NI Switches Help

Edition Date: April 2015

Part Number: 375472H-01

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Symbol Prefix Value
p pico 10 -12
n nano 10 -9
µ micro 10 -6
m milli 10 -3
k kilo 10 3
M mega 10 6
G giga 10 9
T tera 10 12


nV nanovolts 10-9 volts
µV microvolts 10-6 volts
µΩ microohms 10-6 ohms
mΩ milliohms 10-3 ohms
MΩ megaohms 106 ohms
pA picoamps 10-12 amperes
nA nanoamps 10-9 amperes
µA microamps 10-6 amperes
mA milliamps 10-3 amperes


A amps
AC alternating current
AC current The measurement of the electrical current (in amperes) of AC signals. The measurement is typically made using rms averaging.
AC Voltage A voltage that changes as a function of time.
ADE application development environment—A software environment incorporating the development, debug, and analysis tools for software development. LabVIEW, Measurement Studio, and Visual Studio are examples.
American Wire Gauge AWG—A U.S. standard set of non-ferrous wire conductor sizes. Gauge means the diameter. Non-ferrous includes copper and also aluminum and other materials, but is most frequently applied to copper household electrical wiring and telephone wiring. Typical household wiring is AWG number 12 or 14. Telephone wire is usually 22, 24, or 26. The higher the gauge number, the smaller the diameter and the thinner the wire. Since thicker wire carries more current because it has less electrical resistance over a given length, thicker wire is better for longer distances. For this reason, where extended distance is critical, a company installing a network might prefer telephone wire with the lower-gauge, thicker wire of AWG 24 to AWG 26.

AWG is sometimes known as Brown and Sharpe (B&S) Wire Gauge.
analog A signal whose amplitude can have a continuous range of values.
analog bus For SCXI switch modules, see HVAB. For the NI SwitchBlock, see NI SwitchBlock analog bus.
analog bus sharing The simultaneous connection of multiple NI SwitchBlock devices to the same analog bus line, enabled using the Analog Bus Sharing Enable property or NISWITCH_ANALOG_BUS_SHARING_ENABLE attribute.
API, application programming interface
  1. A standardized set of subroutines or functions along with the parameters that a program can call.
  2. A set of functions exported by a library.


backplane Circuit board that installed modules or cards connect to at the back of a chassis or carrier.
bandwidth The range of frequencies present in a signal, or the range of frequencies to which a measuring device can respond.
Break-Before-Make A type of switching contact that is completely disengaged from one terminal before it connects with another terminal.
bus, buses The group of electrical conductors that interconnect individual circuitry in a computer. Typically, a bus is the expansion vehicle to which I/O or other devices are connected. Examples of PC buses are the PCI, AT(ISA), and EISA bus.


