National Instruments C Series Modules

The manufacturer National Instruments has numerous series organized by the type of instrument, one of which is the C Series. This series is comprised of Voltage Output Modules, Universal Analog Input Modules, CAN Interface Modules, Digital Modules, Current Output Modules, Counter Input Modules, Temperature Input Modules, Current Input Modules, Digitizer Modules, Voltage and Current Input Modules, Sound and Vibration Input Modules, Strain/Bridge Input Modules, LIN Interface Modules, Voltage Input Modules, Relay Output Modules, and more. It is also worth noting that several specifications for a C Series system are determined by the modules and other specifications are determined by the backplane. Other specifications can even be based on the configuration of the system. The NI C Series is designed for the CompactDAQ and CompactRIO platforms.

The C Series National Instruments modules have varying channel counts, sample/update rates, and front connector types. Some of the modules are conformal coated and/or enclosed as well. This series delivers input and/or output capabilities for CompactDAQ or CompactRIO systems. These modules also tend to be affordable.

The C Series Serial Interface Modules add serial ports such as RS-232 or RS-485/RS-422 to CompactRIO systems, which enables users to communicate with serial instruments. The serial ports can be accessed from the field-programmable gate array to provide versatility to the user. These modules have separate buffers on each port that minimize FPGA space and make programming an easier process.

Another type of C Series module that makes up a large portion of the series is the C Series Digital Modules. These modules can interact with several types of instruments, industrial switches, and transducers. These Digital Modules come equipped with an LED for each channel that shows the status of the channel. While utilizing the C Series Digital Modules with CompactRIO, the user can utilize LabVIEW FPGA to design customized high-speed counter/timers, pulse generation, digital communication protocols, etc. These modules are also ideal for many types of industrial and benchtop settings.

The C Series Voltage Input Modules have the capability to measure input voltage signals for CompactDAQ or CompactRIO systems and have features such as overcurrent protection for high-voltage applications and isolation. The channel count ranges from 3 to 32 single-ended or differential analog input channels and some modules have simultaneous sampling.

There are also the National Instruments C Series Temperature Input Modules, which take measurements from thermocouples and resistance temperature detectors. The C Series Temperature Input Modules have a maximum of 16 channels. These modules additionally have open-thermocouple detection, anti-aliasing filters, and cold-junction compensation. Several of the NI C Series Temperature Input Modules have noise immunity, isolation for safety, and high common-mode voltage range as well.

National Instruments Data Acquisition (DAQ) Devices

National Instruments manufactures a variety of products, including several types of data acquisition or DAQ devices. These include affordable singular DAQ devices with a minimal channel count, modular DAQ devices with a medium channel count, and modular DAQ systems with a high channel count. These devices are created for several platforms and buses, such as CompactDAQ, CompactRIO, PXI, PXI Express, PCI, and PCI Express. DAQ devices measure an electrical or physical spectacle like voltage, temperature, current, sound, or pressure via a computer.

A complete DAQ system is comprised of DAQ measurement hardware, sensors, and a computer that has programmable software. NI DAQ is different from other DAQ options because it has customizable software that is simple to utilize, extremely precise analog designs, cohesive signal conditioning, and a large variety of high-quality DAQ hardware. Several applications that NI DAQ devices are ideal for include design validation and verification, asset condition monitoring, research and analysis, diagnostics and repair, PC-based control and automation, and manufacturing and quality test.

The more affordable singular USB DAQ devices have eight single-ended channels, a maximum sample rate from 10 kS/s to 100 kS/s, 12, 14, or 16 bits resolution, and either 6 mV or 26 mV analog input absolute accuracy. The USB DAQ devices are tiny with a minimal weight and can be used with DAQExpress application software.

The modular DAQ devices with a medium channel count are the CompactDAQ Chassis, CompactDAQ Controller, and conditioned I/O modules. The CompactDAQ Chassis control the timing, data transfer, and synchronization between C Series I/O modules and an external host. A CompactDAQ system includes a chassis linked to the user’s PC via USB or Ethernet, then filed with at least one conditioned I/O module that delivers direct sensor connectivity. There are also CompactDAQ Controllers available that can run a Windows or real-time OS without requiring another device or link.

