National Instruments NI 6520 User manual

USER GUIDE
NI 6520/6521
This document describes how to use the National Instruments 6520 and
6521 data acquisition (DAQ) devices with NI-DAQ 7.5 or later. Device
specifications can be found on ni.com/manuals. If you have not already
installed the DAQ device, refer to the DAQ Getting Started Guide for
installation instructions.
The NI 6520/6521 contains five Form A single-pole single-throw (SPST)
non-latching relay outputs, three Form C single-pole double-throw (SPDT)
non-latching relay outputs, and eight channel-to-channel isolated industrial
inputs. The relay’s maximum switching power is 60 W/60 VA. The
maximum switching voltage for the NI 6520 is 30 Vrms/42.4 Vpk/
60 VDC. The maximum switching voltage for the NI 6521 is
150 VDC/VAC.
The NI 6520/6521 uses positive logic. A digital high energizes the relay,
and a digital low deenergizes it. In the energized state, the common (COM)
contact is connected to the normally open (NO) contact. For Form C relays
in the deenergized state, the common contact is connected to the normally
closed (NC) contact. For Form A relays in the deenergized state, the
common contact is not connected to the normally open (NO) contact. When
the board (PC) is powered off, the relays are deenergized.
Caution (NI 6521 Only) This 150 V Measurement Category II module features 1,400 Vrms
isolation between I/O channels and the backplane, as verified by a dielectric withstand test,
5 s maximum. For more information on hazardous voltage requirements and safety
information for the NI 6521, refer to the NI 6521 Isolation Voltages section.
Contents
Configuration .......................................................................................... 2
Functional Overview............................................................................... 3
Safety Information .................................................................................. 4
Related Documentation........................................................................... 6
Features ................................................................................................... 6
Digital Filtering................................................................................ 6

NI 6520/6521 User Guide 2 ni.com
Digital Filtering Example..........................................................7
Programmable Power-Up States.......................................................8
Change Detection .............................................................................9
Change Detection Example.......................................................9
Watchdog Timer...............................................................................10
Digital I/O................................................................................................10
I/O Connector ...................................................................................10
Pin Assignments ...............................................................................11
Signal Descriptions...........................................................................12
Optically Isolated Differential Inputs...............................................13
Sensing DC Voltages ................................................................13
Input Signal Connection Example.............................................14
Mechanical Relays............................................................................15
Relay Forms ..............................................................................15
Non-Latching Relays.................................................................15
Relay Power Ratings .................................................................15
Power-On and Power-Off Conditions .......................................16
Relay Signal Connection Example............................................16
Power Connections ..................................................................................17
Isolation Circuitry.............................................................................17
Input Isolation Circuitry ............................................................17
Output Isolation Circuitry .........................................................17
Isolation Voltages.............................................................................17
NI 6520 Isolation Voltages .......................................................17
NI 6521 Isolation Voltages .......................................................18
Protecting Inductive Loads...............................................................18
Cables and Accessories ...........................................................................20
NI 6520 Accessory Options..............................................................20
PCI-6521 Accessory Options ...........................................................21
PXI-6521 Accessory Options ...........................................................21
Specifications...........................................................................................22
Where to Go for Support .........................................................................22
Configuration
The NI 6520/6521 device is completely software configurable, so it is not
necessary to set jumpers for I/O configuration.
The PCI-6520/6521 is fully compliant with the PCI Local Bus
Specification, Revision 2.1, and the PXI-6521 devices are fully compliant
with the PXI Hardware Specification, Revision 2.1. The PCI/PXI system
automatically allocates all device resources, including the base address and
interrupt level. The NI 6520/6521 base address is mapped into PCI memory
space. It is not necessary to perform configuration steps after the system
powers up.

