Low-Cost Industrial Digital I/O – 150 V
connected to industrial actuators such as pumps, valves, motors, and
relays. NI 6521 devices hold these known safe I/O states after receiving
power, while your computer boots and your software application can
begin running. Programmable power-up states are glitch-free, meaning
the outputs never go through an incorrect state during power up.
You can configure each individual output line as logic high or logic
low. NI 6521 devices store the settings in onboard nonvolatile memory
and implement the power-up states automatically after power is applied
to the device.
Hardware
High-Reliability Industrial Feature Set
NI 6521 devices offer a set of high-reliability features designed for
demanding automation applications:
• Isolation provides an extended voltage range for direct connection
to industrial sensors and actuators
• Programmable power-up states provide safe operation when
connected to pumps/valves/motors/relays
• Watchdog timer detects computer or application crashes and
ensures safe recovery to know digital I/O states
Watchdog Timers for Fault Detection and Recovery
Watchdog timers are an innovative technology that provide protection
against a wide variety of fault conditions:
• Change detection triggers your application and returns I/O data
after a digital event to minimize processor usage
• Computer crash – total OS crash
• Programmable input filters eliminate glitches/spikes and remove noise
• Application crash – software application ceases to respond
• Driver crash – device driver ceases to respond
Direct Sensor Connection with Isolation
Isolation, a form of built-in signal conditioning, provides several
advantages such as an extended voltage range for direct connection to
industrial sensors and actuators. Isolation also improves signal quality
and protects computer circuitry. NI 6521 devices provide channel-to-
channel isolation where every channel is physically and electrically
separated from the others. This isolation breaks ground loops, improves
common-mode voltage and noise rejection, and permits the two parts of
the circuit to be at different voltage levels. Many industrial applications
require isolation to protect the electronics from transient voltage spikes
and provide greater common-mode noise rejection in electrically noisy
environments containing machinery and inductive loads.
• PCI bus failure – communications cease to respond
With watchdogs, the digital outputs go to a safe state when a fault
condition is detected. Watchdogs are important whenever the module
is connected to actuators such as pumps, valves, motors, and relays.
NI 6521 devices monitor the software application; if the application
fails to reset the watchdog timer within the time limit, the device
automatically sets the output lines to a user-defined safe state.
NI 6521 devices remain in the watchdog state until the watchdog timer
is disarmed by the application and new I/O values are written, the
NI 6521 is reset, or the computer is restarted.
Trigger Your Application
with Change Detection
With change detection, you can automatically trigger your software
application to perform a digital read operation upon a digital change of
state. A digital change of state is defined as the rising edge (0 to 1
transition) or falling edge (1 to 0 transition) on one or more digital lines.
To minimize the effects of noisy input lines, use programmable input
filters in combination with change detection to eliminate spurious
change detection events caused by noise or glitches. NI-DAQmx also
includes multithreaded streaming technology so digital change detection
events can occur independent of other data acquisition activities such as
analog input or output events.
Using change detection, you can monitor for digital events with minimal
processor usage. No polling is necessary because the digital I/O module
generates an interrupt to automatically wake up your application. Using
NI-DAQmx software technology, the NI 6521 devices notify the software
application when the event is detected, causing the application to
automatically perform a read operation.
Figure 1. NI 6521 Hardware Block Diagram
Glitch-Free Startup with
Programmable Power-Up States
Using programmable power-up states, you can configure the initial
NI 6521 output states in software to ensure glitch-free operation when
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