PXI-6281 18-Bit 625 kS/s Multifunction PXI DAQ Module
The PXI-6281 is a high-accuracy M Series PXI multifunction data acquisition module for laboratory automation, research, design validation and production testing. It combines precision analog input, analog output, digital I/O, counter/timer and triggering functions in a single 3U PXI module.
The module provides 16 single-ended or eight differential analog inputs with 18-bit resolution. It supports a maximum single-channel sampling rate of 625 kS/s and an aggregate multichannel rate of 500 kS/s.
The PXI-6281 also includes two 16-bit analog outputs, 24 digital I/O lines and two 32-bit counter/timers. Its combination of resolution, flexible input ranges and multifunction I/O makes it suitable for precision voltage measurement and automated mixed-signal testing.
Key Features of the PXI-6281
- 16 single-ended analog input channels
- Eight differential analog input channels
- 18-bit analog input resolution
- 625 kS/s maximum single-channel sampling rate
- 500 kS/s maximum aggregate multichannel rate
- Programmable input ranges from ±100 mV to ±10 V
- Two 16-bit analog voltage outputs
- Up to 2.86 MS/s analog output update rate
- 24 TTL/CMOS-compatible digital I/O lines
- Hardware-timed correlated digital I/O
- Two 32-bit general-purpose counter/timers
- Analog and digital triggering
- Six DMA channels
- PXI trigger and clock synchronization
- One 68-pin VHDCI front-panel connector
- SCXI and SCC signal-conditioning compatibility
- NI-DAQmx software support
PXI-6281 Technical Specifications
| Product Model | PXI-6281 |
|---|---|
| Part Number | 779121-01 |
| Product Family | NI M Series high-accuracy multifunction DAQ |
| Product Type | PXI multifunction I/O module |
| Single-Ended Analog Inputs | 16 |
| Differential Analog Inputs | 8 |
| ADC Resolution | 18 bits |
| Maximum Single-Channel Sample Rate | 625 kS/s |
| Maximum Multichannel Sample Rate | 500 kS/s aggregate |
| Analog Input Sampling | Multiplexed, nonsimultaneous |
| Input Coupling | DC |
| Analog Input Ranges | ±0.1 V, ±0.2 V, ±0.5 V, ±1 V, ±2 V, ±5 V and ±10 V |
| Maximum Analog Input Level | ±11 V relative to AI GND |
| Analog Output Channels | 2 voltage outputs |
| DAC Resolution | 16 bits |
| Maximum Analog Output Update Rate | 2.86 MS/s with one channel |
| Analog Output Range | ±10 V |
| Digital I/O | 24 lines |
| Hardware-Timed Digital I/O | 8 lines on Port 0 |
| Maximum Digital Waveform Rate | 10 MHz, system dependent |
| Counters/Timers | 2 × 32-bit |
| DMA Channels | 6 |
| Triggering | Analog and digital |
| PXI Synchronization | PXI_TRIG, PXI_STAR and PXI_CLK10 |
| Front-Panel Connector | 1 × 68-pin VHDCI |
| Module Format | 3U, single-slot PXI module |
| Bus Interface | PXI hybrid compatible |
| Software Driver | NI-DAQmx |
| NI Suggested Replacement | PXIe-4309 for precision analog input applications |
| Availability Status | Discontinued by NI |
PXI-6281 Part Number and Accessories
| Item | Part Number | Description |
|---|---|---|
| PXI-6281 | 779121-01 | 18-bit, 16-channel high-accuracy PXI multifunction DAQ module |
| SHC68-68-EPM Cable | Varies by length | Shielded 68-pin VHDCI cable for connecting the module to a terminal accessory |
| SCB-68A | 782536-01 | Shielded screw-terminal block for custom analog and digital wiring |
| CB-68LP | 777145-01 | Low-profile 68-pin connector block with screw terminals |
| BNC-2120 | 779014-01 | Shielded BNC terminal accessory for laboratory signal connections |
| PXIe-4309 | Verify current configuration | NI-recommended precision PXI Express analog input replacement |
The PXI-6281 uses one 68-pin VHDCI front-panel connector. Select the terminal block according to the required shielding, connector layout, signal type and fixture design.
The selected cable must be compatible with VHDCI M Series DAQ hardware. Inspect the connector carefully before installation because the high-density pins can be damaged by misalignment or excessive force.
18-Bit High-Accuracy Analog Input
The PXI-6281 uses an 18-bit analog-to-digital converter to provide greater measurement resolution than conventional 16-bit multifunction DAQ modules.
Across a nominal ±10 V range, the 20 V input span is represented by 262,144 ADC codes. Smaller programmable input ranges provide finer theoretical voltage increments for low-level signals.