©National Instruments Corporation 3 NI 6520/6521 User Guide
The NI 6521 is designed to meet the requirements of UL/IEC 61010-1 and
is UL listed.
Refer to the driver software documentation for configuration instructions.
After the NI 6520/6521 device and the driver software are installed, the
DAQ device appears under the Devices and Interfaces branch of the MAX
configuration tree.
If the DAQ device does not appear in MAX, use the following
troubleshooting guidelines.
• Verify that you are using the correct version of NI-DAQ
(NI-DAQmx 7.5 or later). To download the most recent National
Instruments drivers, go to ni.com/drivers.
• Press <F5> to refresh the MAX window, or close and re-open MAX.
• Reboot the computer.
• Power off and unplug the computer or chassis, and install the device in
a different slot. Refer to the DAQ Getting Started Guide for installation
instructions and safety guidelines.
Functional Overview
Figure 1 illustrates the key functional components of the NI 6520/6521
device.
Figure 1. NI 6520/6521 Block Diagram
I/O Connector
PCI/PXI/CompactPCI Bus
Flash
Memory
PCI Bus
Interface
Photocouplers
Data/Control Data/Control
Configuration
Control
Relays
10 MHz
Clock
8 Digital
Inputs
8 Digital
Outputs
DIO Lines
Programmable
Power-Up States
Watchdog Timer
Change
Detection
Digital
Filtering
Industrial Digital
I/O Control FPGA

NI 6520/6521 User Guide 4 ni.com
Safety Information
The following section contains important safety information that you must
follow when installing and using National Instruments DIO devices.
Do not operate the device in a manner not specified in this document.
Misuse of the DIO device can result in a hazard. You can compromise the
safety protection built into the DIO device if it is damaged in any way. If
the DIO device is damaged, return it to National Instruments for repair.
Do not substitute parts or modify the DIO device except as described in this
document. Use the DIO device only with the chassis, modules, accessories,
and cables specified in the installation instructions. You must have all
covers and filler panels installed during operation of the DIO device.
Do not operate the DIO device in an explosive atmosphere or where there
may be flammable gases or fumes. If you must operate the DIO device in
such an environment, it must be in a suitably rated enclosure.
If you need to clean the DIO device, use a soft, nonmetallic brush. Make
sure that the DIO device is completely dry and free from contaminants
before returning it to service.
Operate the DIO device only at or below Pollution Degree 2. Pollution is
foreign matter in a solid, liquid, or gaseous state that can reduce dielectric
strength or surface resistivity. The following is a description of pollution
degrees:
• Pollution Degree 1 means no pollution or only dry, nonconductive
pollution occurs. The pollution has no influence.
• Pollution Degree 2 means that only nonconductive pollution occurs in
most cases. Occasionally, however, a temporary conductivity caused
by condensation must be expected.
• Pollution Degree 3 means that conductive pollution occurs, or dry,
nonconductive pollution occurs that becomes conductive due to
condensation.
You must insulate signal connections for the maximum voltage for which
the DIO device is rated. Do not exceed the maximum ratings for the DIO
device. Do not install wiring while the DIO device is live with electrical
signals. Do not remove or add connector blocks when power is connected
to the system. Avoid contact between your body and the connector block
signal when hot swapping modules. Remove power from signal lines
before connecting them to or disconnecting them from the DIO device.

©National Instruments Corporation 5 NI 6520/6521 User Guide
Operate the DIO device at or below the measurement category1marked
on the hardware label. Measurement circuits are subjected to working
voltages2and transient stresses (overvoltage) from the circuit to which they
are connected during measurement or test. Installation categories establish
standard impulse withstand voltage levels that commonly occur in
electrical distribution systems. The following is a description of installation
categories:
• Measurement Category I is for measurements performed on circuits
not directly connected to the electrical distribution system referred to
as MAINS3voltage. This category is for measurements of voltages
from specially protected secondary circuits. Such voltage
measurements include signal levels, special equipment, limited-energy
parts of equipment, circuits powered by regulated low-voltage sources,
and electronics.
• Measurement Category II is for measurements performed on circuits
directly connected to the electrical distribution system. This category
refers to local-level electrical distribution, such as that provided by a
standard wall outlet (for example, 115 V for U.S. or 230 V for Europe).
Examples of Measurement Category II are measurements performed
on household appliances, portable tools, and similar DIO devices.
• Measurement Category III is for measurements performed in the
building installation at the distribution level. This category refers to
measurements on hard-wired equipment such as equipment in fixed
installations, distribution boards, and circuit breakers. Other examples
are wiring, including cables, bus-bars, junction boxes, switches,
socket-outlets in the fixed installation, and stationary motors with
permanent connections to fixed installations.
• Measurement Category IV is for measurements performed at the
primary electrical supply installation (<1,000 V). Examples include
electricity meters and measurements on primary overcurrent
protection devices and on ripple control units.
1 Measurement categories, also referred to as installation categories, are defined in electrical safety standard IEC 61010-1.
2 Working voltage is the highest rms value of an AC or DC voltage that can occur across any particular insulation.
3 MAINS is defined as a hazardous live electrical supply system that powers equipment. Suitably rated measuring circuits may
be connected to the MAINS for measuring purposes.