capacitance The ability to hold an electrical charge.
card See NI SwitchBlock card.
carrier See NI SwitchBlock carrier.
  1. Pathway from a CPU or, on a network, between computers.
  2. A channel can also be an input connection to a data acquisition system or to an instrument, such as an oscilloscope or logic analyzer.
  3. Pin or wire lead to which you apply or from which you read an analog or digital signal. Analog signals can be single-ended or differential. For digital signals, channels group to form ports. Ports usually consist of either four or eight digital channels.
characteristic impedanceA transmission line parameter that determines how propagating signals are transmitted or reflected in the line.
CMOSComplementary-Metal-Oxide Semiconductor
cold junction, cold-junction, cold-junction compensation
  1. A method of compensating for inaccuracies in thermocouple circuits.
  2. An artificial reference level that compensates for ambient temperature variations in thermocouple measurement circuits. IC temperature sensors are linear and their output is expressed as mV/°C. A 10 mV/°C sensor, for example, outputs 250 mV at 25 °C. Thermistor outputs, however, are nonlinear. Therefore, thermistor output is specified as the voltage range over a defined temperature range (x volts at 50 °C to y volts at 0 °C).
cold switching The process of closing the relay contacts before applying voltage and current, as well as removing voltage and current before opening the contacts.
COMThe common terminal of a multiplexer.
contact bounce The intermittent and usually undesired opening of mechanical relay contacts during closure, or closing of contacts during opening. The contact bounce period depends upon the type of relay and varies from .5 ms for small reed relays to 10-20 ms for larger relays.
contact life The maximum number of expected switch or relay closures with a given voltage/current load before failure.
contact material The material of which the contacts of a relay are made.
contact potential A voltage produced between contact terminals due to the temperature gradient across the relay contacts.
contact rating The maximum voltage, current, and power capacities of relay contacts under specified environmental conditions.
contact resistance The electrical resistance across closed contacts.
contactor An electric switch for controlling a motor or other electric device.
Controller An intelligent device (usually involving a CPU) that is capable of controlling other devices.
crosspoint switch An integrated circuit term; a switch that connects the signal on an input bus to one or more output buses. Also referred to as a switch matrix.
current The rate of flow of electric charge measured in amperes.
current drive The ability to supply a given output current. Refer to current sinking.
current drive capability The amount of current a digital or analog output channel is capable of sourcing or sinking while still operating within voltage range specifications.
current excitationThe current a DMM can generate to perform a variety of measurements including resistance measurements.
current loop A communications method that transmits data as current flow over relatively long distances and through environments with relatively high noise.
current sinking The ability of a signal generator or output of a DAQ device to dissipate current for analog or digital output signals. Refer to current drive.
current sourcing The ability of a DAQ device or instrument device to supply current for external devices, such as sensors or conditioning units. ICP is one technique for supplying DC current to transducers with built-in amplifiers.


  1. Data acquisition—The process of collecting and measuring electrical signals from sensors, transducers, and test probes or fixtures and inputting them to a computer for processing.
  2. Data acquisition—The process of collecting and measuring the same kinds of electrical signals with A/D and/or DIO devices plugged into a computer, and possibly generating control signals with D/A and/or DIO devices in the same computer.
DC direct current—Although the term speaks of current, many different types of DC measurements are made, including DC voltage, DC current, and DC power.
DC coupled Allowing the transmission of both AC and DC signals.
DC current The electric current of a DC signal.
DC gain error The gain error on the DC component of a signal. This specification is not applicable when the device has AC input coupling.
DC offset The DC voltage or current present on a signal.

A typical technique in many instruments is to put the instrument in DC offset calibration mode, where the external signal is removed. Then, the internally generated DC offset signal is measured, and in some cases, stored, and compensated for.
DC parametric measurement The use of Ohm's Law, which states that the current through a conductor is directly proportional to the voltage and inversely proportional to the resistance, to measure one DC characteristic by forcing another. For instance, you can force a current in order to measure terminal voltage, or vice versa.
DC voltage The direct current (non-changing) component of a voltage. In practice, the DC voltage should not change over the period of observation, that is, the measurement time.
dielectric constant The ratio of a capacitance using a given dielectric to the capacitance using a vacuum as a dielectric.
differential thermal EMFthe voltage difference between two junctions
DIO digital input/output
DMM digital multimeter—A digital instrument capable of measuring several different fundamental electrical characteristics, most often voltage, resistance, and current.
DPDTdouble-pole double-throw
drive currentthe current that flows through the coil of an electromechanical relay to move the armature
dry circuit switching Switching below specified levels of voltage and current to minimize any physical and electrical changes in the contact junction.
dry reed relay A glass enclosed, hermetically sealed, magnetically actuated contact. Typical atmosphere inside the glass enclosure is nitrogen.
DUT device under test


