USB is the standard peripheral interface for PCs and has billions of installations. Since the emergence of USB data acquisition instruments in 1998, digital oscilloscopes and other micro-USB instruments have been introduced into the market, forming a family of instruments with their own characteristics, and began to enter the mainstream of measuring instruments.

introduction

In 1993, Intel's fifth-generation microprocessor Pentium came out. It used a PCI bus of 32 bits, a clock of 33 MHz, and a bandwidth of 132 MB/s to 264 MB/s. In 1995, in order to solve the bottleneck of the traditional peripheral bus of PC, it could not adapt to the bottleneck of Pentium chip. The general-purpose serial bus (USB) was introduced by PC supplier Compaq, DEC, IBM and software company Microsoft, and the USB implementation forum (USB-IF) was established. ), began to promote and certify USB. The USB 1.1 version has a low rate of 1.5 Mbps and a full-speed data rate of 12 Mbps. The main controller divides the bus transmission time into frames, each frame is 1 ms, and transmits multiple transactions to multiple devices within the frame time. The mechanical connection of the USB bus is very simple. The cable is a 4-core shielded cable. One pair of twisted pairs (D+, D-) transmits signals, and the other pair of twisted pairs (VBUS, power ground) transmits +5V DC voltage; There are two types of A (or small A) and B (or small B). Plug-and-play (ie, hot-swap) of USB devices is an advantage. For the first time a user inserts a USB device, the device can be used by manually or automatically installing the driver. A USB host controller port can connect up to 127 devices with a distance of no more than 5 meters between devices. In 2000, the USB 2.0 version of the specification was introduced, which increased the data rate of the USB bus to 480 Mbps and was backward compatible. On the high-speed USB bus, the main controller divides each frame into 1 micro-frames for 1ms, and each micro-frame 125μs can complete more transactions, increasing the total data rate by 40 times.

The USB interface specification classifies a variety of PC peripherals into similar categories. For example, a mouse, a keyboard, etc. belong to the HID class, the audio products belong to the Audio class, and the CD, the hard disk, and the flash memory belong to the MassStorage class. Microsoft's Windows operating system provides drivers for common categories for automatic installation. USB devices that are not supported by Windows (such as measuring instruments) need to provide their own drivers, or the installer packages the applications and drivers together to complete the device installation at one time. The USB-IF is responsible for authenticating the hardware and programs that conform to the USB interface specification and issuing the certification mark. The driver is also certified by Microsoft Hardware Device Quality (WHQL) for automatic installation under the Windows operating system.

After 2000, desktop PCs added USB ports, and laptops even installed two USB ports. Hundreds of millions of PCs with USB interfaces are now running, with billions of PC peripherals and other devices, including USB measuring instruments using USB interfaces. Because the USB interface has a large number of PC peripherals, USB interface support products, including controller chips, integrators and bridges, cables and connectors, drivers and installers, development tools, etc., are very popular, plus USB The interface is easy to install, the data rate is wide, easy to expand, plug and play, low cost, etc., and more electronic products using the USB interface appear.

1. Emerging USB measuring instrument family

The USB interface into the measuring instrument began in 1998, when IOtech and NI first used the USB interface in their data acquisition instruments, and many well-known instrument companies accepted the USB interface. The easiest way is to add USB as a peripheral interface, because most of the desktop measuring instruments are equipped with embedded microprocessors, and some desktop measuring instruments use the Windows platform with built-in Pentium processor, so installing the USB interface is a natural matter. In 2000, Yokogawa began installing USB interfaces on digital oscilloscopes. After that, Agilent, LeCroy, and Tektronix also equipped USB interfaces on digital oscilloscopes. In 2003, USB was widely accepted as one of the standard interfaces by desktop, portable, and modular measuring instruments because Agilent and NI added support for USB in the I/O layer of the widely used Virtual Instrument Software Architecture (VISA). Traditional IEEE488 interface instruments and RS232 interface instruments can be connected to desktop and notebook computers via USB-IEEE488 and USB-RS232 converters.