NI 6520/6521 User Guide 6 ni.com
Related Documentation
The following documents contain information that you may find helpful as
you use this user guide:
•DAQ Getting Started Guide—This guide describes how to install the
NI-DAQ software, the DAQ device, and how to confirm that the device
is operating properly.
•NI-DAQmx Help—This help file contains information about using
NI-DAQmx to program National Instruments devices. NI-DAQmx
is the software you use to communicate with and control NI DAQ
devices.
•Measurement & Automation Explorer Help for NI-DAQmx—This
help file contains information about configuring and testing DAQ
devices using Measurement & Automation Explorer (MAX) for
NI-DAQmx, and information about special considerations for
operating systems.
•DAQ Assistant Help—This help file contains information about
creating and configuring channels, tasks, and scales using the DAQ
Assistant.
Note You can download these documents from ni.com/manuals.
Features
Digital Filtering
Use the digital filter option available on the NI 6520/6521 input lines to
eliminate glitches on input data. When used with change detection, filtering
can also reduce the number of changes to examine and process.
You can configure the digital input channels to pass through a digital filter
after the photocouplers, and you can control the timing interval the filter
uses. The filter blocks pulses that are shorter than half of the specified
timing interval and passes pulses that are longer than the specified interval.
Intermediate-length pulses—pulses longer than half of the interval but less
than the interval—may or may not pass the filter.
The filter operates on the inputs from the photocouplers. Photocouplers
turn on faster than they turn off and pass rising edges faster than falling
edges. As a result, the photocouplers can add up to 100 μs to a high pulse
or subtract up to 100 μs from a low pulse.
Note A 100 μs change is typical when IF= 16 mA, and RL= 10 kΩ.

©National Instruments Corporation 7 NI 6520/6521 User Guide
Table 1 lists the pulse widths guaranteed to be passed and blocked.
You can enable filtering on as many input lines as is necessary for your
application. All filtered lines share the same timing interval, which ranges
from 100 μs to 200 ms.
Internally, the filter uses two clocks: the sample clock and the filter clock.
The sample clock has a 100 ns period. The filter clock is generated by a
counter and has a period equal to one half of the specified timing interval.
The input signal is sampled on each rising edge of the sample clock, which
is every 100 ns. However, a change in the input signal is recognized only if
it maintains its new state for at least two consecutive rising edges of the
filter clock.
The filter clock is programmable and allows you to control how long a
pulse must last to be recognized. The sample clock provides a fast sample
rate to ensure that input pulses remain constant between filter clocks.
Digital Filtering Example
Figure 2 shows a filter configuration with an 800 ns filter interval (400 ns
filter clock).
Figure 2. Digital Filtering Example
Table 1. NI 6520/6521 Digital Filter Timing Intervals
Filter Interval
Pulse Width Passed Pulse Width Blocked
Low Pulse High Pulse Low Pulse High Pulse
tinterval tinterval + 100 μs tinterval – 100 μs(tinterval/2) + 100 μs(tinterval/2) – 100 μs
HHHHH
HLLHH
HLLHH
A
B
C
External
Signal
Filter
Clock
Sample Clock (100 ns)
External
Signal
Sampled
Filtered
Signal

