Digital I/O Overview
Hardware-Timed Digital I/O
Power-Up States
Hardware-timed digital I/O, also referred to as high-speed digital Some applications require that the digital I/O device power up with
I/O, implies that the writing and reading of digital lines is controlled a determined value on the data lines. This procedure ensures that
by an external or internally generated clock signal. The clock signal equipment connected to the digital I/O device is not damaged during
latches the digital data into or out of a buffer, which can be streamed the time after power is applied and before the software application
at high speed to the computer. With hardware-timed digital I/O, begins running. All NI digital I/O devices power up with a known
software commands are similar to buffered data acquisition state on the digital I/O lines, either logic high, logic low, or high
command and an array of data is passed with each read or write impedance. NI 6527 devices offer software programmable power-up
operation. NI 653x devices can be used in either a hardware-timed or states on each line that can be configured using a software utility
a software-timed manner.
located in the Developer Zone.
For more information on NI 653x devices see page 432.
Handshaking Digital I/O
For more information, visit ni.com/info and enter ex95u3.
Change Notification and Pattern Matching
Handshaking digital I/O refers to transfers of digital data between the To monitor digital lines without continuously polling their state, you
digital I/O device and a peripheral. Both parties in the transfer can use Change Notification on NI 6527 devices to notify your
control a handshaking line and can pause the operation if they are application when one or more digital input lines changes from low to
unable to keep up with the data rate. NI 650x devices offer high, high to low, or both. You can configure NI 650x devices to
8255 Mode 2 bidirectional transfers at typical constant sustainable provide pattern matching when the input lines on a port match a
rates of 1 to 10 kbytes/s using a 233 MHz Pentium computer.
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.
For more information on high-speed handshaking, see the NI 653x
devices on page 432.
NI offers a complete array of digital I/O products for connection to any digital data bus or device.
376 National Instruments • Tel: (800) 433-3488 • Fax: (512) 683-9300 • info@ni.com • ni.com
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