National Instruments 653X User Manual page 64

ACK
REQ
Reference
Point
1 The 653X device asserts the ACK signal when ready to accept data.
2 The peripheral device can then strobe data into the 653X device by asserting the
REQ line. This 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 653X device to latch input data.
5 The 653X device reasserts the ACK signal when it has space and is ready for
another input. A programmable delay can be inserted here.
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653X device terminology differs from 8255 terminology.
• Input—The REQ line carries the 8255 STB (Strobe) input signal, and
the 653X device ACK line carries the 8255 IBF (Input Buffer Full)
output signal.
• Output—The REQ line carries the 8255 ACK input signal, and the
653X device 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
3-13
Chapter 3 Timing Diagrams
653X User Manual