NI 6520/6521 User Guide 8 ni.com
In periods A and B, the filter blocks the glitches because the external signal
does not remain steadily high from one rising edge of the filter clock to the
next. In period C, the filter passes the transition because the external signal
remains steadily high. Depending on when the transition occurs, the filter
may require up to two filter clocks—one full filter interval—to pass a
transition. Figure 2 shows a rising (0 to 1) transition. The same filtering
applies to falling (1 to 0) transitions.
Programmable Power-Up States
The default power-up state for Form C relays is deenergized (control bit is
low), which makes the contact connect at COM and NC. The Form C relay
status is user-configurable. If the power-up state of a Form C relay is
configured to energized (control bit is high), the contact connects at COM
and NO after power-up.
Figure 3. Programmable Power-Up States for Form C Relays
The default power-up state for Form A relays is deenergized (control bit is
low), which makes the contact open at COM and NO. The Form A relay
status is user-configurable. If the power-up state of a Form A relay is
configured to energized (control bit is high), the contact connects at COM
and NO after power-up.
Figure 4. Programmable Power-Up States for Form A Relays
User-configurable power-up states are useful for ensuring that the
NI 6520/6521 power up in a known state.
To use MAX (recommended) to program the power-up states, select
the device and click the Properties button. Refer to the software
documentation for information about how to program the power-up states
using NI-DAQ with LabVIEW or other National Instruments application
development environments (ADEs).
Note The response time of programmable power-up states is 400 ms.
NC
NO
COM NC
NO
COM
Control Bit Is High (1)
Control Bit Is Low (0)
NOCOM NOCOM
Control Bit Is High (1)
Control Bit Is Low (0)

©National Instruments Corporation 9 NI 6520/6521 User Guide
Change Detection
You can program the NI 6520/6521 device to send an interrupt when a
change occurs on any input line.
The DIO device can monitor changes on selected input lines or on all input
lines. It can monitor for rising edges (0 to 1), falling edges (1 to 0), or both.
When an input change occurs, the DIO device generates an interrupt, and
the NI-DAQ driver then notifies the software.
Note Excessive change detections can affect system performance. Use digital filtering to
minimize the effects of noisy input lines.
The DIO device sends a change detection when any one of the changes
occurs, but it does not report which line changed or if the line was rising or
falling. After a change, you can read the input lines to determine the current
line states. The maximum rate of change detection is determined by the
software response time, which varies from system to system.
An overflow bit indicates that an additional rising or falling edge has been
detected before the software could process the previous change.
Refer to the software documentation for information about how to set up
and implement the change detection.
Change Detection Example
Table 2 shows a change detection example for six bits of one port.
This example assumes the following line connections:
• Bits 7, 6, 5, and 4 are connected to data lines from a four-bit TTL
output device. The DIO device detects any change in the input data
so you can read the new data value.
• Bit 1 is connected to a limit sensor. The DIO device detects rising
edges on the sensor, which correspond to over-limit conditions.
Table 2. Change Detection Example
Bit
7 6 5 4 3 2 1 0
Changes to detect — —
Enable rising-edge detection yes yes yes yes no no yes no
Enable falling-edge detection yes yes yes yes no no no yes

NI 6520/6521 User Guide 10 ni.com
• Bit 0 is connected to a switch. The software can react to any switch
closure, which is represented by a falling edge. If the switch closure is
noisy, enable digital filtering for this line.
In this example, the DIO device reports rising edges only on bit 1, falling
edges only on bit 0, and rising and falling edges on bits 7, 6, 5, and 4. The
DIO device reports no changes for bits 3 and 2. After receiving notification
of a change, you can read the port to determine the current values of all
eight lines. You cannot read the state of any lines that are configured for
change detection until the change detection interrupt occurs.
Watchdog Timer
The watchdog timer is a software configurable feature used to set critical
outputs to safe states in the event of a software failure, a system crash, or
any other loss of communication between the application and the
NI 6520/6521 while power is provided to the PC/board.
When the watchdog timer is enabled, if the NI 6520/6521 does not receive
a watchdog reset software command within the time specified for the
watchdog timer, the outputs go to a user-defined safe state and remain in
that state until the watchdog timer is disarmed by the application and new
values are written, the NI 6520/6521 is reset, or the computer is restarted.
The expiration signal that indicates an expired watchdog will continue to
assert until the watchdog is disarmed. After the watchdog timer expires, the
NI 6520/6521 ignores any writes until the watchdog timer is disarmed.
You can set the watchdog timer timeout period to specify the amount of
time that must elapse before the watchdog timer expires. The counter on the
watchdog timer is configurable up to (232 – 1) ×100 ns (approximately
seven minutes) before it expires.
Digital I/O
I/O Connector
The 37-pin D-SUB connector on the NI 6520/6521 provides access to eight
isolated digital inputs and 8 relay outputs. For easy connection to the digital
I/O connector, use the National Instruments 37-pin D-SUB accessories. For
a list of compatible 37-pin accessories, refer to the Cables and Accessories
section.
Caution Do not make connections to the digital I/O that exceed the maximum I/O
specifications. Doing so may permanently damage the NI 6520/6521 and the chassis. Refer
to the Signal Descriptions section and the device specifications for information about
maximum input ratings.

