National Instruments 653 Series User Manual page 67

Chapter 3 Timing Diagrams
ACK
REQ
Reference
Point
1 The NI 653X asserts the ACK signal when ready to accept data.
2 The peripheral device can then strobe data into the NI 653X by asserting the REQ
line. This assertion can happen before or after ACK is asserted.
3 Asserting the REQ signal causes the ACK signal to deassert.
4 Deasserting the REQ signal causes the NI 653X to latch input data.
5 The NI 653X reasserts the ACK signal when it has space and is ready for another
input. A programmable delay can be inserted here.
NI 653X User Manual
NI 653X terminology differs from 8255 terminology.
• Input—The REQ line carries the 8255 STB (Strobe) input signal, and
the NI 653X ACK line carries the 8255 IBF (Input Buffer Full) output
signal.
• Output—The REQ line carries the 8255 ACK input signal, and the
NI 653X ACK line carries the 8255 OBF (Output Buffer Full) output
signal.
1
2
ACK and REQ are shown as active low.
Steps 1-5 are repeated for each transfer.
Figure 3-9. 8255-Emulation Input Handshaking Sequence
3
5
4
Action Steps
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