After practical application, it proves that the USB interface is a simple and convenient and low-cost interconnection technology in the measuring instrument, and it has achieved remarkable results in the data acquisition system. Traditionally, the data acquisition system card of the PC platform needs to occupy the ISA or PCI slot, and a large number of cables that are taken out from the card to the sensor. When the data collection is increased, the hardware and software resources such as the number of PC slots, addresses, and interruptions are increased. Restriction, poor scalability, poor anti-electromagnetic interference performance, difficult installation and disassembly, and uneconomical cost. With the increase of the USB interface and data rate of the PC and the drop in the price of the USB interface chip, the USB connector and cable are cheaper, resulting in the introduction of a large number of USB data acquisition systems, ranging from $100 to $1,000, which is very popular among users. Welcome. In fact, this kind of expansion bus or peripheral bus with PC configuration has a precedent for the successful experience of measuring instrument systems. PCI, PXI, VXI and the upcoming LXI various bus instrument expansion applications are fully utilized. It has played a significant role in the high penetration rate of PCs, low product cost, and ease of use. The PC market is much larger than the measuring instrument market, and the development and manufacturing resources are extremely rich. These are advantages and are not available in the measuring instrument industry. In the 1960s, the measuring instrument industry independently developed the GPIB bus (IEEE 488), which was the result of HP's 10 years of hard work. It is worth noting that IEEE 488 is still the bus of choice for desktop instruments. Since the IEEE488 bus, NI has proposed the concept of virtual instrumentation, and borrowed the ship to the sea to make full use of the hardware and software resources of the PC for measuring instruments. The results have been impressive, which has led to the development of measuring instruments.

However, the USB interface is not driven by the instrument company or organization in the extended application of the measuring instrument. The situation is different from the IEEE488, VXI, PXI and other buses. They are organized and standardized, and have an influence and promotion on the measuring instrument industry. . USB instruments belong to the market of measurement instrument companies, and the initial progress is slow. In recent years, the number of products has increased significantly. Especially after the release of USB2.0, USB measurement instruments are generated from data acquisition to digital multimeters, digital oscilloscopes, logic analyzers, and arbitrary waveforms. The development of devices, digitizers, protocol analyzers, etc., has formed a wide variety of measuring instrument entities that are small in size, light in weight, affordable, and moderate in performance. With the popularity of PCs, a class of PC-based instruments has emerged in measuring instruments. It is built into virtual panel instruments, referred to as PCI instruments, by means of measurement cards and PC resources inserted into the peripheral slots of PCs. After the emergence of notebook PCs and pocket PCs, they do not have peripheral slots for measuring instruments, only the various interfaces available for measuring instruments, including floppy disks, hard disks, PC cards, infrared, parallel, serial, 1394, One to four types of interfaces such as Ethernet and USB. After the actual test, USB instruments are most popular among users in the middle and low-end measuring instruments, forming a class of USB instruments and entering the mainstream of measuring instruments.

It is worth mentioning that USB instruments are popular products that enter the market of measuring instruments with plug-and-play and economical benefits, its mechanical components (such as connectors, cables) and electrical characteristics (such as timing, synchronization). They are not at the same level as IEEE488, VXI, and PXI instruments. At the same time, there is no USB instrument specification, only the USB specification.

2, USB data collector

Among the USB instruments, the USB data collector is the earliest and the most diverse, with products ranging from simple modules to multiple card chassis. The simple USB data logger module typically has more than 8 analog input channels, 12 to 14-bit resolution, and a sampling rate of less than 100 KS/s for simple measurement and recording of I/O. The module is connected to the USB port of the PC through a USB cable. After downloading the driver program from the CD, it can work. The analog signal from the sensor is input from the module terminal block. To compare and understand the features of these low-cost USB data loggers, Advanced Technology Editors of Test & Measurement World (TM&W) recently tested three low-priced ($150) products, which are NI The company's USB  6008, DT's DT  9810 and MC's PMD  1208 FS. The main features of USB  6008 are as follows:

8-channel single-ended module input, resolution 12-bit (differential), 11-bit single-ended), sampling rate up to 10KS/s, step-by-step A/D converter;

Input voltage range ±20V (differential), ±10V (single-ended), DC power supply is supplied by USB interface (5V/200mA);

Drivers for Windows, MacOS, LinuxOS operating systems, data logging software, plug-and-play installation software, LabVIEW or C programming;

Digital I/O 12 way, 5MHz counter;

Dimensions: 63.5 mm × 85.1 mm × 23.1 mm, weight: 59 g.

