Differences between an NI HSDIO-Based and a Programmable FPGA- Digital Instrument


In the past, when facing a complicated test requirement, engineers used an easily available instrument that somewhat met their needs and made the instrument fit their applications or tests. If such an instrument was not available or was not flexible enough, engineers had to build a tester themselves, from scratch, which may have been a more cost-effective but also time- and effort-consuming process. NI now provides options to better suit individual application needs. On one hand, the NI high-speed digital I/O (HSDIO) platform features instrument-class devices set up perfectly for applications such as semiconductor test, where it is important to be able to sweep parameters such as data deskew for characterization of chips. On the other hand, NI FlexRIO instruments provide a programmable FPGA, which is ideal for emulating master and slave implementations of protocols, real-time decision-making and closed loop tests. Eventually, the application dictates which method to use, nevertheless, this paper discusses the unique benefits of each approach to help engineers make the correct decision when using a digital instrument from NI. This paper focuses on NI HSDIO and NI FlexRIO features and applications.





NI HSDIO devices are instrument-class, which means they include detailed specifications that an engineer might expect on comparable high-quality instruments. These devices typically offer features such as eye diagrams, skew specifications, voltage threshold accuracy, and jitter. Figure 1 shows an eye diagram included with the NI PXIe-6548 high-speed digital module.

Figure 1. Eye Diagram of the NI PXIe-6548 at 200 Mbit/s

NI-HSDIO Driver Built-In Features

The NI HSDIO driver provides several built-in features for advanced testing, such as data delay for timing calibration and hardware compare for bit error rate testing. Some of these features are listed below.


NI HSDIO instruments provide advanced timing capabilities that offer not only general-purpose digital pattern I/O at finer resolutions but also the capability to characterize semiconductor chips by sweeping timing parameters such as clock frequencies or data deskews. Some of the included features are

  • Data delay on data lines, clocks, and trigger/event lines 1
  • Source- or system-synchronous clocking
  • DDS-based internal clock source with sub-Hz resolution (Gundog and Retriever)
  • RTD compensation (Lab and Gundog)

1The NI PXIe-6547 and NI PXIe-6548 feature three banks of data delay, where independent values of deskew can be applied to three banks of physical channels.

      Comparison and Waveform Control

NI HSDIO devices feature built-in comparison features and additional logic states that enable bit error rate testing and compatibility with semiconductor standard patterns and files such as WGL and STIL. With the hardware compare features, engineers can define generation and comparison patterns for stimulus-response tests where the HSDIO device performs a real-time analysis of the response data and reports errors to the user.

These devices also feature a complex scripting engine that engineers can use to loop and link prestored digital patterns with “zero dead time” or no lag between consecutive waveforms. They also can use scripting to perform analysis statements such as IF and WAIT for applications that require the option of choosing between multiple preloaded waveforms or other complex waveform manipulations.

Some of the comparison features are

  • Advanced logic states: 1, 0, Z, H, L, X 1
  • Onboard hardware compare 1

1 NI 655x and NI 6544/45/47/48 only

2 NI 656x and NI 6547/48 only


      Voltage Levels

NI 655x devices feature enhanced voltage ranges and programmability for tests that require nonstandard logic levels or for tests such as voltage threshold testing and diode testing. These features are ideal for semiconductor characterization or general-purpose digital I/O. These voltage-specific features include

  • -2 to 5.5 V
  • 10 mV programmability
  • Programmable high and low levels


NI HSDIO devices feature a synchronization and memory core (SMC) daughter card that allows multiple devices to be synchronized within hundreds of picoseconds. The NI-TCLK API allows intuitive and simple synchronization, making channel expansion for analog or digital instruments trivial. Built-in features also include programmable skew between synchronized instruments, offering enhanced channel-to-channel and board-to-board deskew control. The image below shows three simple VIs that engineers can use to synchronize multiple mixed-signal instruments.


The NI Digital Waveform Editor is a graphical software tool engineers can use to visualize digital signals and easily create, edit, and modify digital waveforms for customized interfacing and test applications.