E Series A standard architecture for instrumentation-class, multichannel data acquisition devices.
electrical lifeThe number of switch cycles, under load, before the contact resistance of a relay rises above 1 Ω.
electromechanical relayA type of relay composed of a coil, an armature mechanism, and contacts.
EMF electromotive force—The electrical force present without a load on the circuit.
endpoint programming Programming technique that uses two inputs which are the physical channels that connect together. NI Switch Executive will automatically use of any channels that have been reserved for routing to create this connection. For example, specifying to connect column 0 (c0) to column 1(c1) would be used in endpoint programming. In order for the connection to be made, at least one row (r0), must be configured as reserved for routing. This programming technique is simple regardless of how many modules are added to the system, assuming channels are reserved for routing appropriately.
expansion bridge Accessory for the NI SwitchBlock that connects the analog buses of adjacent carriers so that the carriers share a single analog bus.
explicit path programming Programming technique that requires every individual path to be explicitly closed. NI Switch Executive will not automatically route through devices or channels that have been reserved for routing. For example, to connect column 0 (c0) to column 1 (c1), two separate connection paths must be defined: one to connect column 0 to a row, and the second to connect the row to column 1. Explicit path programming becomes more complicated as additional modules are required in the route.


fault In a component or circuit, a defect such as a short-circuit to ground or an unintended open circuit. Also, a failure condition caused by such a defect.
fault insertion unit A device designed to insert fault conditions.
FET switchField-Effect Transistor—A type of relay composed of several CMOS transistors. A voltage applied to the control circuitry connects the source and drain of a transistor network.
formA classification of relays categorized by the number of poles, throws, and default position of the relay.
FIU fault insertion unit
frequency The basic unit of rate, measured in events or oscillations per second using a frequency counter or spectrum analyzer. Frequency is the reciprocal of the period of a signal.
front panel
  1. The interactive user interface of a VI. Modeled after the front panel of physical instruments, it is composed of switches, slides, meters, graphs, charts, gauges, LEDs, and other controls and indicators.
  2. The physical front panel of an instrument or other hardware
Functions palette The LabVIEW palette containing block diagram structures, constants, communication features, and VIs.
fundamental The component of a periodic wave whose frequency, f0, is the greatest common divisor of the harmonic frequencies, and the inverse of the wave period T.


general-purpose topologyA topology composed of multiple isolated relays used to connect one input to one output.
GND ground
  1. A pin.
  2. An electrically neutral wire that has the same potential as the surrounding earth. Normally, a noncurrent-carrying circuit intended for safety.
  3. A common reference point for an electrical system.


handshakingA type of scanning similar to synchronous scanning except that the switch sends a digital pulse back to the other device after each scan list entry has been executed.
harmonic Pertaining to whole-number multiples of the fundamental frequency of a sound or signal.
HIL hardware-in-the-loop
hot switching The process of closing the relay contacts after applying voltage and current, as well as applying voltage and current before opening the contacts.
HVABhigh-voltage analog bus


I/O input/output—Transfer of data to/from a computer system involving communications channels, operator interface devices, and/or data acquisition and control interfaces.
  1. The electrical characteristic of a circuit expressed in ohms and/or capacitance/inductance.
  2. resistance
inductance The characteristic of a coil that generates a voltage due to changes in the current. An inductor creates a voltage that is the derivate of the current, while a capacitor creates a voltage that is the integral of the current.
inrush current The peak instantaneous current drawn by a circuit or device when first turned on.
insertion loss The attenuation of signals due to the impedance when passing the signals through a switching module or system. Specified as a decibel value (dB) over a frequency range.
integrated relay test Integrated feature for NI SwitchBlock that verifies the proper operation of each individual relay.
isolation voltage The voltage that an isolated circuit can normally withstand, usually specified from input to input and/or from any input to the amplifier output, or to the computer bus.


LabVIEWLaboratory Virtual Instrument Engineering Workbench—A program development application based on the programming language G and used commonly for test and measurement purposes.
latching The ability to keep a relay contact in its current state if power is removed.
latching relayA type of electromechanical relay without a default position that remains in its last position when the drive current stops flowing.
LEDlight-emitting diode
load impedance Resistance, capacitance, and (often) inductance presented by a load to an output amplifier. The recommended load impedance is the minimum resistance and maximum capacitance that the circuitry connected to the analog output should have.
load regulation The ability of a power supply to keep its output voltage or current at a constant value with a changing output load.
LVDS low voltage differential signaling. A low-noise, low-power, low-amplitude method for high-speed digital data transfer.