Contrast tests believe that their electrical indicators are in the same grade, except that the input voltage range of DT-9810 is +2.4V, and higher input voltage range should use USB products with higher price. They differ slightly in software. Although they all provide data logging software, users can read stored data from a PC monitor, but only DT-9810 has a digital I/O port that can control the module, and provides oscilloscope display and digital multimeter software. , as well as the application programming interface and LabVIEW virtual instrument under Windows. The PMD-1208FS TracerDAQ software stores and displays data in EXCEL or limited file formats, provides oscilloscope displays, and has internal self-calibration capabilities. This type of USB data acquisition module provides engineering technicians and students with low-cost, high-quality, lightweight, and easy-to-connect data logging and acquisition tools.

The mid-to-high-end USB data logger has essentially the same features as the PCI/VXI/PXI data logger, such as 16-bit resolution, sampling rate of 100KS/s, 32 analog inputs, and 32 digital I/O. Due to the relatively simple interconnection of the USB interface, the mid-to-high-end USB data collector has a price advantage. The current measurement instrument companies that produce PCI/VXI/PXI data collectors supply similar products of the USB interface. It is worth noting that there are data acquisition companies that produce a separate chassis with a built-in digital signal processor DSP interface and a flash memory USB data acquisition system, such as Hacker's USB2-DAQ/DIO data acquisition system, due to its strong computation and Control capability, plug-in 16 to 32 analog inputs, 12 to 16-bit resolution, 100 KS/s sampling rate, 24-bit digital I/O lines, 12 cards per chassis, real-time for motor systems control. In addition, the top suppliers of digital multimeters also attach importance to the use of USB interfaces. Keithley has introduced the KUSB3116 data acquisition module with indicators of 8 analog inputs, 16-bit resolution, and 500KS/s sampling rate. Fluke has a USB interface on the 189 digital multimeter that has sold a lot of sales, and enhances the data recording function. With the FlukeView3.0 software package, the 189DMM has the ability to store, display and analyze data. It can be connected by laptop and USB cable. Taiwan 189DMM and constitute a data recording system. The 189DMM has 0.025% voltage reading accuracy and more than 20 different measurement functions. A large number of sensor resources make the data recording system more popular than the single digital multimeter.

3, USB digital oscilloscope

The USB digital oscilloscope is another type of USB instrument, most of which is a palm-sized structure that can be combined with a notebook computer to form a portable digital oscilloscope. At present, there are a variety of products with bandwidths below 200MHz. The simplest one is the single-channel pen type USB, and the more complicated is the dual-channel handheld USB digital oscilloscope. They avoid the desktop, portable, and handheld digital oscilloscopes that already have an advantage in the oscilloscope market, and the world of miniature digital oscilloscopes, which are inexpensive and available for $100 to $200. The performance of such miniature digital oscilloscopes is not worse than that of handheld digital oscilloscopes, and of course they cannot be required to have as comprehensive indicators as portable digital oscilloscopes.

A pen-type USB digital oscilloscope USBScope50 produced by Elan has 100MHz analog bandwidth, 8-bit resolution, single sampling rate of 50MS/s, re-sampling rate of 1GS/s, single channel input, input voltage range of 0.3V/3V/ 30V, with X10 double head (1MΩ/15pF), Windows98SE/2000/XP digital oscilloscope software, 300V isolation protection between input and USB interface, using USB1.1 or USB2.0 interface. The USBScope50 has no mechanically adjustable knobs and switches, and all electrical parameters are programmed via a soft panel. When multiple inputs are required, multiple products of the same model can be stacked, and the delay between channels can be adjusted to match to achieve parallel triggering. The USBScope50 actually forms a simple virtual oscilloscope for simple waveform display.
Stingray's handheld DS1M12 is a multi-purpose, low-frequency USB digital oscilloscope with an A/D converter with 12-bit resolution and 1MS/s sampling rate, which can be used at 20MS/s oversampling rate. The two input signals are simultaneously sampled, the analog bandwidth is 250KHz, and each input has 32KB of waveform storage. Two applications are available: EasyScope for digital oscilloscopes and EasyLogger for data acquisition. The DS1M12 features digital oscilloscopes, data loggers, spectrum analyzers, digital voltmeters, frequency meters, and arbitrary waveform generators. Third-party applications can be downloaded under the Windows operating system, using the most common LabVIEW programming language, providing Windows and Linux driver software. After loading both EasyScope and EasyLogger applications, the DS1M12 has the following key features:

◆ Under EasyScope software, time base 2μs/div to 50ms/div (14 steps total), vertical range 10mV/div to 5V/div (6 steps total), X and Y direction cursor, metric display (Min/Max /Average/Vrms/F), spectrum display, measurement download, screen download, waveform generation (sine, square, triangle, sawtooth, user-defined waveform).

