Digital I/O Overview
Digital I/O Overview
Voltage Levels
For laboratory and test and measurement applications, the most
commonly used logic levels are 5 V TTL and TTL-compatible
CMOS. In industrial control applications, 24 V levels with isolation
are common and digital I/O devices may include electromagnetic
relays, motor controls, switches, valves, or pumps.
NI offers digital logic levels from nonisolated 5 V TTL/CMOS on
NI 650x devices, up to 28 V input and 60 V output on NI 6527
devices. NI 6527 devices also offer channel-to-channel optical
isolation and higher current switching capability on the outputs
(120 mA). For even higher voltage and isolation levels, the NI SCXI-1162,
SCXI-1162HV, and SCXI-1163 modules provide signal conditioning
from 5 V TTL/CMOS up to 240 VAC/VDC in optically isolated banks
of four lines each.
Figure 1. Test DIO device hardware and signal connections using
a Measurement & Automation Explorer test panel.
Connectivity
For shielded screw-terminal connections to your NI 6527 or NI 6508
device, use the SCB-100 connector block and shielded SH100-100-F
cable. Unshielded connector blocks and cables are also available, such
as the CB-100 kit which includes an unshielded ribbon cable. For
custom connectivity, the CA-1000 accessory enclosure can be used
for convenient connection to pushbutton/rocker switches, banana
jack cables/probes, and LED indicators. Refer to Digital I/O
Accessories and Cables on page 382 for more details and options.
Software-Timed Digital I/O
Software-timed digital I/O, also referred to as unstrobed or static
digital I/O, implies that the writing to digital output lines and
reading from digital input lines is performed by software
command. A single write or read operation is performed with each
software command. Depending on the voltage and current
requirements, you can use either NI 650x or NI 6527 software-
timed devices to monitor and control switches, relays, actuators,
annunciators, fans, lights, and motors. The maximum speed for
software-timed digital I/O operations depends on the computer
processor speed, communication bus, and operating system. For
highest performance and greatest determinism in the timing of
your software-timed digital I/O, use a PXI system running
LabVIEW Real-Time.
Data Acquisition and
Signal Conditioning
Tech Tip
Q: I am controlling digital lines and relays, and I need to set the
power-up states of each line in software. Can I do this?
A: You can do this with the NI 6527 isolated digital I/O devices. You
can configure the power-up state of each output line independently
with a utility in the Developer Zone. For more information, see
page 377 or visit
ni.com/info
and enter ex95u3.
Figure 2. You can use LabVIEW to create powerful DIO applications. This figure
represents the graphical user interface of a software timed DIO application on an
NI 650x device. See page 43 for more information about LabVIEW.
National Instruments • Tel: (800) 433-3488 • Fax: (512) 683-9300 • info@ni.com • ni.com
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