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DAN-113 Datasheet(PDF) 2 Page - Exar Corporation |
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DAN-113 Datasheet(HTML) 2 Page - Exar Corporation |
2 / 6 page DATA COMMUNICATIONS APPLICATION NOTE DAN113 2 EXAR Corporation, 48720 Kato Road, Fremont, CA 94538 TEL 1-510- 668-7000 – FAX 1-510-668-7017 - www.exar.com User-Mode Application Windows NT protects user from accidentally crashing the operating system by allowing access to the hardware only through device driver, which means there is no way to directly perform read/write operations on the hardware device. Thus the system reliability is increased. Application read/write operation, in general, can be either synchronous or asynchronous (see Microsoft SDK for more detail). In synchronous read/write operation, the application makes a request to send or receive data. The operation does not return until the data transmission either is complete or has been terminated due to an error that must be reported to the caller. Thus the application is blocked until the operation is complete. Meantime, the application does not pay attention to other tasks. In asynchronous read/write operation, the application makes a request to send or receive data and leaves without waiting the transmission completion. The purpose of asynchronous read/write operation is to introduce more parallelism to the application by allowing I/O operations to fully overlap the computation. Although asynchronous read/write operation is more difficult to write, debug, and maintain, it is still perform better and offer higher performance. The application that uses asynchronous read/write operation may provide an event handle in a data structure (overlapped mode). The event will be signaled upon completion of the I/O operation. In another way, the application may provide a callback function that is called when the I/O operation is complete. In the callback function, you must complete the data communication. Microsoft SDK document provides more detailed information to implement either method. This non-standard serial driver has registered in I/O manager with the DOS compatible device names that are COM5, COM6, COM7, COM8, COM9, COM10, COM11 and COM12, which are related with Ch0, Ch1, Ch2, Ch3, Ch4, Ch5, Ch6 and Ch7 of XR17C158 chip correspondingly. COM1, COM2, COM3, and COM4 are avoided here because PC may already use these device names for motherboard serial ports. From the document of Windows NT DDK, it is found that COM ports above COM9 require that the device name be specified explicitly as a device on the local machine, for example, “\\\\.\\COM10”. The first step that application needs to do is to call Windows API function, CreateFile, to open the corresponding serial port (using the symbolic device names above). The function return, if successful, is a file handle. The application uses the file handle to access the hardware device for all subsequent I/O operations. The application takes responsibility to close the port by using CloseHandle after the operation is over. If the application needs to receive data from a particular port, ReadFile function can be used. This operation takes the handle of a file obtained from CreateFile, a |
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