Actual accuracy also depends on gain error, offset error, noise, temperature, calibration and source impedance. ADC resolution should therefore be considered together with the complete accuracy specification.
16 Single-Ended or Eight Differential Inputs
The module can be configured for as many as 16 single-ended analog inputs or eight differential inputs. The selected measurement mode depends on the signal source, grounding and noise environment.
Single-ended mode provides the maximum channel count and is appropriate for short connections to ground-referenced sources with compatible ground potential.
Differential mode measures the voltage between two input conductors. It generally provides better rejection of common-mode noise and is preferred for floating sources, low-level signals and longer cables.
625 kS/s Single-Channel Sampling
The PXI-6281 supports a maximum sampling rate of 625 kS/s when acquiring one analog input channel. This provides sufficient speed for many laboratory, industrial and automated test signals while maintaining 18-bit converter resolution.
The module uses a multiplexed input architecture. It does not digitize every analog input simultaneously, so a short time interval exists between conversions of adjacent channels.
500 kS/s Multichannel Sampling
When scanning multiple analog inputs, the PXI-6281 supports an aggregate rate up to 500 kS/s. This rate is shared across all channels in the scan list.
For example, eight channels sampled at equal rates can each be acquired at an average rate up to approximately 62.5 kS/s before considering settling requirements and application overhead.
High source impedance or large voltage changes between adjacent channels can require a slower conversion rate to allow the multiplexer and input amplifier to settle accurately.
Programmable Input Ranges
The PXI-6281 provides bipolar input ranges of ±100 mV, ±200 mV, ±500 mV, ±1 V, ±2 V, ±5 V and ±10 V. Selecting a range close to the expected signal amplitude improves effective use of the ADC span.
The selected range must include signal offset, expected noise and transient conditions. Do not apply voltages beyond the module’s maximum working and overvoltage protection specifications.
Low-Level Voltage Measurement
The ±100 mV range and 18-bit ADC make the PXI-6281 suitable for conditioned low-level voltage signals. Applications can include bridge-amplifier outputs, laboratory sensors and precision device-characterization measurements.
For very small signals, use shielded twisted-pair wiring and differential inputs. Temperature gradients, thermoelectric voltages and external noise can become significant compared with the measured signal.
Direct connection of thermocouples, strain gauges and other specialized sensors may require external excitation, amplification, filtering, linearization or cold-junction compensation.
Two 16-Bit Analog Outputs
The PXI-6281 includes two 16-bit analog voltage outputs for generating test stimuli, control signals, setpoints and waveforms.
A single output channel supports update rates up to 2.86 MS/s. When both channels are used, the available update performance depends on the selected hardware-timed configuration.
The outputs are intended for voltage signals and limited-current loads. Use a suitable external buffer or power amplifier when the load requires more current than the DAQ module can supply.
24 Digital I/O Lines
The module provides 24 programmable digital I/O lines arranged as three 8-bit ports. Individual lines can be used to monitor logic signals or control external devices through suitable interface circuitry.
Applications include reading switches, controlling multiplexers, monitoring fixture status, enabling equipment and communicating logic states with a device under test.
The lines provide TTL/CMOS logic and should not be connected directly to 24 V industrial signals. Use external level conversion or an industrial digital I/O module when higher field voltages are involved.
Correlated Digital I/O
The eight lines on Port 0 support hardware-timed correlated digital input and output at rates up to 10 MHz, subject to system and bus performance.
Correlated digital operations can share timing and triggering with analog input, analog output and counter tasks. This enables synchronized acquisition of analog and digital test data.
Ports 1 and 2 are primarily used for static digital I/O and programmable timing interface functions.
Two 32-Bit Counter/Timers
The PXI-6281 includes two general-purpose 32-bit counter/timers. They support edge counting, frequency measurement, period measurement, pulse-width measurement, two-edge separation and pulse generation.
The counters can also be used with supported quadrature encoders for position and direction measurements. Counter signals are accessed through programmable-function interface terminals on the front connector.
Analog and Digital Triggering
Analog triggering allows an acquisition to respond when a voltage crosses a selected level or enters or leaves a defined window. Digital triggering uses a logic transition or internally routed event.
Hardware triggers improve timing repeatability compared with starting an operation only through software. They are useful for transient recording, device validation and synchronized stimulus-response tests.
PXI Timing and Synchronization
The PXI-6281 can route clocks and trigger signals through the PXI chassis backplane. Supported resources include PXI trigger lines, PXI_STAR and the PXI reference clock.
Multiple modules can share a trigger or reference clock for coordinated acquisition and generation. The onboard phase-locked loop can synchronize measurement timing to the PXI chassis clock.
Six DMA Channels
The module provides six DMA channels that can be used by analog input, analog output, digital input, digital output and the two counter/timer subsystems.
