Analysis of the Application of PXIE Bus in Multichannel High Speed Data Acquisition Streamer Disk System

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1. Introduction

With the development of measurement and control technology and the continuous expansion of its application scope, data acquisition methods and storage technologies have undergone tremendous changes in content and form. In this process, the test requirements of multi-channel, high sampling rate and real-time storage of a large amount of data pose challenges to the existing test system. How to effectively store and analyze a large amount of data in the test process has become the first issue for engineers to consider. This paper introduces the combination of data acquisition card based on PXIE bus and RAID0 disk array. Combining LabVIEW 2009 as the development platform, guided by flexible testing technology, it provides readers with a solution of a data acquisition system with multi-channel, high sampling rate, and real-time storage of a large amount of data.

2. Current situation

At present, multi-channel and high sampling rate data acquisition cards are widely available in the market. Such acquisition cards will generate a large amount of test data instantly. At this stage, the data storage medium is mainly SATA interface hard disk, and SATA interface hard disk is limited by the system bus bandwidth, for example, PCI bus is 133MB/s, PCIe bus is 4GB/s, PXie bus is 6GB/s. Therefore, if we want to build a multi-channel, high sampling rate, real-time data storage system with reliability, adaptability, flexibility and expansibility, the technical indicators of system bus, data transmission mode, system software architecture and hardware are very important.

3. System scheme

In order to build a multi-channel, high sampling rate, real-time data storage test system, our system scheme is described from both software and hardware aspects.

3.1 Hardware system

The hardware platform is based on the PXI of NI. The relatively mature multi-channel, high sampling, and large amount of data storage scheme of the PXI platform is: PXIE data acquisition card+RAID disk array. The highest data throughput of this scheme is 400-600MB/s, which fully meets the construction of such systems and ensures the reliability of system operation.

3.2 Software system

The software platform is based on NI's LabVIEW 20 DAQmx9.0. LabVIEW 2009 provides a new generation of data storage format, TDMS 2.0, with a maximum storage speed of 400MB/s. It uses super adaptability to meet the requirements of a large amount of data generated instantaneously in the process of data collection and ensure the real-time storage of large amounts of data in the system.

3.2.1 Software architecture

The efficiency of the software architecture will directly affect the execution efficiency of the program. Thanks to the multi-core processor and the multithreading operation mechanism of LabVIEW, engineers can use different threads to complete the acquisition, processing, and storage of input signals. By improving the flexibility of the software architecture, the program execution efficiency can be improved to maximize the performance of the PXI system.

3.2.2 Data storage type

Generally, when the sampling rate is small and the amount of data is small, you can choose to read DBL data directly. When the sampling rate is high and the amount of data is large, the measurement data can be read in the format of Raw 1D I16 or 2D I16. Among them, 1D I16 occupies the smallest memory, each sampling point occupies 2 bytes of memory, and each DBL sampling point occupies 8 bytes of memory. Therefore, when the amount of data is large, I16 format can significantly reduce the memory and CPU usage. The two formats have their own advantages and can play their advantages in different situations, making the system super adaptable and flexible.

3.2.3 Data transmission mechanism

Using IRQ or DMA to transmit the collected data can greatly reduce the CPU utilization, thus comprehensively improving the system performance.

3.2.4 Data storage mode

Generally, the buffer allocated by the API function of the system handling file operations is too small, and the performance is low when reading large files. According to the principle of flexibility, API functions that support the "disable buffering" mode can be selected in the programming process to avoid the speed bottleneck of the software streaming disk.
In LabVIEW 2009, functions that support streaming disks in non buffered format include TDMS, binary and Win32 API functions. Turning on Disable Buffer can significantly improve the speed of streaming disks.

On LabVIEW 2009 and DAQmx 9.0 platforms, you can use DAQmx TDMS2.0 to integrate the streaming disk module DAQmx Configure Logging (TDMS),

This API function is more efficient, simple to program, and takes up less system resources. It directly transfers the calibration information of the original data and channels from the board card to the hard disk via DMA, with a maximum speed of 1.2GB/s. In the process of acquisition, if it is not necessary to read the sampling value of the channel, the "Operation" of this module can be selected as "Record", so that the sampling value will be directly stored as a TDMS 2.0 file. Such flexible and efficient storage mode appropriately explains the flexibility required by flexible testing technology.

4. Project cases

A test system can synchronously sample 64 channels of signals in the system, which can be saved in real time at a maximum sampling rate of 3.3MS/s (all channels), and can also display and save real-time data at 300KS/S (all channels). For PXI hardware system, NI's 18 slot 3U PXIe-1075 chassis, PXIe-8130 controller, PXIe-6124, etc., PXIe-6124 synchronous sampling rate of 4MS/s per channel, 16 bit resolution, and PXie interface of bandwidth from one exclusive module to the controller are selected; The RAID 0 disk array composed of NI-8262 and HDD-8264 is selected as the data storage device, and the maximum data throughput can reach about 600MB/s.

At the same time, the system improves the channel trigger function for users, including analog trigger, digital edge trigger, analog reference trigger, etc., to meet various needs in different situations. In addition, the software adopts the state machine framework structure and modular programming style, which facilitates the revision and management of software functions and fully realizes the flexibility and expansion required by flexible testing technology.

The data acquisition function is encapsulated as a function module, and the measured data is saved to the hard disk in real time through DAQmx Configure Logging (TDMS) VI, which reduces the utilization of CPU and memory, and improves the execution efficiency of the program. The specific implementation method is shown in Figure 4.

5. Conclusion

This paper describes the implementation method of multi-channel, high sampling and real-time data storage and the matters needing attention. Based on a large number of data and a system example, it deeply analyzes the reliability, adaptability, flexibility and expansibility of multi-channel high-speed data acquisition. By introducing the data storage format of NI TDMS, it provides a reference for engineers to develop a multi-channel, high sampling rate, large amount of real-time data storage test system.

keyword: PXIE bus High speed data acquisition Streaming disc system Reference address: Analyze the Application of PXIE Bus in Multi channel High speed Data Acquisition Streaming Disk System

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Recommended reading Latest update time: 2024-05-02 23:26

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