National Instruments 653X - Manual (Page 67)

Chapter 3 Timing Diagrams
653X User Manual 3-16 ni.com
Figure 3-12. 8255 Emulation Output State Machine
Wait
For
Data
Wait
For
REQ
Programmable
Delay
Wait
For
REQ
When REQ
Unasserted
When 6533 Device has
data to output, output data.
When REQ
Asserted
Initial State: ACK Cleared
Output Data,
Then Send ACK
Clear ACK