DMA transfers data between the module and host memory with limited processor intervention. This allows several hardware-timed tasks to run concurrently while preserving host resources for test execution, analysis and logging.
SCXI and SCC Signal Conditioning
The PXI-6281 can be used with compatible SCXI or SCC signal-conditioning hardware to add amplification, filtering, isolation and sensor-specific measurement capabilities.
External signal conditioning can support thermocouples, strain gauges, accelerometers and higher-voltage signals. The conditioned output must remain within the PXI-6281 input and working-voltage limits.
NI-DAQmx Software Support
The PXI-6281 is controlled with the NI-DAQmx driver. NI-DAQmx provides functions for configuring channels, sample clocks, triggers, digital ports, analog outputs and counters.
Supported programming environments include LabVIEW, LabWindows/CVI and compatible C, C++, C# and Python applications. NI Measurement & Automation Explorer can identify the module, manage calibration information and run basic test panels.
Because the PXI-6281 is discontinued, verify compatibility between the module, operating system, PXI controller and NI-DAQmx version before installation.
Typical PXI-6281 Applications
- Precision voltage measurement
- Automated production testing
- Laboratory data acquisition
- Design verification and validation
- Low-level signal measurement
- Sensor acquisition with external conditioning
- Mixed-signal functional testing
- Electronic component characterization
- Research and educational systems
- Data logging
- Frequency and pulse measurement
- SCXI-based measurement systems
Precision Voltage Data Logging
The 18-bit ADC and flexible input ranges make the PXI-6281 suitable for logging voltage signals where measurement detail is more important than very high channel density.
Long-duration applications should use a sample rate appropriate for the signal bandwidth. Lower sampling rates reduce data-storage requirements and may improve rejection of unwanted high-frequency noise when combined with suitable filtering.
Automated Production Testing
In a production test system, the PXI-6281 can measure device voltages, generate analog stimuli, monitor digital states and perform counter measurements.
PXI triggering and DMA transfers support repeatable automated sequences. The module can be combined with switching, power and measurement instruments to create a complete functional test system.
Research and Design Validation
The module can acquire precision voltage signals while its analog outputs provide control levels or test stimuli. Digital I/O can control fixture states, and counters can measure frequency or pulse signals.
These capabilities reduce the number of separate instruments required for experimental systems and early design-validation fixtures.
PXI-6281 Troubleshooting
The PXI-6281 is not detected in NI MAX
Confirm that the module is fully installed in a compatible PXI or hybrid slot. Check chassis power, controller communication and NI-DAQmx installation, then restart the system and rescan the chassis.
Analog readings are noisy
Use shielded twisted-pair wiring and differential input mode where appropriate. Separate low-level signals from digital, motor and mains wiring, and check the grounding arrangement.
A floating signal gives unstable readings
Provide an appropriate bias-current return path between the input and analog ground. A floating source without a return path can drive the input amplifier outside its valid common-mode range.
Measurements change when additional channels are scanned
Reduce the channel-conversion rate and allow more settling time. High source impedance and large voltage differences between consecutive channels can increase multiplexing errors.
The actual rate is lower than 625 kS/s
The 625 kS/s specification applies to single-channel acquisition. The maximum aggregate rate for multichannel scanning is 500 kS/s and must be divided across the channels in the task.
An analog output does not reach the requested voltage
Check load resistance, current demand and external wiring. Test the output without the load to determine whether excessive loading is causing the voltage drop.
A digital line cannot control an industrial load
The digital lines provide logic-level signals and cannot directly switch 24 V or high-current loads. Add a compatible relay driver, transistor interface or isolated industrial digital module.
A hardware trigger does not start the acquisition
Verify the selected trigger source, polarity, threshold and terminal route. Confirm that the task has been started and is waiting for the trigger before the event occurs.
Counter measurements are incorrect
Check the PFI terminal assignment, signal level, grounding and counter mode. Verify whether the application requires edge counting, frequency, period, pulse-width or encoder measurement.
PXI-6281 Comparison with Similar DAQ Modules
| Model | Primary Configuration | Best Suited For |
|---|---|---|
| PXI-6280 | 16 AI, 18-bit, 625 kS/s, no AO | Precision analog acquisition without output generation |
| PXI-6281 | 16 AI, 2 AO, 24 DIO, 18-bit | Precision multifunction voltage measurement and control |
| PXI-6284 | 32 AI, 48 DIO, 18-bit | Higher input and digital channel density |
| PXI-6289 | 32 AI, 4 AO, 48 DIO, 18-bit | High-accuracy systems requiring more multifunction I/O |
| PXIe-4309 | 32 AI, flexible resolution up to 28 bits | Modern high-accuracy PXI Express analog acquisition |
PXI-6281 vs PXI-6280
Both modules provide 16 analog inputs with 18-bit resolution and a maximum single-channel sampling rate of 625 kS/s. The PXI-6281 adds two analog outputs and additional multifunction capabilities.
