Chapter 3
Timing Diagrams
©
National Instruments Corporation
3-11
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 NI 653
X
latches data in from the I/O connector on the active REQ edge before
reading the data. For output, after writing the data, the NI 653
X
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 NI 653
X
can perform handshaking I/O with devices that contain the
8255 chip, including the National Instruments NI PC-DIO-24/PnP,
NI 650
X
family, and NI PC-DIO-96/PnP. Performing the 8255-emulation
protocol with your NI 653
X
is similar to 8255 or 82C55 Programmable
Peripheral Interface (PPI).
Note
The NI 653
X
does not emulate the bidirectional protocol of an 8255 device.
The NI 653
X
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 NI 653
X
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 polarities of ACK
and REQ are programmable, but are shown as active high signals in the following
diagrams.
