National Instruments 653X - Manual (Page 69)

Chapter 3 Timing Diagrams
653X User Manual 3-18 ni.com
Using the Level-ACK Protocol
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
Reference
Point Action Steps
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