DOCUMENT:Q82643  24-SEP-1999  [win95x]
TITLE   :Serial Communications Overview in Windows 3.1
PRODUCT :Microsoft Windows 95.x Retail Product
PROD/VER:WINDOWS:3.1,3.11
OPER/SYS:
KEYWORDS:

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The information in this article applies to:

 - Microsoft Windows versions 3.1, 3.11 
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SUMMARY
=======

In Microsoft Windows version 3.1, Terminal and other Windows communications
applications work as described in the following paragraphs.

MORE INFORMATION
================

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.


KBCategory: kbusage kbtool
KBSubcategory: win31 wincomm windrvr

Additional query words: 3.10 3.11

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Keywords          :  
Technology        : kbWin3xSearch kbZNotKeyword3 kbWin310 kbWin311
Version           : WINDOWS:3.1,3.11

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