Finally, there are the modular DAQ systems with a high channel count, which are the PXI models. A PXI system includes a chassis that delivers cooling, power, and a communication bus for modular instruments or I/O modules. These modules can be controlled two ways: through an external PC or an embedded controller. PXI products allows users to simply synchronize modules and measurement with the common signals on the backplane of a PXI Chassis. NI has created over 600 PXI products with a wide range of measurement capabilities. PXI products solely contain the important instrumentation circuitry. These modular instruments bring performance in a small size that will reduce space on the user’s benchtop or manufacturing floor.


Understanding Specifications Part 2

To prevent interference on receiving apparatus, for example, audio and TV receivers or computer systems, signals generated in the line supply and the radiated electromagnetic field of radio frequency from electrical equipment may not exceed certain limits. For this the IEC makes recommendations. A special committee of the IEC, the CISPR (International Special Committee on Radio Interference), has published several definitions concerning measuring sets and measurement procedures for the various types of interference-producing equipment.

Although the CISPR is an international committee, some countries have developed their own standards.  In Germany the VDE committee (Verbandes Deutscher Elektrotechniker) has set its own requirements. The VDE requirements may not be incompatible with the IEC recommendations, and frequently reference is made to both.

In the Philips PM 3240 specifications, it will be clear that the prime function of an oscilloscope is not to generate radio­frequency (rf) signals. The instrument only produces rf signals as a parasitic effect.




For S&I products standard environmental conditions for operation are recommended for commonly existing applications. These conditions are based on IEC Puhl. 359: Expression of the functional performance of electronic measuring equipment. This is intended, with other conditions, to unify methods used in making and clarifying statements on the functional performance of electronic measuring equipment. For each condition must be specified:

  • Reference condition
  • Rated range of use
  • Limit range of operation
  • Storage and transport condition

Normally, apparatus intended for different conditions of use are subjected to the same climatic and mechanical conditions for storage and transport. During use apparatus may also be subjected to mechanical stresses, due to handling and local transport.



IEC/CISPR prepares technical recommendations for the maximum rf interference limits.

As radio frequencies (rf) are considered the frequencies in the range from IO kHz to 3000 GHz.

Two categories of apparatus are distinguished:

  • Apparatus intentionally using or generating rf: radio frequency equipment
  • Apparatus producing rf as a parasitic effect

The limits may depend on the field of application of the apparatus. Classification of ISM Radio Frequency Equipment CISPR Recommendation No. 39 specifies limits for Industrial, Scientific, and Medical (ISM) radio frequency equipment.

Class I

  • Equipment, manufactured in series, which meet the limits of Class I.
  • The limits of Class I correspond to the limits of CISPR recommendation no. 39.
  • Equipment meeting these limits may disturb rf communication under certain conditions.
  • National authorities may impose administrative formalities for operation; for example, notification of operation by the user.

Class II

  • Equipment, manufactured in series, which meet the limits of class II. The limits of class II are more stringent than the limits of class I.
  • Equipment meeting these limits will generally not cause interference. National authorities refrain from administrative procedure for operation. Equipment other than class I and class II will generally be subject to national regulations. ISM equipment of this class consists generally of large equipment which must be tested at the location of operation.

For S & I radio frequency equipment it is recommended that the requirements of class II be met. If this is not feasible, the requirements of class I (CISPR 39/1) shall be met.



Understanding Specifications

The 15-MHz portable dual-trace oscilloscope Philips PM 3226 is a compact, lightweight instrument featuring simplicity of operation, for a wide range of use in servicing, research, and educational applications. Other features include provision for chopped or alternate display of Y signals, automatic triggering, mains triggering, and triggering on the line and frame sync pulses of a television signal. The cathode-ray tube displays a useful screen area calibrated into 8 x 10 divisions by an external graticule.

All circuits are fully transistorized and mounted on printed circuit boards for ease of maintenance. The straightforward design and layout combine simple operation with a high degree of reliability.

