Serial Communications Overview in Windows 3.1 (82643)

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1. Applications open and initialize an available communications port using the standard Windows application program interface (API). Windows keeps track of what ports are in use by other applications, and will return an error code if a requested port is already in use.
   
   Windows applications do not need to know how to access specific types of communications hardware; this is handled in a device independent fashion by the communications API and driver.
   
   Windows applications can select bit rates up to 57.6 kilobits per second (Kbps) via the communications API.
2. Windows applications send/receive data to/from the COM port using the standard Windows API documented in the SDK.
   
   To service active port connections, Windows applications should set a timer or use the EnableCommNotification() API. EnableCommNotification() will send a window message to the application when data buffers are filled or empty. This prevents the application from unnecessarily clogging the system when waiting for data transmissions. Applications should NOT rely on PeekMessage() when servicing open ports.
3. Windows sends/receives application data to/from the communications driver (COMM.DRV). In Windows 3.1, the data is sent and received between Windows and the driver in blocks, rather than individual characters. This reduces transmission overhead and improves performance. Windows 3.00 used individual characters.
4. The communications driver sends/receives data to/from the COM port. The COM driver supports 8250 compatible universal asynchronous receiver transmitters (UARTs). Also, the Windows 3.1 COM driver enables the FIFO buffer of 16550A compatible UARTs in order to reduce interrupt overhead and improve data throughput. Special communications devices may require a replacement communications driver to support extra features or a different hardware interface.
5. In enhanced mode, the virtual communications device (VCD) virtualizes COM port hardware to arbitrate access by multiple applications. The VCD notifies virtual machines (VMs) when their COM ports require servicing.
   
   In enhanced mode, COMBUFF buffers data between the communications driver and physical port. This prevents multitasking applications from losing data when they do not currently have CPU time.
6. Finally, data is received and transmitted by the physical COM port to/from the attached device (for example, modem, direct connection, fax card, and so on.). Some COM ports have onboard data buffers, such as those using the 16550A UART, to improve communications throughput and reduce errors.