minimum of t

 ns. 

With REQ edge disabled, it has to be held t

 after the next active going ACK signal edge 

occurs.

Figure 3-35.  Long Pulse Output Handshaking Sequence

Initial State

ACK is deasserted. 

1

The 653X device sends an ACK pulse with programmable width to indicate that 
it has data to output. Assuming the peripheral device has deasserted the REQ 
signal. Otherwise, the ACK signal remains asserted until the peripheral device 
deasserts the REQ signal.

2

The peripheral device can latch the data on the rising or falling edge of the ACK 
pulse, or anytime before asserting the REQ signal.

3

Once the data is latched, the peripheral device must respond with an active-going 
REQ signal edge. 

ACK

REQ

1

3

2

ACK and REQ are shown as active high.
Steps 1-3 are repeated for each transfer.

*

= programmable pulse width

*

Initial State