National Instruments 653X - Manual (Page 75)

Chapter 3 Timing Diagrams
653X User Manual 3-24 ni.com
Using Protocols Based on Signal Edges
The 653X device can communicate via pulses on the ACK and REQ lines.
The three edge protocols are:
• Trailing-edge protocol—The trailing edge of the ACK or REQ pulse
indicates that the 653X device or peripheral device is ready for a
transfer.
• Leading-edge protocol—The rising edge of the ACK or REQ pulse
indicates that the 653X device or peripheral device is ready for a
transfer
• Long-pulse protocol—This is a variant of the leading-edge protocol,
with the additional option of using a data-settling delay. If your
application requires a large minimum pulse width, you would want to
use this protocol. In this case, the programmable delay is used to
increase the ACK pulse width instead of delaying the ACK pulse.
You can also use long-pulse protocol to handshake with an actual 8255 or
82C55 PPI. You must set the ACK and REQ signals to active low and select
a minimum pulse width of 500 ns for your 8255 or 82C55.