Figure 2. Tools such as the Script Editor and Digital Waveform Editor make creating and generating waveforms with driver-defined devices easy.

Using a configurable environment such as NI LabVIEW SignalExpress, engineers can set up and implement a driver-defined digital device with little or no programming to acquire and generate data, log data, perform analysis on acquired data, and generate reports.

TD-Scan for NI is software engineers can use to import standard semiconductor pattern files such as waveform generation language (WGL) and standard test interface language (STIL). TD-Scan for NI software as well as NI HSDIO hardware features such as hardware compare and extended digital states make the process of taking logic and patterns from standard digital pattern files and generating them on hardware simpler.




The programmability of voltage and timing parameters on NI HSDIO devices makes them ideal for chip characterization, where sweeping these parameters is required to characterize chips completely. Figure 3 shows the front panel of a LabVIEW VI, where the clock rate is swept from a few megahertz to 200 MHz to characterize the behavior of the digital-to-analog converter (DAC) under these varying clock rates. Similarly, deskew or voltage can also be swept for further chip characterization.

Figure 3. This application characterizes a digital-to-analog converter for dynamic and linear parameters.

      General-Purpose Interfacing

Advanced features such as programmable deskew and voltages make NI HSDIO devices suitable for characterization, but the feature-rich driver also provides an out-of-the-box experience for engineers needing a general-purpose interface to a variety of devices under test (DUTs). The NI HSDIO platform is ideal for general-purpose digital pattern generation, video or image signal generation and acquisition, or logic analysis. NI also provides ready-made solutions for digital audio and video testing called NI Video Measurement Suite (VMS), NI Picture Quality Analysis (PQA), and NI AudioMASTER.


Why NI FlexRIO?


Peer-to-Peer Streaming

With NI peer-to-peer data streaming technology, engineers can continually transfer data to and from PXI Express NI FlexRIO FPGA modules at rates greater than 800 MB/s with minimal latency. High-performance data switches on NI PXI Express chassis offer high-bandwidth communication, while routing data from one module directly to another (without transferring data through the host controller) minimizes the latency of the transfer.

Peer-to-peer transfers are supported between multiple PXI Express NI FlexRIO FPGA modules and between select NI PXI Express digitizers and PXI Express NI FlexRIO FPGA modules. Figure 3 depicts a peer-to-peer system with an NI PXIe-5122 digitizer and two NI PXIe-7965R NI FlexRIO FPGA modules for distributed serial data processing.

Figure 4 – Data can be to and from and instrument to an FPGA board for on the fly processing. 

With the LabVIEW FPGA Module, engineers can access these peer-to-peer streams through simple first-in-first-out (FIFO) nodes. An easy-to-use API on the host controller sets up a peer-to-peer stream between multiple NI FlexRIO FPGA modules after configuring the peer-to-peer FIFOs.

Figure 5. The NI-P2P driver offers simple, high-level access to the high-performance capabilities of peer-to-peer streaming, while intuitive nodes on the FPGA block diagram simplify data transfer.

IP Integration Node

While LabVIEW is an effective tool for FPGA programming, engineers may have existing hardware description language (HDL) intellectual property (IP) that they need to integrate into NI FlexRIO hardware applications. The HDL Interface Node, shown in Figure 6, provides a simple, inline interface to HDL code.

Figure 6. With the IP Integration Node, engineers can import existing IP into NI FlexRIO devices.



      Digital Protocol Test

Consider the lowest layers of the OSI model – the physical and data link layers. Rather than designing this new bus from scratch, engineers often start with the physical or electrical attributes of an existing bus technology and then customize the higher-level functionality. The physical layer comprises the transmission medium, such as channels, voltages, and connectivity, and the data link layer acts as the functional means of transferring data over the physical layer, including signaling and flow control. Two distinct and incompatible bus architectures may share a physical layer, but they can have very different data link layers.

