National Instruments 653X - Manual (Page 63)

Chapter 3 Timing Diagrams
653X User Manual 3-12 ni.com
Using Asynchronous Protocols
All handshaking protocols except burst are asychronous. The asynchronous
protocols include 8255 emulation, level ACK, leading edge, trailing edge,
and long pulse.
When using these protocols, you have the following options:
• You can change the polarity of the ACK and REQ signals (except for
8255-emulation). The diagrams in this chapter show active-high
signals.
• You can set a programmable delay, from 0 to 700 ns, programmable in
increments of 100 ns. Use the programmable delay to insert wait states
if you have a slow peripheral device. A delay increases the duration of
each transfer. The location of the delay in the handshaking sequence
differs from protocol to protocol. In addition, a delay increases the
minimum spacing between consecutive transfers.
• You can enable request-edge latching, where in input, the 653X device
latches data in from the I/O connector on the active REQ edge before
reading the data. For output, after writing the data, the 653X device
latches data out of the I/O connector on the active REQ edge. The
active edge of the REQ is determined (rising or falling) by the
handshaking protocol and the REQ polarity.
Using the 8255-Emulation Protocol
Your 653X device can perform handshaking I/O with devices that contain
the 8255 chip, including National Instruments PC-DIO-24/PnP,
650X family, and PC-DIO-96/PnP. Performing the 8255-emulation
protocol with your 653X device is similar to 8255 or 82C55 Programmable
Peripheral Interface (PPI).
Note The 653X devices does not emulate the bidirectional protocol of a 8255 device.
The 653X device can perform back-to-back transfers much faster than a
true 8255-based device. If your peripheral device requires more time
between transfers, configure the 653X device to add a data-settling delay
between transfers.
Note In the 8255-emulation protocol, ACK and REQ are active low, reflected in the
following timing diagrams. For all other handshaking I/O protocols, the polarity of the
ACK and REQ are programmable, but are shown as active high signals in the following
diagrams.