The PM 3240 portable hf oscilloscope enables the measurement of signals at a high sensitivity (5 mV/div) over a large bandwidth (50 MHz). There is a wide choice of display possibilities, such as one channel, two channels alternately or chopped, two channels added, with normal and inverted position for one input signal, and a main and delayed time base. The PM 3240 oscilloscope features a tapless power supply with low dissipation. This power supply works on any ac mains voltage between 90 and 264 V, or any dc voltage between 90 and 200 V, thus obviating the need for adjusting the instrument to the local mains voltage. All these features make the oscilloscope suitable for a wide variety of applications.


This specification is valid after the instrument has warmed up for 15 minutes. Properties expressed in numerical values with tolerances stated are guaranteed by the manufacturer.  Numerical values without tolerances are typical and represent the characteristics of an average instrument.

Generally, for the more sophisticated oscilloscopes additional information is given, particularly regarding the conditions under which the specifications are valid.


Besides the bandwidth of an amplifier, the “phase characteristic” also is of utmost importance. The phase characteristic shows the phase shift between the input and output signal as a function of the frequency. Mathematically, it can be proved that for an “ideal” amplifier response both the phase and the frequency characteristic must be Aat. However, an “idea” phase behavior is related to the frequency response.

For X-Y measurements, the importance of the phase characteristics becomes obvious. If the X and Y amplification channels show different characteristics, then, without any phase shift in the signals to be measured (for example, the same signal), a phase shift is nevertheless displayed on the screen, usually at the higher frequencies. The reason for this is because the Y channel includes a delay line, whereas the X channel does not. The fraction of the Y axis cut off by the ellipsoid denotes sin cp, with cp being the phase shift in degrees.

Environmental testing is often referred to as testing an apparatus according to M2C1 or M2C2 rules. With few exceptions, this statement is meaningless to the customer.  In the case of the Philips PM 3240 oscilloscope, slightly more is specified. The specifications state that the requirements are fulfilling the IEC 68 Db recommendation, which does not say very much. For this reason, a glossary of operation conditions and test procedures for operating, storage, and handling is given.



What Does IEC Mean?

IEC stands for the International Electrotechnical Commission, which is affiliated   to the International Organization for Standardization (ISO). The standards recommended by the IEC include:

  • The way of specifying instruments
  • Operating conditions
  • Test procedures
  • Safety requirements

These standards are important because every manufacturer who follows the IEC recommendations is specifying the same type of products in the same way, and this results in more clarity for the customers. Also, the Philips Product Division for Scientific and Industrial Equipment (S&I) follows the IEC recommendations, and a Quality Manual detailing the IEC requirements is binding within the S&I Division.



Measurement Pitfalls

Very often hum is present on the signals under test. This can be easily determined from the screen, because the hum is related to the line frequency. If a signal shows a kind of unexpected amplitude modulation, switching back the time-base setting to about 5 to 10 or 20 ms/div, and switching over the trigger source selector to MAINS (or LINE), will generally result in a stable picture in the event of hum.

For example, imagine a 250-kHz square-wave signal whose amplitude is rather vague. Switching back the TIME/orv to 5 ms/div and triggering from MAINS results in a stable display of a 20-ms envelope. This means that a 50-Hz hum is present on the square-wave signal.

Another example is a photograph (double-exposed) of a case where the power supply of a pulse generator is defective. The lower trace shows the varying amplitudes of the pulses. Switching the oscilloscope to 5 ms/ div and MAINS triggering showed the cause of the trouble immediately, a 50-Hz dip in the output which can only be caused by the power supply of the pulse generator (upper trace).




A pitfall sometimes met in X-Y measurements is different coupling or the input channels, for example, X via channel A is dc-coupled and Y via channel B is ac-coupled.

The specification of a PM 3240 oscilloscope for ac-coupled input is stated by giving an “input RC time” of about 22 ms. With 1- MO input resistance this means a coupling capacitor of 22 nF. From this it follows chat for 7.23 Hz a phase shift of 45° occurs (l /21rfC = 106 0) compared to a dc-coupled input. This must be borne in mind and avoided when operating the oscilloscope in the X- Y mode.

Now picture an X-Y display of a same sine-wave signal of 20 Hz. One channel is dc-coupled; the other ac-coupled. The fraction cut off the Y axis is arc sin <p, with <p being the phase shift between the X and Y signals. From the illustration can be read arc sin <p = i; so that <p = 22°.