Decoupling the physical layer from the data link layer is one approach to minimize the amount of custom hardware in communication protocol or bus test systems. FPGAs are ideal for implementing protocols at the data link layer, though their I/O pins may not be optimized for the desired physical layer. This physical layer likely requires additional silicon, perhaps similar to that used in the DUT. With a modular connection between the FPGA and the physical layer over a standard interface, a single FPGA can be programmed to implement a variety of protocols, while various physical layers can be swapped in and out through the standard interface. NI FlexRIO, which provides user-programmable FPGA modules combined with interchangeable I/O adapter modules, is an ideal platform for decoupling the physical and data link layers. Engineers can implement protocols on the FPGA in the abstract, graphical LabVIEW FPGA programming environment, which requires no specialized knowledge of low-level HDLs. I/O adapter modules can be purchased or customized through a module development kit. For example, National Instruments offers the NI 6584 RS422 NI FlexRIO adapter module for RS422 physical layer test. Data link levels such as high-level data link control (HDLC) or synchronous data link control (SDLC) can be implemented on the FPGA, whereas the physical layer is represented by an adapter module such as the NI 6584.

      Custom Tests/Triggering

Many applications require custom algorithms. A very simple example of this is the implementation of a custom serial trigger, which may be challenging to implement on a logic analyzer that has a fixed trigger personality. This logic analyzer might have parallel triggers or an 8-bit serial trigger, but a specific application may require a 16-bit trigger or even a trigger that can dynamically change. Implementing these custom triggers is not easy, if not impossible, on traditional boxed instruments such as logic analyzers or pattern generators. An FPGA in this case becomes an ideal solution because engineers can program the FPGA to their custom requirements, whether it is a 16-bit serial trigger or even a complex video trigger.

Other applications that require such customized algorithms include complex bit error rate testing, where engineers not only require the ability to compare digital data on the fly but also inject errors or implement error correction algorithms such as hamming encoding. Again, in this situation, NI FlexRIO is an ideal solution because engineers can program the reconfigurable FPGA to perform bit error rate tests, dynamically inject errors, or implement custom error correction algorithms all using one piece of hardware.

      Applications Requiring Multiple Time Domains

Several applications require more than one clock, or even completely independent time domains. A traditional boxed instrument usually has one high-frequency timebase. Any acquisition or generation patterns need to be clocked at a derivative of that master timebase. Although this approach has solved most application challenges in the past, it has been an unacceptable workaround for some. NI FlexRIO provides an ideal environment to solve this problem.

Both NI LabVIEW and FPGAs are inherently parallel, so the combination of the two makes it simple to implement parallel loops running in different time domains. Consider an application that requires a digital I/O instrument to generate a single port of SPI communication at 5 MHz and to acquire 10 channels of TTL digital data at 100 MHz for a finer resolution on acquisition. Traditionally, it was possible to use just a 100 MHz clock for both the generation and acquisition and generate a sample at every 20 edges of the clock to get a 5 MHz effective generation pattern. However, if the frequency of the desired generation pattern was changed to 4.7 MHz, the above-mentioned implementation is difficult. If engineers use an FPGA-based approach, they can isolate the generation and acquisition loops – in other words, they can implement the generation loop at exactly 4.7 MHz and the acquisition loop at 100 MHz while keeping them completely independent of each other. Moreover, engineers can use a combination of external and internal clocks to achieve more flexibility.

Table 1 shows the main differences between some of the NI HSDIO and programmable FPGA digital instruments.