Choose the PXI-6280 when only precision acquisition is required. Select the PXI-6281 when the test system must also generate analog stimulus or control voltages.
PXI-6281 vs PXI-6289
The PXI-6281 provides 16 analog inputs, two analog outputs and 24 digital I/O lines. The PXI-6289 doubles the analog input count to 32 and provides four analog outputs and 48 digital I/O lines.
Select the PXI-6281 for lower-channel-count precision systems. Choose the PXI-6289 when additional analog and digital channels are more important than minimizing module cost and wiring complexity.
PXI-6281 vs PXI-6251
The PXI-6281 provides 18-bit analog input resolution and sampling up to 625 kS/s. The PXI-6251 provides 16-bit resolution but supports a higher maximum single-channel rate of 1.25 MS/s.
Select the PXI-6281 when measurement resolution and low-level accuracy are priorities. Choose the PXI-6251 when greater acquisition speed is more important.
PXI-6281 vs PXIe-4309
NI identifies the PXIe-4309 as the suggested replacement for precision analog input applications. The PXIe-4309 provides 32 analog input channels, flexible resolution up to 28 bits and higher acquisition performance.
The PXIe-4309 is not a direct functional replacement for every PXI-6281 application because it does not reproduce the same analog output, digital I/O and counter configuration. Separate modules may be required to replace those functions.
Recommended Related Products
- PXI-6280 18-Bit Precision Analog Input Module
- PXI-6284 32-Channel High-Accuracy DAQ Module
- PXI-6289 High-Accuracy Multifunction DAQ Module
- PXI-6251 High-Speed Multifunction DAQ Module
- PXIe-4309 High-Accuracy Analog Input Module
- View More NI PXI and PXI Express Modules
Selecting the Right High-Accuracy DAQ Module
Choose the PXI-6281 when the application requires 18-bit precision analog input together with two analog outputs, digital I/O and counter/timer functions.
Select the PXI-6280 when analog outputs are unnecessary, the PXI-6284 for additional input density or the PXI-6289 for more analog outputs and digital lines. Consider the PXIe-4309 when building a new precision PXI Express acquisition system.
Why Choose the PXI-6281?
- Provides 18-bit analog input resolution
- Supports 16 single-ended or eight differential inputs
- Offers sampling rates up to 625 kS/s
- Includes low-level ranges down to ±100 mV
- Provides two 16-bit analog outputs
- Includes 24 digital I/O lines
- Supports two 32-bit counter/timers
- Provides analog, digital and PXI triggering
- Uses six DMA channels for concurrent tasks
- Integrates with NI-DAQmx and LabVIEW
Frequently Asked Questions
What is the PXI-6281?
The PXI-6281 is an 18-bit M Series PXI multifunction DAQ module with analog input, analog output, digital I/O and counter/timer capabilities.
What is the PXI-6281 part number?
The standard NI part number for the PXI-6281 is 779121-01.
How many analog inputs does it provide?
The module provides 16 single-ended analog inputs or eight differential analog inputs.
What is the maximum analog input sampling rate?
The maximum single-channel sampling rate is 625 kS/s. Multichannel acquisition supports an aggregate rate up to 500 kS/s.
Does the PXI-6281 sample every channel simultaneously?
No. It uses a multiplexed ADC, so the selected channels are converted sequentially rather than at exactly the same instant.
What analog input ranges are supported?
The module supports bipolar ranges from ±100 mV through ±10 V, including ±0.2 V, ±0.5 V, ±1 V, ±2 V and ±5 V.
Does it provide analog outputs?
Yes. The PXI-6281 includes two 16-bit analog voltage outputs with a maximum single-channel update rate of 2.86 MS/s.
How many digital I/O lines are included?
The module provides 24 digital I/O lines. Eight Port 0 lines support hardware-timed correlated digital waveform operations.
Which connector does the module use?
The PXI-6281 uses one 68-pin VHDCI front-panel connector.
Which software controls the PXI-6281?
The module is controlled through NI-DAQmx and can be programmed with LabVIEW, LabWindows/CVI and compatible text-based environments.
What is the recommended replacement for the PXI-6281?
NI recommends the PXIe-4309 for modern high-accuracy analog input applications. Additional modules may be required to replace the PXI-6281’s analog output and digital functions.
Request a Quote for the PXI-6281
Contact us for current availability, condition, lead time and project pricing for the PXI-6281 18-Bit 625 kS/s Multifunction PXI DAQ Module. We can also help identify compatible VHDCI cables, terminal blocks, PXI chassis and modern high-accuracy replacement modules.