This result can be checked by calculation. For 20 H z, the X e o f a 22-nF capacitor is 0. 36 Mil. Together with the I -Mil input resistance of the oscilloscope, this means that tan cp = 0.36 (current through capacitor is 90° ahead of the voltage across it) and arc tan 0.36 = 22°. The readout from the screen is thus a fair approximation. Already it will be clear that at I kHz this effect is no longer noticeable.



Common-mode signals can be rejected by operating the oscilloscope in the A – B mode. The A – B mode must never be used for rejecting the line supply voltage.

This application of the A – B mode could be considered when signals have their zero level directly coupled to the line supply voltage. This is found in a great number of TV receivers. The illustration shows a simplified power supply of a typical TV receiver, equipped with both tubes and transistors. Node A of the Graetz diode bridge D is directly connected to the common (v) or chassis.

But in the oscilloscope the common (v) and the ground (-. b) are very often connected. For this reason, the common connection of the oscilloscope cannot be connected to the chassis of the TV receiver. This would mean that the 220-V terminals are connected to ground (.!.)  via the diodes of bridge D, causing a short circuit. At the very least the fuses VL would be blown, but very often the diodes D have to be replaced as well. Also, depending on the local 220-V supply, the filaments of half of the tubes may temporarily be connected to double the voltage.

Now, one may initially think that since both oscilloscope inputs have I-MO input impedance, if the A – B mode is used without ground connections, everything will be alright. But if the operator accidently touches both instruments at the same time, a lethal electrical shock may be received. Thus, for safety this procedure must never be permitted.

The only solution to this when making measurements on TV receivers is to use an isolation transformer, with separated primary and secondary windings. This not only applies when connecting an oscilloscope to a TV receiver, but also for other equipment, such as TV pattern generators and digital voltmeters.

By means of the separation transformer, the TV receiver is made to float with respect to sound. Nevertheless, care should always be taken when working on TV receivers.



National Instruments LabVIEW

LabVIEW software, created by National Instruments in the 80’s, can be used with Windows, Mac, Unix, Linux, and other operating systems in many types of applications. This feature makes LabVIEW a versatile software, ideal for users with a variety of requirements. With the use of LabVIEW, users can create industrial equipment and intelligent machines more quickly. National Instrument’s embedded design platform is a combination of a software stack, integrated and easily customized hardware, and an ecosystem of users and IP. NI LabVIEW can also be used to help teach engineering students in the classroom or lab. When students use LabVIEW, they can improve their discovery rates, find answers more quickly, and become more successful.

LabVIEW has numerous modules and different variations available. These include: LabVIEW NXG, SignalExpress, the LabVIEW Real-Time Module, LabVIEW Datalogging and Supervisory Control Module, LabVIEW FPGA Module, LabVIEW MathScript RT Module, LabVIEW Control Design and Simulation Module, Vision Development Module, LabVIEW Statechart Module, LabVIEW SoftMotion Module, and the LabVIEW Robotics Module. For example, LabVIEW NXG is well-suited for applications, such as creating production test systems, designing wireless communications systems, confirming or verifying electronic designs, and more.

Additionally, LabVIEW has three editions: LabVIEW Base, LabVIEW Full, and LabVIEW Professional. The LabVIEW Base edition is suggested for desktop measurement applications and comes with device drivers for NI hardware and third-party devices, as well as simple math and signal processing features. The LabVIEW Full Edition is suggested for inline advanced math and signal processing. This edition is necessary for signal processing add-ons and real-time and FPGA hardware. LabVIEW Professional is ideal for applications needing code confirmation. The LabVIEW Professional Edition comes with code and application deployment features, as well as numerous software engineering add-ons. Both the LabVIEW Full and LabVIEW Professional editions support Windows, Max, and Linux. The LabVIEW Base Edition, however, only supports Windows. Every edition of LabVIEW is available in several languages: English, Simplified Chinese, French, Korean, German, and Japanese.

There is also an NI LabVIEW Student Edition, which has the same functionality as the LabVIEW Full Development System. This version, however, comes with LabVIEW MathScript RT and LabVIEW Control Design and Simulation modules. The graphical setup ensures that creating working code will be accomplished swiftly. This software additionally has the capability to link to hardware and other types of applications, such as Microsoft Excel. The LabVIEW Student Edition offers a free trial for six months if students would like to try it out first.


