Research And Realization Of Temperature Instrument Based On USB Port

Verification of temperature element is the foundation of temperature accurate measure and precise control. From early on, verification of temperature element always is a difficult job, normally the verification of thermocouple and resistance is operated manually, and needs ten more different types and materials to be converted according to international standard. A large amount of data operated manually, which is low efficiency and high error rate. The low efficiency of verification impacts on quality and normal work. In order to solve this, this article designs a management system of temperature instrument based on USB.USB(Universal Serial Bus)is a new interface technology applied in computer. It simplifies the connection between PC and external equipment, and makes it easier to extend PC interface. USB is as hot spot in computer and embedded field, which speed up the development PC external. From USB 1.1 to 2.0, and then to USB OTG (on-the-go) now, USB has been improved and to be mature. USB has high speed, low cost, low consumption, plug and play and easy to maintain and etc. it not only becomes standard interface on PC main board, but also is standard protocol of external devices, such as keyboard, mouse, screen, printer, DC and etc. and occupied the market of middle and low speed external equipment of PC USB has following advantages:①easy to use. In USB system, all USB devices could be"plug and play"; USB host could identify status of device dynamically, and assign address and parameters automatically to connected devices.②fast. Fast is main characteristic of USB technology, Ver 1.1 standard has full rate of 12Mb/s and low rate 1.5Mb/s modes. The main mode is full rate, which speed is 100 times of series port, and is 10 more times of parallel port interface. USB 2.0 could provide up to 480Mb/s data rate, and could develop more kinds of equipments on it, include high pix of DV, next G scanner and printer.③easy to extend. It could connect up to 127 external devices, besides all external devices use standard USB port connector, simplified installation process very much.④independent power supply USB bus could supply 5V, 500mA. So, new equipments do not need special AC supply that reduce the cost and improve quality-cost rate.⑤flexible usage. In order to fulfill different requirements of external devices, USB provides 4 different data transmission modes: Control transmission, Synchronization, Interrupt, and Bulk transmission.⑥Support many external device working simultaneously. Data bandwidth could be from several K to 480Mb/s, and it could allow transmit real time and none real time data in one cable, transmit several data stream between host and external and several external devices operated simultaneously.HID (Human Interface Device)equipments. This kind of device must support standard USB descriptor and USB device request. If necessary, device could also define its special descriptor and request, these are called Device Definition Descriptor and Device Definition Request. Moreover, a full device also demonstrates its using method of interface and end point. HID device has following characteristics:①exchanged data stores in structure of report, device must support HID report format;②every event could carry small or middle amount of data. Each event of low rate device could have 8 B mostly, full rate the most is 64 B, high rate is 1024B;③has limit of highest transmission rate. Low rate device could finish one event in 10 ms, the highest rate is 800B/s; full rate device is 1 ms each event, the highest rate is 64KB/s; high rate device mostly finish one event in 125μs, the highest rate is 24.576MB/s;④no assurance of transmission rate. When insert USB device, host will request all kinds of descriptor to identify devices. In order to recognize one device as HID, which should follow HID standard when define the descriptor. Chart 1 demonstrates the relationship of descriptors. In fact, each device could have many interface descriptor to implement multi interface device, and that each interface descriptor should have multiple end point descriptor.This artical discusses the development of the communication classes between HID application programms and USB peripherals by using HID which comply with USB standards. And such developed communication classes consider firmware, drivers and applications as individual units which usually are treated as interlocking and non-separating group. The communication classes also solve the problem of device driver programming which embedded-device programmers will encounter when they are selecting USB solutions. USB devices using the discussed technique in this artical, which eliminates the driver programming and installation and finally makes a real mean for "plug and play", will need no additional drivers for their own running request just like USB keyboard and mouse.This artical also discusses the development of thermal instrument managed system, include thermal element calibrator, device managed system and thermal instrument verify managed system. The thermal element calibrator module communication with the host via USB interface, the application protocol implementing based on the discussed technique in this artical. This system has high automatic grade when verify temperature element, it makes verification period short clearly. Comparing with original verification manually, now the efficiency improves greatly, normally 5 verification points, 3 hours could finish all work, it achieve the automatization of temperature verification process and the target of data processing in micro-computer.