Digital I/O Overview
Digital I/O Overview
Hardware-Timed Digital I/O
Hardware-timed digital I/O, also referred to as high-speed digital
I/O, implies that the writing and reading of digital lines is controlled
by an external or internally generated clock signal. The clock signal
latches the digital data into or out of a buffer, which can be streamed
at high speed to the computer. With hardware-timed digital I/O,
software commands are similar to buffered data acquisition
command and an array of data is passed with each read or write
operation. NI 653x devices can be used in either a hardware-timed or
a software-timed manner.
For more information on NI 653x devices see page 432.
Power-Up States
Some applications require that the digital I/O device power up with
a determined value on the data lines. This procedure ensures that
equipment connected to the digital I/O device is not damaged during
the time after power is applied and before the software application
begins running. All NI digital I/O devices power up with a known
state on the digital I/O lines, either logic high, logic low, or high
impedance. NI 6527 devices offer software programmable power-up
states on each line that can be configured using a software utility
located in the Developer Zone.
For more information, visit
ni.com/info
and enter ex95u3.
Handshaking Digital I/O
Handshaking digital I/O refers to transfers of digital data between the
digital I/O device and a peripheral. Both parties in the transfer
control a handshaking line and can pause the operation if they are
unable to keep up with the data rate. NI 650x devices offer
8255 Mode 2 bidirectional transfers at typical constant sustainable
rates of 1 to 10 kbytes/s using a 233 MHz Pentium computer.
For more information on high-speed handshaking, see the NI 653x
Change Notification and Pattern Matching
To monitor digital lines without continuously polling their state, you
can use Change Notification on NI 6527 devices to notify your
application when one or more digital input lines changes from low to
high, high to low, or both. You can configure NI 650x devices to
provide pattern matching when the input lines on a port match a
specific pattern. In a PXI LabVIEW Real-Time application, change
notification and pattern matching can be used to deterministically
trigger execution for high-reliability real-time event response.
Data Acquisition and
Signal Conditioning
devices on page 432.
NI offers a complete array of digital I/O products for connection to any digital data bus or device.
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