Using Protocols Based On Signal Edges; Using The Trailing-Edge Protocol - National Instruments NI 653 Series User Manual

Chapter 3 Timing Diagrams

Using Protocols Based on Signal Edges

Using the Trailing-Edge Protocol

Reference
Point
Initial State ACK is deasserted. The NI 653X waits for the peripheral device to pulse REQ to
indicate it has data.
1 The NI 653X sends an ACK pulse of programmable width when ready to receive
data.
2 After receiving the trailing edge of the ACK pulse, the peripheral device can
strobe data into the NI 653X and pulse the REQ.
3 The NI 653X sends another ACK pulse when ready for another input.
NI 653X User Manual
The NI 653X can communicate using 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 NI 653X or peripheral device is ready for a transfer.
• Leading-edge protocol—The rising edge of the ACK or REQ pulse
indicates that the NI 653X or peripheral device is ready for a transfer.
• Long-pulse protocol—This protocol 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, 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.
ACK
Data Latched
REQ
Initial State
Steps 1–2 are repeated for each transfer.
Figure 3-20. Trailing-Edge Input Handshaking Sequence
Data Valid
1
2
ACK and REQ are shown as active high.
Action Steps
3-22
3
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