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Chapter 3 Timing Diagrams
© National Instruments Corporation 3-15 653X User Manual
Figure 3-11.
8255 Emulation Output Handshaking Sequence
Reference
Point Action Steps
1 When the 653X device has data to output, it asserts the ACK signal, then waits for
the peripheral device to assert REQ to indicate it is ready to accept data
2
The peripheral device asserts a REQ signal to accept the data.
3
The peripheral device can receive the data on the falling or rising edge of the ACK
signal or any time in between before the next rising edge on REQ.
4
The REQ signal edge in step 2 causes the ACK signal to return to deassert.
5
The rising REQ signal edge enables a new transfer to occur. The peripheral device
should wait until it has received data before deasserting the REQ signal. The
peripheral device can also wait for the ACK signal to deassert before deasserting
the REQ line.
6
The 653X device reasserts the ACK signal when it has data and is ready for
another output. A programmable delay can be inserted here.
ACK
REQ
1
2
5
4
6
3
ACK and REQ are shown as active low.
Steps 1-6 are repeated for each transfer.