â—† Under EasyLogger software, sampling rate (100K samples/second to 100S/sample), Y-axis calibration (mV/V/user-defined), X-axis calibration (time or sample number), 3 screen cursors, 4 pop-up or E-mail alerts, data download (CSV, BMP, text, binary), call history documents, and add memos to users.

◆ Hardware also has 32KB waveform storage, pulse parameter trigger (edge, width, maximum/minimum), delay trigger, arbitrary waveform generator (10-bit resolution, ±3.5V output), size: 116mm × 30mm × 100mm, Weight: 160 grams.

Acute's DS-1000USB digital oscilloscope is a pocket-sized PC-based oscilloscope with an emphasis on analog bandwidth. It works under the USB2.0 interface and has most of the functions of a desktop digital oscilloscope:

â—†Using 9-bit resolution and 200MS/s sampling rate, equivalent sampling rate of 5GS/s, sensitivity of two inputs is 2mV/div to 10V/div;

â—† When external triggering and TV triggering, the bandwidth can be limited from 200MHz to 20MHz, pulse parameter triggering and complex waveform triggering;

â—† can form up to 6 stack inputs, with 64K/512K sample storage capacity, single FFT transform, math operation, record/playback waveform processing functions, Internet control;

â—† Data output methods are WORD, EXCEL, HTML, etc.

4, USB instrument expansion, real-time synchronization control

There are certain limitations when using USB as an interface to the instrument system. Because USB devices operate independently, when multiple USB devices are connected together through a single port, there is no synchronization between them. The USB bus is designed based on an asynchronous point-to-point architecture where devices cannot work together in real time. For instrument systems, real-time synchronous control is often required. At this point, the USB interface is abandoned and the costly interfaces such as VXI or PXI are selected. In fact, both VXI and PXI are extensions of the PC bus, and the most important extension is the addition of timing and synchronization. Therefore, with the existing results of the USB interface, the timing and synchronization functions are increased, and the application range of the USB interface instrument is expanded. Recently, Fiberbyte has introduced a USB-inSync (USB Synchronization) technology that retains all the features of the original USB bus, adds real-time synchronization control, and expands the application of the USB interface in the instrument system. It can be regarded as an innovation of the USB interface. . It transforms the USB bus into an operating platform that is synchronic, deterministic, and scalable, providing a low-cost, easy-to-use interconnect bus for instrumentation systems. The main features of USB-inSync are as follows:

Can use the original standard USB cable, port, connector, hub and other resources;

All USBinSync devices are connected to the same USB port and interlocked into a synchronous system;

Up to 127 discrete devices can be connected at the same time, and the synchronization time difference is within ±5 ns;

Any USB-inSync device in the interconnect device can be selected as a timing reference for controlling other devices;

With plug-and-play capability, the synchronous control and processing of interconnected devices is managed by the PC via the USB port;

16-bit analog input, 16-bit digital I/O, and digital triggering during data acquisition;

Ordinary USB devices are fully compatible with USB-inSync devices to form a measurement instrument system;

The PC host needs to run Windows XP or Windows 2000 operating system, Pentium III or higher processor, 128 MB or more memory, 10 MB or more hard disk space, and USB 1.1 or above main controller.

Fiberbyte has started to supply USB-inSync data acquisition system. DAQ2500X is the first product. It has 4 independent 16-bit resolution, 100KS/s analog input channels, and independent A/D in each channel. The converter, powerful first-in, first-out (FIFO) processing capability can perform burst simultaneous sampling, the 100KS/s sampling rate of 4 groups of A/Ds constitutes 1MB/s data throughput rate, and the module supports both program-controlled digital I/O and Trigger for real-time data collection. It should be pointed out that the USB interface is designed to work in parallel for a variety of devices with different functions. The PC host performs sequential interaction processing on the device through the asynchronous bus. This asynchronous operation hinders direct interaction between devices and establishes multiple devices. The ability to synchronize and process is very limited. The DQA-2500X using USB-inSync has its own local clock and establishes phase lock on the USB-inSync device inside the PC's USB network. The main hub acts as a timing controller to achieve time synchronization and data processing between the devices. The main hub makes phase compensation based on the phase difference of each device to ensure that the devices are fully synchronized. When the number of networking devices changes, the timing and synchronization process is automatically re-adjusted. As long as the Windows operating system recognizes the new device, the synchronization relationship is established immediately. USB-inSync technology also provides decisive digital triggering to enable test system start and stop to be performed in a controlled manner, and to extend timing and synchronization to other slave hubs within the system, establishing master-slave timing and synchronization aisle.

