NI PXIe-6356 on a test bench: what the eight-channel DAQ card measures and why the "OK" LED alone is not enough for a used inverter, servo drive or I/O module
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Why is the "OK" LED on an inverter not enough? We discuss the capabilities and limitations of the 8-channel NI PXIe-6356 DAQ card used on a test bench to objectively check automation equipment.
NI PXIe-6356 on a test bench: what the eight-channel DAQ card measures and why the "OK" LED alone is not enough for a used inverter, servo drive or I/O module
A frequency inverter lands on the bench, the green LED is on, and the buyer asks whether the unit is definitely working. The LED alone does not answer that. A DAQ measurement card in the PXI Express standard is a replaceable module in a chassis that measures and generates electrical signals. On a test bench it is used to check automation equipment objectively.
The LED on the housing of an inverter, drive or module only tells you that the power supply and part of the logic are working. It does not tell you whether the inputs, outputs and measurement paths work correctly. An analog output with a damaged output stage or a digital input with a shifted threshold can look completely normal until someone measures it. That is why a measurement is needed before used equipment is sold, and when buying used parts it is worth knowing what such a measurement looks like.
Below we describe what the PXIe-6356 card can do, what its limitations are, and what to watch out for when planning a test bench around it. We start with the inputs and outputs, because their ranges determine what can be checked at all.
Analog inputs: what the card can measure
The analog inputs determine how accurately you will see the signal from the device under test. The PXIe-6356 card has 8 analog input channels, simultaneously sampled, with 16-bit resolution. The maximum sampling rate is 1.25 MS/s per channel, and the timing accuracy is 50 ppm of the sampling rate. The available input ranges are ±10 V, ±5 V, ±2 V and ±1 V.
The inputs work only in differential mode (DIFF), and the card does not support SCC carriers. With the card powered on, the input impedance is more than 100 GΩ in parallel with 10 pF. With the card powered off, it is 2 kΩ between AI+ and AI GND and between AI- and AI GND. The common-mode rejection (CMRR) at 60 Hz is given as 75 dB, and the crosstalk between adjacent channels is -80 dB. The input bias current is ±10 pA.
Analog outputs and their limitations
The analog outputs let you apply a known signal to the input under test, for example a control voltage. The card has 2 such outputs with a range of ±10 V, ±5 V or ±external reference. The output current is ±5 mA, so the outputs are for applying control signals, not for powering loads.
The output FIFO holds 8191 samples and is shared by the channels in use. The card supports non-periodic waveforms, periodic waveforms regenerated from the card's FIFO, and periodic waveforms regenerated from the computer buffer, with dynamic updates. The power-on output state is ±5 mV. When the card is powered on and off, a glitch of up to 1.5 V peak appears on the outputs for 200 ms. Connect the device under test to the outputs only when the card is powered, and disconnect it before the card is powered off.
Digital inputs and outputs, counters
I/O modules and drives communicate with digital signals, so this part of the card is the one used most often. The card has 24 digital lines in total: 8 in port P0.<0..7> and 16 PFI lines. Each terminal can be set in software as an input or an output. The input high level is in the range 2.2-5.25 V and the low level in the range 0-0.8 V. The output current is up to 24 mA for P0.<0..7> and up to 16 mA for the PFI lines. Input voltage protection is ±20 V on a maximum of two pins. The card also has input filters (debounce), selectable separately for each input.
The card contains 4 counters/timers with 32-bit resolution. They measure edge counting, pulse, pulse width, semi-period, period and two-edge separation. They also support position measurement from a quadrature encoder, X1, X2, X4. In addition there is a frequency generator with one channel, base clocks of 20 MHz, 10 MHz and 100 kHz, and dividers from 1 to 16. The base clock accuracy is 50 ppm. The FIFO holds 127 samples per counter, and data is transferred by DMA (scatter-gather) or programmed I/O.
| Parameter | PXIe-6356 |
|---|---|
| Analog inputs | 8 simultaneously sampled channels, differential mode only |
| Analog input resolution | 16 bits |
| Maximum sampling rate | 1.25 MS/s per channel |
| Timing accuracy | 50 ppm of the sampling rate |
| Input ranges | ±10 V, ±5 V, ±2 V, ±1 V |
| CMRR (at 60 Hz) | 75 dB |
| Crosstalk between adjacent channels | -80 dB |
| Maximum working voltage of the AI+ input | ±11 V for all ranges |
| Measurement category | Measurement Category I |
| Analog outputs | 2 |
| Analog output range | ±10 V, ±5 V or ±external reference |
| Analog output current | ±5 mA |
| Digital lines | 24 in total, 8 in port P0.<0..7> |
| Counters/timers | 4, 32-bit resolution |
| Power requirement | 4.7 W at +3.3 V and 15.4 W at +12 V |
| +5 V terminal current capability | 1 A max. |
| Module format | x1 PXI Express |
| Slot compatibility | x1 and x4 PXI Express or hybrid slots |
| I/O connector | one 68-pin VHDCI |
On our test bench we use exactly this card to check used frequency inverters, servo drives and I/O modules before sale.
