Chapter 3
Timing Diagrams
3-12
ni.com
NI 653
X
terminology differs from 8255 terminology.
•
Input—The REQ line carries the 8255 STB (Strobe) input signal, and
the NI 653
X
ACK line carries the 8255 IBF (Input Buffer Full) output
signal.
•
Output—The REQ line carries the 8255 ACK input signal, and the
NI 653
X
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 NI 653
X
asserts the ACK signal when ready to accept data.
2
The peripheral device can then strobe data into the NI 653
X
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 653
X
to latch input data.
5
The NI 653
X
reasserts the ACK signal when it has space and is ready for 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.