5, wireless USB is about to become a new member

The application of USB1.1 and USB2.0 has been very common and the number is huge. According to incomplete statistics, there are now 1.5 billion USB cables spread across different applications. By 2006, this number will double to more than 3 billion, mainly from portable audio products, digital cameras, printers, mobile hard drives, and flash storage. body. The new laptops began to be equipped with wireless Ethernet, and the application of a large number of USB peripherals also prompted wireless USB to enter the market as soon as possible to reduce the interconnection between USB devices and PCs. In order to promote the development of wireless USB, Agrere Systems, HP, Intel, Microsoft, NEC, Philips Semiconductors, Samsung Electronics and other seven companies formed the Wireless USB Promotion Organization (WUSU-PG) in the past few years to develop a wireless USB specification. The organization is supported by more than 100 members, and the draft Wireless USB specification has been submitted to USB-IF this year and is scheduled to be released before the end of this year. The first products to comply with the Wireless USB standard will appear as discrete chips, including controller chips, adapters, transceivers, etc., and then completed by a more integrated single chip.

The Wireless USB specification is built on the Ultra Wideband (UWB) wireless multimedia (WiMedia) aggregation platform, which uses UWB as a carrier to transmit and receive USB specification information. UWB short-range wireless communication mode is a carrier-free ultrashort pulse sequence modulated wave, occupying GHz-level bandwidth, UWB has become the IEEE802.15.3a standard. Wireless USB connects to the WiMedia aggregation platform through the protocol adaptation layer, builds a USB2.0-compatible application software stack, shares the UWB RF protocol, and is recognized by IEEE802.15.3a. The wireless USB transmitted and received through UWB provides the equivalent high-speed USB data transmission rate, the bandwidth reaches 480Mbps within 3m distance, and reaches 110Mbps within 10m distance. Incidentally, the wireless method of the IEEE1394 interface frequently used by measuring instruments is also built on the UWB specification. It is foreseeable that more devices will be wirelessly transmitted and received through the UWB radio. They will be aggregated at the WiMedia layer. Regardless of the protocol signals they originally used, they will be converted into the same UWB signal after the convergence layer and transmitted by the UWB physical layer. Share extremely wide UWB radios. UWB has been tried by the United States as a short-range LAN communication without applying for frequency license. Wireless USB sharing UWB radio has achieved satisfactory wireless transmission and reception, and maintains the characteristics of USB2.0. Of course, due to non-cable interconnection, USB2 The DC supply in .0 is no longer present.

Wireless USB uses a measuring instrument to wait for a while, first need to be used and successful on the PC, and the cost is reduced to be similar to USB2.0, and then use the mature wireless USB chip to make a wireless USB instrument. At the same time, wireless USB needs to be approved by the government's radio management department as an open frequency band, and wireless USB measuring instruments can be popularized. It is expected that this period will be 2 to 3 years. It is believed that UWB technology will also make progress at this time. The transmission and reception distance will exceed 3m, and wireless USB will also benefit accordingly, maintaining a data transmission rate of 480Mbps outside the distance of 3m.

6, the conclusion

USB instruments have entered the market for measuring instruments for seven years, and early progress has been slow. Since the introduction of the USB2.0 high-speed interface, USB instruments have advanced at a faster pace in terms of variety, performance, applications, etc., not only limited to USB devices with common indicators, but also have more high-end USB instruments with features such as timing. And synchronous expansion, GHz-level time domain reflectometers, etc. If the miniaturization and miniaturization of USB instruments are successful, more miniature measuring instruments will certainly appear. The availability of wireless USB instruments will increase the mobility of measuring instruments. USB instruments have become the mainstream of measurement instruments, while promoting the development of traditional instruments toward miniaturization and miniaturization.