©National Instruments Corporation 11 NI 6520/6521 User Guide
The output lines on the NI 6520/6521 consist of electromechanical
non-latching relays. For information about connecting to these signals for
Form C and Form A relays, refer to the Relay Signal Connection Example
section.
The input lines on the NI 6520/6521 consist of input photocouplers, which
react to a voltage differential rather than only to a voltage increase. For an
input channel, if the voltage difference between two input lines is greater
than logic threshold, the reading on the digital line will be logic high. Refer
to the Input Signal Connection Example section for more information on
input connection.
For information on safely connecting to the NI 6521, refer to the 37-Pin
High-Voltage Accessory Safety Kit Installation Guide shipped with your
device, and also available on ni.com/manuals.
Pin Assignments
Figure 5 shows the pin assignments for the 37-pin connector on the
NI 6520/6521 device.
The naming convention for each input pin is P0.X, where Xis the line
number and a +or –indicates whether the terminal is positive or negative.
The naming convention for each output pin is P1.XNO, P1.XCOM, and
P1.XNC for Form C, where Xis the output channel number, and P1.XNO
and P1.XCOM for Form A, where Xis the output channel number.
Note For input ports, you can connect signals to the two pins of each line, regardless of
which one has the higher voltage. Output lines are electromechanical relays and act as
bi-directional switches.

NI 6520/6521 User Guide 12 ni.com
Figure 5. NI 6520/6521 Pin Assignments
For more information on signals on the NI 6520/6521, refer to the Signal
Descriptions section.
Signal Descriptions
Table 3. NI 6520/6521 Signal Descriptions
Pin Signal Name Description
1, 20, 4, 23, 7, 25, 9, 27 NO.<7..0> Normally Open—These are the normally open
terminals of the relays.
3, 22, 6 NC.<2..0> Normally Closed—These are the normally closed
terminals of the relays. This is the default state for
the Form C relays.
2, 21, 5, 24, 8, 26, 10, 28 COM.<7..0> Common—These are the common terminals of the
relays.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
P1.0NO
P1.0COM
P1.0NC
P1.2NO
P1.2COM
P1.2NC
P1.4NO
P1.4COM
P1.6NO
P1.6COM
NO CONNECT
P0.0+
P0.0–
P0.2+
P0.2–
P0.4+
P0.4–
P0.6+
P0.6–
P1.1NO
P1.1COM
P1.1NC
P1.3NO
P1.3COM
P1.5NO
P1.5COM
P1.7NO
P1.7COM
NO CONNECT
P0.1+
P0.1–
P0.3+
P0.3–
P0.5+
P0.5–
P0.7+
P0.7–

©National Instruments Corporation 13 NI 6520/6521 User Guide
Optically Isolated Differential Inputs
The input lines on the NI 6520/6521 device provide 8 lines of optically
isolated digital input. These inputs contain an AC input photocoupler and a
resistor for current limiting. The maximum input voltage (VIN) on these
lines is 30 VDC.
(NI 6520 Only) The input (VIN) range on the channels is –30 VDC to
+30 VDC, for signals connected between P0.X+ to P0.X–, where Xis
line <0..7>.
Sensing DC Voltages
The NI 6520/6521 can sense DC signals from 11 V to DC power supply
levels up to 30 V.
Input voltages between –4 V to +4 V produce a logic low. Input voltages
between –30 V to –11 V or between +11 V to +30 V produce a logic high.
The logic state of signals between –11 V to –4 V and +4 V to +11 V is not
defined. This corresponds to standard industrial 24 V logic thresholds.
There should never be more than a 30 V difference in P0.X+ and P0.X–
terminals. Exceeding 30 V differential voltage could cause damage to the
photocouplers.
(NI 6521 Only) The 30 V differential voltage can have up to ±120 V
common-mode voltage compared to chassis/digital ground of the NI 6521
device.
12, 30, 14, 32, 16, 34, 18,
36
P0.<7..0>+ Isolated input ports, positive terminals—Each of
these terminals serves as the signal input. You can
connect either end of the input signal to it, regardless
of whether the level is positive or negative.
13, 31, 15, 33, 17, 35, 19,
37
P0.<7..0>– Isolated input ports, negative terminals—Each of
these terminals serves as the signal input. You can
connect either end of the input signal to it, regardless
of whether the level is positive or negative.
Table 3. NI 6520/6521 Signal Descriptions (Continued)
Pin Signal Name Description

