National Instruments 653 Series User Manual

Also see for 653 Series: User guide

Contents

National Instruments 653 Series User Manual Manual pdf 72 page image

Chapter 3 Timing Diagrams© National Instruments Corporation 3-17 NI 653X User ManualUsing the Level-ACK ProtocolIn level-ACK protocol, the NI 653X asserts the ACK signal when ready fora transfer and holds the ACK signal level until an active-going edge occurson the REQ line. After the REQ edge occurs, the NI 653X deasserts theACK signal until the device is ready for another transfer.Figure 3-14. Level-ACK Input Handshaking SequenceReferencePoint Action StepsInitial State ACK is deasserted. The NI 653X waits for an active REQ to indicate that theperipheral device is ready. The peripheral device may optionally drive the firstdata at this time. The transfer cannot begin until the peripheral asserts REQ; theperipheral 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 NI 653Xbefore the pulse occurs to avoid missing the pulse.1 The NI 653X waits until it has space for data, then it asserts ACK.2 The peripheral device can then strobe data into the NI 653X by first deassertingthen asserting the REQ signal. The NI 653X waits for an active-going transitionon the REQ line. ACK stays asserted, indicating the NI 653X is ready, until theactive-going REQ occurs.3 The active-going REQ signal edge deasserts the ACK signal and causes theNI 653X to latch input data.4 To slow down the data transfer, you can insert a programmable delay before theACK signal is asserted.ACKREQ1234ACK and REQ are shown as active high.Steps 1-4 are repeated for each transfer.Initial State