National Instruments 653X Manuale Utente
Chapter 3
Timing Diagrams
© National Instruments Corporation
3-13
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.
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.
653X device ACK line carries the 8255 OBF (Output Buffer Full)
output signal.
Figure 3-9. 8255 Emulation Input Handshaking Sequence
Reference
Point
Action Steps
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.
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.
another input. A programmable delay can be inserted here.
ACK
REQ
1
2
4
3
5
ACK and REQ are shown as active low.
Steps 1-5 are repeated for each transfer.