National Instruments DIO 6533 - Manual (Page 65)

Chapter 5 Signal Timing
DIO 6533 User Manual 5-6 © National Instruments Corporation
In input mode, the 6533 device asserts the ACK signal low when ready
to accept data. The peripheral device can then strobe data into the 6533
device by pulsing the REQ line low. The falling REQ signal edge causes
the ACK signal to deassert, and the rising REQ signal edge causes the
6533 device to latch input data. Afterward, the 6533 device reasserts the
ACK signal low when ready for another input. Figure 5-5 shows an
input transfer in 8255 emulation mode.
Figure 5-5. 8255 Emulation Mode Input
Output
Note: 6533 device terminology differs from 8255 terminology. In output mode,
the 6533 device REQ line carries the 8255 ACK input signal, and the
6533 device ACK line carries the 8255 OBF output signal. Both lines are
active low.
Wait
For
Space
Wait
For
REQ
Programmable
Delay
Wait
For
REQ
When REQ Unasserted,
Latch Input Data
When 6533 Device
Has Space For Data
Clear
ACK
When REQ
Asserted
Initial State
ACK Set
Send
ACK