Chapter 3
Timing Diagrams
©
National Instruments Corporation
3-13
653
X
device terminology differs from 8255 terminology.
•
Input—The REQ line carries the 8255 STB (Strobe) input signal, and
the 653
X
device ACK line carries the 8255 IBF (Input Buffer Full)
output signal.
•
Output—The REQ line carries the 8255 ACK input signal, and the
653
X
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 653
X
device asserts the ACK signal when ready to accept data.
2
The peripheral device can then strobe data into the 653
X
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 653
X
device to latch input data.
5
The 653
X
device 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.