make-before-breakBreaking and completing two paths simultaneously.
matrixA topology in which you can connect multiple inputs to multiple outputs organized as columns and rows.
MAXMeasurement & Automation Explorer—A controlled, centralized configuration environment that allows you to configure all of your NI devices.
Measurement Category The following is a description of measurement categories:
  1. Measurement Category I is for measurements performed on circuits not directly connected to MAINS 1 . This category includes signals such as voltages on a printed wire board (PWB) on the secondary of an isolation transformer.
  2. Measurement Category II is for measurements performed on circuits directly connected to the low-voltage installation. This category refers to local-level distribution such as that provided by a standard wall outlet.
  3. Measurement Category III is for measurements performed in the building installation. This category is a distribution level referring to hardwired equipment that does not rely on standard building insulation.
  4. Measurement Category IV is for measurements performed at the source of the low-voltage (<1,000 V) installation.
mechanical lifeThe number of switch cycles before the contact resistance of a relay rises above 1 Ω.
minimum switch load In hot switching, the minimum power required to sufficiently close and maintain connection between electromechanical relay contacts.
MOSFETMetal-Oxide-Semiconductor Field-Effect Transistor
multicard device NI SwitchBlock device composed of two or more cards.
multiplex To assign more than one signal to a channel.
multiplexerA topology in which you can connect one input to multiple outputs or one output to multiple inputs.
muxsynonym for multiplexer


NCnormally closed
NI SwitchBlock High-density expandable switch carrier and interconnectable cards that enable greater switching capacity per slot than standalone PXI switch modules. The NI SwitchBlock operates within a PXI chassis or, when connected to a PXI peripheral slot or a PXI Express hybrid peripheral slot, within a PXI Express chassis.
NI SwitchBlock analog bus A shared set of traces on the carrier backplane used to internally route connections between cards in the carrier. This bus is shared across multiple carriers when the carriers are connected using the NI 2806 expansion bridge.
NI SwitchBlock analog bus line A single trace of the analog bus on an NI SwitchBlock carrier. See NI SwitchBlock analog bus.
NI SwitchBlock card High-density relay card that can be combined with other NI SwitchBlock cards in software to operate as a single device.
NI SwitchBlock carrier Subchassis that operates within a PXI/PXI Express chassis and holds interconnectable NI SwitchBlock cards.
NI SwitchBlock device One or more cards configured in software to function as a single device.
NOnormally open
nonlatching relayA type of electromechanical relay with an initial position of normally closed maintained by the force of a spring or permanent magnet while no drive current flows.


overcurrentA current above the optimum, operational, or rated current.
overtemperatureA temperature above that at which a component, circuit, device, or system is rated.


pathThe route a signal follows through the switch from input terminal to output terminal.
path resistance The resistance of a complete signal path from source to destination. This includes resistance of wiring, switching, and input and output connectors.
PCBprinted circuit board
power consumption limit Maximum amount of power that a module or carrier can draw from a chassis, or that a card can draw from a carrier, without damaging the NI switch products.
power dissipation limit Maximum amount of power that can be lost as heat from signals or from power consumed from the backplane without damaging the NI switch products.
power limits Maximum amount of power, including power consumption and power dissipation, that can route through a module, carrier or carrier slot without damaging the NI switch products.
PXIPCI eXtensions for Instrumentation—A modular, computer-based instrumentation platform.
PXI ExpressPCI Express eXtensions for Instrumentation—The PXI implementation of PCI Express, a scalable full-simplex serial bus standard that operates at 2.5 Gbps and offers both asynchronous and isochronous data transfers.
PXI Express-compatible moduleA modified PXI module that is compatible with existing PXI chassis slots and PXI hybrid chassis slots. PXI Express-compatible modules preserve hardware and software compatibility, with the exception of local bus features.