NI 6520/6521 User Guide 14 ni.com
Input Signal Connection Example
Figure 6 and Figure 7 show signal connections for a supply connected to an
isolated input. The power supply can be any DC voltage within the
NI 6520/6521 device range. Refer to the device specifications for
information about voltage ranges.
Figure 6. NI 6520 Input Signal Connection Example
Figure 7. NI 6521 Input Signal Connection Example
P0.X +
P0.X –
0 V~ +30 V
or
0 V~ –30 V
2.43 kΩ
+3.3 V
AC Input
Photocoupler
+3.3 V
P0.X +
P0.X –
Signal
Common
Mode
0 V~ +30 V
or
0 V~ –30 V
2.43 kΩ
Up to 120 V
Common-Mode
Voltage
AC Input
Photocoupler

©National Instruments Corporation 15 NI 6520/6521 User Guide
Mechanical Relays
Relay Forms
Table 4 describes the relay forms available on the NI 6520/6521.
Non-Latching Relays
The relays on the NI 6520/6521 are non-latching. A non-latching relay has
an 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 closed 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.
Non-latching armature relays are useful in control applications when the
switch must return to a known state if power is lost.
Figure 8. Non-Latching Relay States
Relay Power Ratings
Caution The maximum switching current is limited by the maximum switching power, the
maximum voltage, and must not exceed 60 W, 60 VA.
(NI PCI-6520 Only) The maximum power rating for the NI PCI-6520 relays is
60 W or 60 VA, with a maximum current of 2 A for all channels from 0 to
55 °C, external PC ambient.
Table 4. NI 6520/6521 Relay Forms
Form Symbol Description
Form A Single-pole single-throw (SPST) relays with a default state of
normally open.
Form C Single-pole double-throw (SPDT) relays that break the
connection with one throw before making contact with the other
(break-before-make).
NC
NO
NC Position
NC
NO
COM
NO Position
NC
NO
COM

NI 6520/6521 User Guide 16 ni.com
(NI PXI-6521 Only) The maximum power rating for the NI PXI-6521 relays
is 60 W or 60 VA, with a maximum current of 2 A for all channels from 0 to
55 °C, external PXI chassis ambient.
(NI PCI-6521 Only) The maximum power rating for the NI PCI-6521 relays is
60 W or 60 VA. Figure 9 demonstrates the maximum current ratings for the
NI PCI-6521 relays from 0 to 55 °C.
Figure 9. NI PCI-6521 Relay Power Ratings
Power-On and Power-Off Conditions
The relays on the NI 6520/6521 have the default statuses of deenergized
(control bit is low) in both power-on and power-off states. For more
information on default states of the NI 6520/6521 relays, refer to the
Programmable Power-Up States section and the Non-Latching Relays
section.
Relay Signal Connection Example
To connect to the NI 6520/6521 signals for Form C relays, connect NC,
NO, and COM, as shown in Figure 10.
Figure 10. Relay Signal Connection Example for Form C Relays
5550454035302520151050
2.0
1.6
1.2
0.4
0.2
0
Ambient Temperature (°C)
Maximum Current (Amps)
0.6
0.8
1.0
1.4
1.8
2.2
External PC Ambient—All Channels
Load BLoad A
P1.XCOM
P1.XNC
P1.XNO
Power

