NI NI6521

Low-Cost Industrial Digital I/O – 150 V
NI 6521
• 8, ±30 VDC digital inputs with 60 V
channel-to-channel isolation
• 8 nonlatching mechanical relay outputs
• 150 V AC/DC maximum switching voltage
with maximum switching current of 2 A
and maximum switching power of 60 W
• UL listed: UL 61010-1 (IEC 61010-1)
• High-reliability industrial feature set
includes isolation, programmable
power-up states, digital filtering,
watchdog timers, and change detection
• NI measurement services software for
highest productivity and performance
Recommended Software
• LabWindows/CVI
• Measurement Studio
Other Compatible Software
• C, C++
• Microsoft Visual Studio .NET 2003
Measurement Services
Software (included)
• NI-DAQmx driver, Version 7.5 or later
Operating Systems
• Windows 2000/NT/XP
• LabVIEW Real-Time
NI 6521
60 V channel-to-channel
±4 VDC
±11 VDC
Overview and Applications
The National Instruments 6521 products are industrial digital I/O devices
with eight channel-to-channel isolated digital inputs and eight nonlatching
mechanical relay outputs. You can use the eight differential, isolated,
±30 VDC digital inputs to read the status of sensors, actuators, and logic
devices. You can use the bipolar digital input channels to read from both
sinking and sourcing devices. NI 6521 devices register a logic low if the
voltage is between ±4 V, and register a logic high if the voltage is less
than -11 V or greater than +11 V.
NI 6521 devices offer three single-pole double-throw (SPDT) Form C
relays and five single-pole single-throw (SPST) Form A relays. You can
use NI 6521 relay outputs to switch devices requiring up to 150 V AC/DC.
NI 6521 relay outputs have maximum current rating of 2A and maximum
switching power of 60 W.
Note: The switching current is limited by the maximum switching
power and maximum voltage. (current = power/voltage)
NI 6521 devices offer completely programmable configuration and do not
require setting up jumpers or DIP switches. These devices are ideal for use
with a programmable automation controller (PAC) for automotive test,
industrial monitoring, and control applications. With high-current drive, highvoltage outputs, and isolation, you can connect the digital I/O module directly
to a wide array of industrial electronic devices, sensors, and actuators.
NI 6521 devices take advantage of NI-DAQmx measurement services
software (Version 7.5 or later) to speed up application development with
Relay Outputs
Max Voltage
150 V AC/DC
Max Current
Max Power
60 W
Feature Set
many helpful features such as DAQ Assistant, automatic code
generation, and high-performance multithreaded streaming technology.
UL listed: UL 61010-1 (IEC 61010-1)
Keyed connector
Insulation covers
Complete software configuration
High-voltage accessories
NI 6521 devices are UL listed and require the use of a keyed connector
to prevent high-voltage signals from accidentally being connected to a
low-voltage device. For added safety, PCI-6521 includes insulation covers
on both sides of the board to prevent the user from coming in contact
with high-voltage traces. Complete software configuration means that
the user at no point is required to come in the vicinity of high-voltage
traces. Several low-cost, high-voltage accessory options are available
to ensure safe operation.
Channel-to-channel . . . . . . . . . . . . . . . . . . . . . . .60 VDC continuous1
Channel-to-bus . . . . . . . . . . . . . . . . . . . . . . . . . .150 VDC continuous2
Channel-to-earth . . . . . . . . . . . . . . . . . . . . . . . . .150 VDC continuous3
Verified by 620 Vrms dielectric withstand test, 5 s
Verified by 1400 Vrms dielectric withstand test, 5 s
Verified by 850 Vrms dielectric withstand test, 5 s
Low-Cost Industrial Digital I/O – 150 V
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
• Change detection triggers your application and returns I/O data
after a digital event to minimize processor usage
• Programmable input filters eliminate glitches/spikes and remove noise
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-tochannel 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.
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
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.
Watchdog Timers for Fault Detection and Recovery
Watchdog timers are an innovative technology that provide protection
against a wide variety of fault conditions:
• Computer crash – total OS crash
• Application crash – software application ceases to respond
• Driver crash – device driver ceases to respond
• 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.