Calibration and software
Measurement results make sense only if the card is in good metrological condition. The recommended warm-up time before measuring is 15 minutes, and the calibration interval is 1 year. The sampling rate of the digital I/O clock is in the range 0-10 MHz. The FIFO holds 2047 samples for digital waveform generation and 255 samples for digital waveform acquisition.
The card has a built-in NI-DAQmx driver. Before buying the card or building a test bench, check in the NI documentation which driver and operating system versions your chassis supports.
What to watch out for when replacing
The following differences and limitations of the PXIe-6356 card are worth checking before you plan a test bench or order a test card.
- Analog input range (PXIe-6356). The maximum working voltage of the AI+ input is ±11 V for all ranges, and the measurement category is I. Mains voltage or the DC link of an inverter must not be connected directly to the card. Such signals require a suitable measurement circuit.
- Analog outputs (PXIe-6356). The output current is ±5 mA. The output is used to apply a signal to the input of the device under test, not to power loads.
- Input mode (PXIe-6356). Only differential mode (DIFF) is supported, without SCC carriers. The wiring of the measurement relative to ground has to be planned so that every signal has a pair of wires.
- Chassis (PXIe-6356). The card is in the x1 PXI Express format and fits x1 and x4 PXI Express or hybrid slots. It cannot be used in a PXI-1044 chassis, because that chassis does not accept PXIe cards. Before buying, check which slots your chassis has.
- Power supply (PXIe-6356). The manufacturer specifies 4.7 W at +3.3 V and 15.4 W at +12 V. Include this in the chassis power budget, especially when other cards are already installed.
- Connector (PXIe-6356). All signals come out through a single 68-pin VHDCI connector. Connecting the device under test therefore requires a separate cable and a connector block.
Beyond the card parameters, the usual checks of a used part apply. Inspect the housing, the heatsink and any signs of overheating. Check the condition of the connectors, contacts and terminal strips. After power-on, watch how the LEDs behave, and read the label on the housing for the firmware version and part number. The most reliable check is a test on the bench under load. Also document the origin of the part and back up the parameters before replacing it.
User experience from the forum
Users report that when testing one of the card's analog outputs, the voltage did not exceed approx. 1.3 V, even though the other channel worked correctly. An NI employee suggested measuring directly at the card, without the connector block. If the result does not change, that channel may be damaged. Practical conclusion: when a result looks suspicious, first rule out the wiring and accessories, and only then assess the card.
General checking procedure
The sequence below is general and does not replace the documentation of the device under test.
- External inspection: housing, heatsink, connectors, terminal strips.
- Backup of the device parameters.
- Connection to the bench according to the device documentation.
- Power-up and first start.
- Test of inputs, outputs and communication.
- Comparison of the measured values with the expected ones.
- Documentation of the result and of the origin of the part.
FAQ
Can the PXIe-6356 card measure resistance?
An NI support employee wrote on the forum that the card is not designed for measuring resistance, and that PXI DMM modules are usually used for such tasks. As a workaround, the employee described supplying the resistor with a constant current from an external source, reading the voltage drop with the card, and calculating the resistance. The card has no internal shunt resistor, so the shunt resistor has to be connected externally.
Can the card measure current?
The forum states that current can be measured indirectly by measuring the voltage across an external shunt resistor. The card's analog outputs are not a current source, so the resistor has to be supplied with voltage. Remember the ±5 mA limit of the analog output and choose the shunt resistor so that this value is not exceeded.
Summary
On the bench the PXIe-6356 gives 8 simultaneously sampled analog inputs, 2 analog outputs, 24 digital lines and 4 counters. Before you start, check that the chassis accepts PXIe cards, and plan for the power of 4.7 W at +3.3 V and 15.4 W at +12 V. Before measuring, make sure the card has a current calibration and that the signals of the device under test fit within ±11 V and the ±5 mA output limit. Order suitable cables and connector blocks, and record the results together with the origin of the part.
If the verification shows that an inverter, servo drive or I/O module needs to be replaced, we will help you choose a suitable replacement from the used, refurbished or new parts available on the market, including brands such as Allen-Bradley (Rockwell Automation), Siemens, Schneider Electric, Bosch Rexroth and ABB. We also specialize in end-of-life (EOL) parts and service, including older-generation and unusual components, and we act as an intermediary for repairs. Contact us at enetiv.com/kontakt.