Timing Errors with Pulse Measurements

While measuring complex waveforms in digital techniques, mistakes can be made very easily. In this section, examples of this are presented. Some of them are explained in detail, to gain knowledge about the possible reasons for false triggering, which leads to wrong timing displays on the screen.

Imagine the display of a square wave and a double pulse in the correct time relation. Now, if the main time base (MTB) is triggered with the lower trace signal, then in the alternate mode, the lower trace could be started with the first of the double pulses, and the upper trace with the second pulse.

The chopped mode could be used in this case, resulting in a vague, unstable picture that would indicate something is wrong. The way to avoid this is to trigger on the upper-trace signal, which has in each repetition time only one upgoing slope and one downgoing slope of the signals.

The following example is a little more complicated and the possible trigger errors are explained in more detail.  Two pulse trains are generated, together with a square wave. If there is a photograph taken from a four-trace oscilloscope and the upper trace shows the square wave, while the middle and lower traces show the successive signals: a triple and double pulse, respectively. From now on the lower trace is called signal A, the middle trace is signal B, and the upper trace signal C. Triggering takes place at the negative slope of signal C.

Now suppose that only signal A and signal B are displayed and that the oscilloscope is triggered at signal A. Then, in the ALTERNATE mode, the result depends on the position of the electronic channel switch at the start of the first written trace. Thus, either signal A or signal B is written first, but both displays show the wrong timing.

When in the alternate mode the trigger source is channel A, then the sweeps are started at pulse number 1 and pulse number 2 alternately. Now, if with the first sweep, signal A is displayed- this depends on the internal position of the electronic channel selector switch. To make sure that the display shows the correct time relation, the chopped mode can be used. Although this results in a wrong picture, the user will be sure that erroneous triggering takes place and that corrective measures can be taken. If available, the user should utilize the signal the signal internally if a four-trace oscilloscope is available or externally if triggered with a dual-trace oscilloscope.




Bandwidth vs. Rise Time


In digital techniques it can happen that two pulses appear in a time­related sequence, but that the second pulse appears a little later, with a delay, with respect to the first one.

If in one signal the successive pulses do not have the same repetition time, it is said that the signal has jitter.


The bandwidth denotes one of the characteristics of an amplifier and the phase another characteristic. The effect of both can be studied by applying an “ideal” square-wave pulse to the input of the amplifier and studying the output voltage. This can be understood if one considers the square wave as being composed of a series (infinite) of sine waves. For example, imagine a pulse is approximated by 5 harmonics, each with its own amplitude and mutual phase. For an exact replica of the waveform, infinite harmonics are needed. The formula for the waveform is given by the Fourier analysis

V(t) = v4


(sin wt + ½ sin 3wt + !- sin 5wt  + · · ·)


After amplification with a factor A, the output voltage V0(t) will be

V (t) = A · 4

(sin wt + ½ sin 3wt +! sin 5wt + · · ·)

0          V


It will be clear that due to this behavior of the amplifier the “ideal” input square wave appears distorted at the output. For example, the rise time Tr is not 0 but has a certain value. The more harmonics are amplified “linearly” (in the right way: same amplification factor and phase shift), the better the result and the shorter the rise time Tr at the output. The value of Tr depends on the frequency of the harmonics, which are of course determined by the frequency of the square wave.

It can thus be stated that the shorter the rise time Tr of the output pulse as a response to an ideal input pulse, the more harmonics are amplified linearly and the higher the bandwidth B of the amplifier. It can be shown that for an amplifier with a bandwidth B, the rise time Tr is related to it according to

BTr   = 0.35

where B = bandwidth, Hz

Tr = rise time, S

Example: For the Philips 50-MHz oscilloscope PM 3240, the rise time is


Tr=      50 X 106 S

= 7 ns

This oscilloscope thus shows a 7-ns rise time as a response to an “ideal” input square wave. As a matter of fact, this is the way the oscilloscope is tested. A pulse for which Tr <<7ns[for example, Tr  =  l00ps(=  10-10  s)], is applied to the input, and the rise time is measured from the screen.