©National Instruments Corporation 17 NI 6520/6521 User Guide
To connect to the NI 6520/6521 signals for Form A relays, connect NO and
COM, as shown in Figure 11.
Figure 11. Relay Signal Connection Example for Form A Relays
Power Connections
Isolation Circuitry
Input Isolation Circuitry
The optocouplers optically isolate the NI 6520/6521 digital input channels.
Each IC has an optocoupler that provides optical isolation for one channel
of input.
Output Isolation Circuitry
The electromechanical relays provide isolation on the NI 6520/6521 output
channels. Each relay provides isolation at one channel of output.
Isolation Voltages
NI 6520 Isolation Voltages
The terminals of each I/O channel are isolated from other I/O channels,
from the connector shell, and digital ground. Isolation barriers provide
isolation up to 30 Vrms/42.4 Vpk/60 VDC between any two terminals,
except between the two terminals making up a single digital I/O channel.
Caution Do not exceed the isolation voltage limits. Exceeding the voltage limits may
cause injury or permanently damage the NI 6520 components. National Instruments is
not liable for any damage resulting from signal connections that exceed these limits.
Load
P1.XCOM
P1.XNO
Power

NI 6520/6521 User Guide 18 ni.com
NI 6521 Isolation Voltages
Cautions This 150 V Measurement Category II module features 1,400 Vrms isolation
between I/O channels and the backplane, as verified by a dielectric withstand test, 5 s
maximum. Do not use this module for connection to signals or for measurements within
Categories III or IV. Do not connect to MAINS supply circuits greater than 150 VAC. Refer
to the Safety Information section for more information on measurement categories. When
hazardous voltages (>30 Vrms/42.4 Vpk/60 VDC) are present on any signal, all signals
must be considered hazardous. Ensure that external wiring or any circuits connected to the
device are properly insulated from human contact.
The NI 6521 must be used with special keyed cables and accessories. Refer to the 37-Pin
High-Voltage Accessory Safety Kit Installation Guide shipped with your device and the
NI 6520 Accessory Options section for more information.
The PCI-6521 must be installed in a PC that adequately grounds the front panel bracket to
the chassis of the PC.
Do not remove covers from the NI PCI-6521. Doing so can result in electrical shock or
death.
Use the PXI-6521 in a PXI chassis with properly installed PXI filler panels. Do not remove
the filler panels from the NI PXI-6521. Doing so can result in electrical shock or death.
Protecting Inductive Loads
When inductive loads are connected to the relays, a large
counter-electromotive force can occur when the relay disconnects the
circuit. This occurs because of the energy stored in the inductive load.
These flyback voltages can damage the relays and/or the power supply.

©National Instruments Corporation 19 NI 6520/6521 User Guide
To limit these flyback voltages at the inductive load, install a flyback diode
for DC loads or a metal oxide varistor (MOV) for AC loads. For best results,
mount the flyback diode within 18 inches of the load. Figure 12 shows an
external flyback diode installed across an inductive load for DC loads.
Figure 13 shows a MOV installed across an inductive load for AC loads.
Figure 12. Contact Protection for DC Inductive Loads
Figure 13. Contact Protection for AC Inductive Loads
VDC
GND
CHAN
NI 6520/6521
+–
V
Flyback Diode
MOV
Inductive Load
VAC
GND
CHAN
NI 6520/6521

NI 6520/6521 User Guide 20 ni.com
Cables and Accessories
NI 6520 Accessory Options
National Instruments offers the following connectivity options for use with
37-pin safety low voltage (≤30 Vrms/42.4 Vpk/60 VDC) NI 6520 devices.
Screw Terminal Connectivity Options
• CB-37F Connector Block—part number 778672-01 (vertical) and
778673-01 (horizontal)
• CB-37F-LP Unshielded, I/O Connector Block with 37-Pin
D-SUB—part number 779353-01
• With one of the following cabling options:
– SH37F-37M Shielded Cable—part number 778621-01 (1 m) and
778621-02 (2 m)
– SH37F-P-4 37-Pin Female-to-Pigtails Shielded I/O Cable—part
number 778620-04 (4 m)
– R37F-37M-1 37-Pin Female-to-Male Ribbon I/O Cable—part
number 779195-01 (1 m)
Or choose from the following custom connectivity options:
• SH37F-P-4 37-Pin Female-to-Pigtails Shielded I/O Cable—part
number 778620-04 (4 m)
• TB-37F-37CP 37-Pin Crimp & Poke Terminals, Shell with Strain
Relief—part number 779185-01
• TB-37F-37SC 37-Pin Solder Cup Terminals, Shell with Strain
Relief—part number 779184-01
For more information about optional equipment available from National
Instruments, refer to the National Instruments catalog or visit the National
Instruments Web site at ni.com.