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