National Instruments Background

National Instruments was founded in 1976 and has offices in over 50 countries. NI develops modular hardware through an open, software-defined platform. Their hardware aids users in boosting performance in automated test and measurement systems. The unique modular hardware is anything from high-performing RF devices to affordable measurement instruments. The versatile I/O allow users to reconfigure their existing hardware in software and thus prevents them from needing to purchase new devices each time they require a different type of application.

National Instruments also has their own special software, such as LabVIEW and TestStand. LabVIEW is a type of systems engineering software. It can be used for applications that involve test, measurement, and control with quick access to hardware and information. LabVIEW 2018 is suggested for applications such as teaching engineering students or creating smart machines or industrial equipment. LabVIEW NXG is the next generation of LabVIEW and is suggested for applications such as creating wireless communications systems, creating production test systems, confirming electronic designs, or measuring physical systems with sensors or actuators.

On the other hand, TestStand is a type of test management software created to aid users with swiftly creating automated test and confirmation systems. TestStand allows users to create, implement, and deploy test system software. Test sequences can be created in TestStand that integrate code modules written in any programming language.

NI is constantly coming up with new solutions for engineers. For example, they recently announced the first FlexRIO Transceiver for direct RF sampling, the PXIe-5785. This FlexRIO Transceiver is a member of a large family of FlexRIO instruments with Xilinx Ultra-Scale FPGAs that can be programmed by users; these devices aid users with meeting custom standards without the expensive cost for custom designing.

As they create newer, more advanced types of modular hardware, they choose to discontinue some of their older models. National Instruments recently discontinued several of their product lines, including: E and B Series models, SCXI modules, SCXI and PXI/SCXI Chassis, Dynamic Signal Analyzers, Plug-In Motion models, PXI R Series, and EtherCAT Chassis. These series are now obsolete and will no longer be manufactured or sold by NI.


Current Probes and Logic Trigger Probes


Basically, the current probe is a transformer of which the primary winding is the test lead through which the current is measured. The probe head consists of a ferrox-cube core and the secondary windings of the transformer. The core can be split into two parts to clip it simply around the measuring lead. The white-colored part of the probe head can be moved backwards and forwards to clip it around the lead. A voltage is developed in the transformer secondary windings by the magnetic field around the measuring lead. This voltage is fed to an amplifier box, the output of which is fed to the oscilloscope. The output cable from the amplifier must be terminated with 50 fl at the oscilloscope end (low-ohmic system for 75-MHz bandwidth).  Furthermore, if the oscilloscope is set to 50-mV/ div sensitivity, the amplifier box provides calibrated outputs ranging from 1 m A/ div on the screen.



Applications for current probes include:

  • Switching currents in power supplies, turn-on and turn-off currents of systems, transistors, and silicon controlled rectifiers (SCRs). The current probe may reveal more information about overload conditions than voltage transients do.
  • Circuits where minimum loading by the measuring device is necessary. The current probe only loads the lead under test between 0.5 and 2 pF, depending upon its diameter. For most applications the impedance inserted into the lead by the probe can be disregarded.
  • Current difference. This is achieved by feeding two leads through the probe head such that the currents flow in opposite directions. The difference between the currents is measured at a high rejection rate for currents that are common mode. Sensitivity can be increased simply by feeding more turns of wire through the clip-on head. The upper range can be extended by splitting the current path into several parallel wires and feeding only one current path through the probe head.



In the more sophisticated digital practice, it frequently happens that the engineer wants to trigger an oscilloscope on a word or combination of bits (simultaneously) present at several lines, for example, in a BCD parallel output of a frequency counter. Word recognizers, suitable for use in conjunction with oscilloscopes, are readily available as a kind of probe. Usually they have several inputs complying with tetrad or octad length (4 or 8). A binary word can be set by switches, and if this word is detected, the unit initiates a trigger which can be used to start a sweep. Alternatively, the trigger can be used to initiate a sweep after a certain delay time by making use of the digital or analog delayed­sweep facility. In this way the display can be started on a recognizable byte, character, or any unique combination of data in logic circuits. For example, the start of the display can be made to coincide with a control byte coming from a peripheral or a BCD output of a decade counter.