Feature NI 6583 NI 6581 NI 6585 NI PXIe-6544/45 NI PXIe-6547/48 NI 6561/62
Number of single-ended channels 35 (32 + 3 PFI) 54 (7 ports) - 32 + 4 PFI 32 + 4 PFI -
Number of LVDS channels 19 (16 + 3 PFI) - 35 (32 + 3 PFI) - - 16 (8 in DDR mode) + 4 PFI
Single-ended voltage levels 1.2 to 3.3 V with 10-bit resolution 1.8, 2.5, and 3.3 V selectable or 1.8 to 5.5 V external reference - 1.2, 1.5, 1.8, 2.5, and 3.3 V 1.2 to 3.3 V with 100 mV resolution -
Form factor NI FlexRIO NI FlexRIO NI FlexRIO PXI Express PXI Express PXI only
Maximum data rate 300 Mbit/s 100 Mbit/s 300 Mbit/s 100/200 Mbit/s 200 / 400 Mbit/s 400 Mbit/s
Bidirectionality Per channel with 10 ns turnaround time Per port, deterministic Per channel, deterministic Per channel, software-controlled Per channel per cycle Per channel, software controlled
Open FPGA Yes Yes Yes No No No
Data delay No - can be implemented in FPGA/HDL No - can be implemented in FPGA/HDL No - can be implemented in FPGA/HDL 1 bank 3 banks 1 bank
Hardware compare No – needs to be implemented in FPGA No – needs to be implemented in FPGA No – needs to be implemented in FPGA No 24 channels No
Scripting No – needs to be implemented in FPGA No – needs to be implemented in FPGA No – Needs to be implemented in FPGA Yes Yes Yes
TCLK support No No No Yes Yes Yes
Digital Waveform Editor support No No No Yes Yes Yes
WGL/STIL file importing No No No Yes Yes Yes
High-resolution clock No No No Yes Yes Yes
LabVIEW SignalExpress support No No No Yes Yes Yes
Maximum streaming rate 800 MB/s with PXI Express for NI FlexRIO 800 MB/s with PXI Express for NI FlexRIO 800 MB/s with PXI Express for NI FlexRIO 660 MB/s 660 MB/s 110 MB/s
Connectivity Best (compatible with HSDIO) Best (compatible with HSDIO) Good (not compatible with HSDIO connectors) Best Best Best
Clock importing and exporting Yes Yes Yes Yes Yes Yes
Multiple time domains Yes Yes Yes High-res clock/ oversampling High-res clock/ oversampling Divide-down clock/ oversampling
IP importing capabilities Yes Yes Yes No No No


Table 1. Product Comparison Table


Programming a Simple Digital Generation Task in LabVIEW


Figure 7 shows how to use the NI PXIe-6548 HSDIO stimulus-response instrument for the generation of a 1,000-sample, 16-bit waveform toggling at every clock edge.

Figure 7. Generating a Digital Pattern Using the NI-HSDIO API in LabVIEW

>> To learn more about programming with the NI-HSDIO API, view “Getting Started with NI-HSDIO API

Programmable FPGA

Figure 8 shows how to use the NI 6581 digital adapter module for NI FlexRIO to generate a simple digital pattern on 16 bits at a clock rate of 100 MHz. Engineers can pick the FPGA target from the LabVIEW Project Explorer as well as define a host VI that runs on the PC and an FPGA VI that runs on the target FPGA. DMA calls from the host VI to the target FPGA VI control the transfer of data to and from the host and FPGA target. Engineers also can use FIFOs and DRAM on the FPGA target to store data that is collected or needs to be generated, thus nearly eliminating the DMA transfer constraints such as latency and bandwidth.

Figure 8. Generating a Digital Pattern with NI FlexRIO Using NI LabVIEW

Note: Please see NI 6581 shipping examples for complete codes.

Multiple Options for Digital Test

NI HSDIO and programmable FPGA digital instruments satisfy two very distinct sets of requirements. NI HSDIO devices are ideal for instrument-like performance during which engineers might need to tweak physical parameters such as timing or voltage (for example, in a characterization lab). With programmable FPGA-based instruments, engineers can import existing HDL IP, implement custom tests (such as bit error injection), and emulate protocols – all in a flexible and reconfigurable manner. Engineers should expect better physical specifications from NI HSDIO devices such as eye diagrams and channel-to-channel skew and jitter, among other parameters. And they should expect higher flexibility, IP integration capabilities, and protocol emulation capabilities from a programmable FPGA -based digital instrument.