National Instruments 653X Manuale Utente

In level-ACK protocol, the 653X device asserts the ACK signal when ready 
for a transfer and holds the ACK signal level until an active-going edge 
occurs on the REQ line. After the REQ edge occurs, the 653X device 
deasserts the ACK signal until the device is ready for another transfer.

Figure 3-14.  Level ACK 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 waits until it has space for data, then it asserts ACK.

2

The peripheral device can then strobe data into the 653X device by first 
deasserting then asserting the REQ signal. The 653X device waits for an 
active-going transition on the REQ line. ACK stays asserted, indicating the 
653X device is ready, until the active-going REQ occurs.

3

The active-going REQ signal edge deasserts the ACK signal and causes the 
653X device to latch input data.

4

To slow down the data transfer, you can insert a programmable delay before the 
ACK signal is asserted.

ACK

REQ

1

2

3

4

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

Initial State