BUY ONLINE at or CALL (800) 813 3693 (U.S.)
Low-Cost Industrial Digital I/O – 150 V
Eliminate Noise with
Programmable Input Filters
Programmable input filters remove noise, glitches, and spikes on input,
and provide debouncing for digital switches and relays. This feature is
important for applications in noisy industrial environments to prevent
false readings caused by noise. You can configure the programmable
input filter for each digital line by setting the filter time in seconds.
Any digital noise, glitch, or spike that is shorter than half of the specified
filter time is blocked by the digital I/O device, preventing invalid readings
and false triggers for change detection events.
NI 6521 devices require and are shipped with NI-DAQmx driver software,
Version 7.5 or later. NI-DAQmx software is also available for download
from With NI-DAQmx, you can use your NI digital
I/O device in NI LabVIEW, ANSI C, Microsoft Visual C++, and the
Microsoft .NET languages C# and Visual Basic .NET.
Using NI-DAQmx technology, you can access the full functionality
and state-of-the-art hardware technology of your NI 6521 digital I/O
devices. NI-DAQmx technology speeds up your development with
many features such as automatic code generation to make configuration
and programming easy. The NI 6521 devices take full advantage of key
NI-DAQmx software technologies such as multithreaded streaming
technology for dramatic improvements in I/O performance and ease of use.
• Use DAQ Assistant to guide you to fast, accurate measurements
with no programming.
• Use automatic code generation to create your application in
LabVIEW, C, Visual Basic .NET, or C#.
• Take advantage of multithreaded streaming technology for
1,000X performance improvements.
• Use automatic timing, triggering, and synchronization technology
to make advanced applications easy.
• Visit for more than 3,000 FREE software downloads to
jump-start your project.
• Use the NI-DAQmx functions for software configuration of all
digital I/O features without hardware switches/jumpers.
• Develop your application with easy and open programming in
LabVIEW, ANSI C, Microsoft Visual C++, C#, and Visual Basic .NET.
NI-DAQmx Software Technology
Relay Forms
Relays are classified by number of poles and number of throws. The
pole of a relay is the terminal common to every path. Each position
where the pole can connect is called a throw. A relay can be made of
n poles and m throws. A single-pole single-throw (SPST) relay has one
pole and one throw. A single-pole double-throw (SPDT) relay has one
pole and two throws.
Single-Pole Single-Throw (SPST) Relay
Single-Pole Double-Throw (SPDT) Relay
Figure 2. Relay Diagrams
Form A
SPST relay with a normally open default state
Form C
SPDT relay that breaks the connection
with one throw before making contact with
the other (break-before-make)
Table 1. Relay Descriptions
Nonlatching Relays
A nonlatching relay has a power-off initial position of normally closed
(NC). This position is maintained by the force of a spring or permanent
magnet while no current flows. The normally open (NO) contact is
maintained by the force of a magnetic field while current flows through
the coil. When the current stops, the relay reverts back to its initial NC
position. Nonlatching relays return to the default NC position in case of a
fault or power failure. This feature is particularly useful when controlling
a motor or valve where the motor or valve needs to be turned off in case
of power failure or fault.
Low-Cost 37-Pin Accessories
Several low-cost, high-voltage accessory options are available for
connecting to the 37-pin D-Sub connector of NI 6521 devices. NI 6521
devices ship with an accessory safety kit for keying connectors.
BUY ONLINE at or CALL (800) 813 3693 (U.S.)
Low-Cost Industrial Digital I/O – 150 V
Ordering Information
NI PCI-6521............................................................................779312-01
NI PXI-6521............................................................................779313-01
Additional Accessory Safety Kit............................................779445-01
NI 6521
CB-37F-HVD (779491-01)
(PXI Only) TB-2621 (779444-01)
SH37F-37M (778621-01, 778621-02)
TB-37F-37CP (779185-01)
Includes NI-DAQmx 7.5 or later
For information on extended warranty and value added services, visit
For complete product specifications, pricing, and accessory
information, call (800) 813 3693 (U.S.) or go to
Figure 3. NI 6521, 37-Pin D-Sub I/O Connector
BUY ONLINE at or CALL (800) 813 3693 (U.S.)
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