R-C time constantTime required to charge a capacitor to 63.2 percent of its maximum voltage.
reed relayA type of relay composed of two overlapping ferromagnetic blades hermetically sealed within a gas capsule that is filled with an inert gas.
relay An electrically activated mechanical device that opens and closes electrical contacts.
resource Term that can refer to any of the following: NI SwitchBlock carrier, NI SwitchBlock card, PXI/PXI Express card.
resistance The resistance to the flow of electric current. One ohm (Ω) is the resistance through which one volt of electric force causes one ampere to flow.
RF radio frequency—Refers to frequencies below the infrared range.
RF multiplexer A switch capable of selecting one of many RF channels.
rise time The time for a signal to transition from 10% to 90% of the maximum signal amplitude.


safety interlock Safety feature that prevents the front connector of an installed NI SwitchBlock card from exposing high-voltage signals when disconnected from accessories.
scanThe process of cycling through a predefined scan list to when and how to make or break connections.
scan listA string composed of channel names and characters that define connections, disconnections, triggering, and timing of the scan.
scan rateThe number of relay cycles that can be performed in a given time frame. Applicable when using a scan list. The scan rate specifies the frequency of the following actions (initial state - ch0 is connected to com0):
  • Obtain trigger input, disconnect ch0 from com0, and wait for debounce.
  • Connect ch0 to com0, wait for debounce, send scan advanced signal, and wait for trigger input.
SCXISignal Conditioning eXtensions for Instrumentation. The National Instruments product line for conditioning low-level signals within an external chassis near sensors so that only high-level signals are sent to DAQ boards in the noisy PC environment. SCXI is an open standard available for all vendors.
Seebeck coefficientsTemperature coefficients of specific metal-to-metal junctions.
Seebeck voltageA property of dissimilar materials to develop a voltage offset across their junction, proportional to the temperature gradient across said junction, resulting in a current if there is a complete electrical path between the materials. For switching, the Seebeck effect can become an issue when performing low level signal measurements.
senseThe channel of a DMM used to measure the voltage drop across a resistance in a 4-wire measurement.
settling timeThe time required for a signal to reach a steady state after sending an actuation command to the relay.
shared analog bus line An analog bus line that multiple NI SwitchBlock devices connect to simultaneously. See analog bus sharing.
shortShort-circuit. A low-resistance connection which is established between two points in a circuit, bypassing any paths with higher resistance. For example, such a connection between the conductors of a transmission line. An intentional short (such as a short-circuit fault condition) rarely causes damage, but an accidental or otherwise unintentional short between transmission lines or power rails at different voltage potentials can often cause damage due to excessive current flow.
single-card device NI SwitchBlock device composed of one card.
software trigger scanningA type of scanning where the scan list advances at every call of niSwitch Send Software Trigger.
solid-state relayA type of relay incorporating an LED to control the gate of a photo-sensitive MOSFET.
SPDTsingle-pole double-throw
SPSTsingle-pole single-throw
SSRsolid-state relay
stub Portion of trace attached to a signal path that degrades the signal.
switch A device for routing signals between two points.
switching capacity The maximum current and voltage that a relay is specified to handle.
switching currentThe maximum rated current that can flow through the switch as it makes or breaks a contact.
switching time (speed)The time necessary for a switch to actuate and settle to a known state.
switching voltageThe maximum signal voltage that the switch module can safely maintain.
synchronous scanningA type of scanning where the scan list advances after the switch receives a digital pulse from another device.


terminalNamed location where a signal is either produced (generated) or consumed (acquired).
terminal block A group of several terminals, intended for interconnection of circuits, mounted on a solid insulating block.
thermal EMFThe voltage created by the junction of dissimilar metals that increases and decreases with the rise and fall of the ambient temperature.
topologyOrganizational representation of the channels and relays in a switch module.
transistor A device used to amplify a signal or open and close a circuit.


VA volt-ampere—A measurement of power. The volt-ampere rating is sometimes used if the rated equipment has significant phase shift between the current and voltage. Hence, the volt-ampere rating gives a power rating that ignores this phase shift.
Vbattbattery voltage
VIvirtual instrument
voltage the electromotive force
voltage standing wave ratioThe measure of signal reflection in a propagating signal.
VSWRvoltage standing wave ratio


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