National Instruments DIO 6533 - Manual (Page 66)

Chapter 5 Signal Timing
© National Instruments Corporation 5-7 DIO 6533 User Manual
In output mode, the 6533 device asserts the ACK signal low when
output data is available. The peripheral device can receive the data on
the falling or rising edge of the ACK signal, or any time in between. The
peripheral device must respond with an active-low REQ pulse to
request additional data. The falling REQ signal edge causes the ACK
signal to return to the inactive state, and the rising REQ signal edge
enables a new transfer to occur. Therefore, the peripheral device should
wait until it has received data before raising the REQ signal. The
peripheral device can also wait for the ACK signal to deassert before
raising the REQ signal. Figure 5-6 shows an output transfer in 8255
emulation mode.
Figure 5-6. 8255 Emulation Mode Output
Wait
For
Data
Wait
For
REQ
Programmable
Delay
Wait
For
REQ
When REQ
Unasserted
When 6533 Device
Has Data to Output
When REQ
Asserted
Initial State
ACK Cleared
Output Data,
Then Send ACK