Windows 3.1 Serial Communications Q & A Part 2

• Microsoft Windows operating system versions 3.1, 3.11

SUMMARY

This article contains specific questions and answers regarding the Microsoft Windows operating system version 3.1.

MORE INFORMATION

1. Q. What is the maximum possible bit rate for Windows applications?

   A. Windows 3.1 allows applications to select bit rates as high as
      57.6 kilobits per second (Kbps). However, how well data can be
      transmitted and received at high speeds depends on the variables
      listed below. With Windows 3.1, a 19.2 Kbps XModem transfer should
      run well on a 20MHz 386. Factors affecting top bit rate in Windows
      applications include the following:

      a. CPU Speed. Faster CPUs allow faster data throughput with fewer
      errors. This is because they allow the COM driver to service
      character interrupts faster, which prevents UART (Universal
      Asynchronous Receiver Transmitter) overflow and allows applications
      to handle transfer protocols better. If you experience slow serial
      performance or a loss of characters on a 286-based machine, try the
      following:

       1) Reduce the number of MS-DOS drivers and terminate-and-stay-
          resident (TSR) programs.

       2) Do not load MS-DOS 5.0 into upper memory.

       3) Add the following line to the SYSTEM.INI file:

             FasterModeSwitch=1.

       For more information on FasterModeSwitch and other SYSTEM.INI file
       switches, see the SYSINI.WRI file in the WINDOWS directory.

       b. Transfer Protocols. Data transmission protocols, such as XModem,
       YModem, and ZModem, vary greatly in robustness in multitasking
       environments. Some are designed to be very fast and provide little
       error checking; others are slower but provide error correction.
       XModem and ZModem usually work well with Windows, even at high
       speeds.

       c. Serial Port Hardware. Newer serial ports use the 16550 Universal
       Asynchronous Receiver Transmitter (UART), which contains a buffer to
       reduce overhead and prevent overflow.

      d. System Overhead. The more applications, TSRs, and device drivers
      that are running, the busier the operating system is. This reduces
      overall execution time for every user in the system. If many programs
      and devices are active while trying to transmit data at high speeds,
      reduced throughput and transmission errors are likely to result.

2. Q. What is the maximum possible bit rate for MS-DOS applications
      running in Windows?

   A. MS-DOS serial communications applications are designed to run in
      a non-multitasking environment; therefore, they make assumptions
      about the operating system and devices that may not always be true
      for a multitasking operating system. These applications may allow the
      selection of speeds as high as 115 Kbps. While these speeds may work
      well in MS-DOS alone, reduced data throughput should be expected when
      running these applications in a multitasking operating system.

      The guidelines listed above for Windows serial communications
      applications also apply to MS-DOS-based applications. However, the
      following guidelines are important for MS-DOS-based applications
      running within Windows 386 enhanced mode:

      a. Full-Screen v. Windowed v. Minimized. MS-DOS-based applications
      run best in full-screen mode; they receive foreground priority and
      avoid the overhead of display virtualization. Although MS-DOS-based
      applications running in a foreground window receive foreground
      execution priority, the overhead of updating the window can seriously
      reduce application performance. If you want to run an MS-DOS-based
      communications application in the background during a data transfer,
      run it as an icon rather than in a window.

      b. Foreground and Background Execution Priority. Windows allocates
      execution time for MS-DOS applications according to foreground and
      background priorities that are supplied in a .PIF file. If the
      application is not getting the time slices it needs to process
      incoming data, errors occur during data transfer. Increase the
      application's background execution priority in the .PIF file to
      prevent errors when receiving data in the background. Make sure that
      background execution is specified in the PIF file.

      c. Lock Application Memory Option. To allow more MS-DOS-based
      applications to run simultaneously, Windows 3.1 386 enhanced mode
      swaps application memory to disk. This can slow the execution of the
      MS-DOS-based applications and significantly reduce communications
      performance. You can disable this swapping of application memory in
      the MS-DOS communications application's .PIF file by checking the
      Lock Application Memory check box in the Advanced Options dialog box
      of the PIF Editor. Note that this setting applies only if a permanent
      swap file is allocated and 32-bit disk access is enabled.

3. Q. Does Windows 3.1 support the 16550 UART buffer?

   A. Many serial port adapters and add-in modems now use the 16550
      Universal Asynchronous Receiver Transmitter (UART), which contains a
      16-byte character buffer to reduce interrupt overhead and errors
      during high-speed serial transmissions. The Windows 3.1 COM driver
      enables this buffer for Windows serial communications applications in
      order to reduce interrupt overhead and improve serial throughput at
      high speeds. Windows applications do not need to call a special API
      to receive this support, it is automatically enabled. To determine if
      serial ports use the 16550 UART, run the Microsoft Diagnostics
     (MSD.EXE) outside of Windows 3.1.

      Windows 3.1 does NOT enable the 16550 buffer for MS-DOS
      communications applications. When running in Windows 386 enhanced
      mode, 16550-aware MS-DOS communications applications may fail to
      detect and enable the UART buffer. This is because Windows is doing
      its own buffering of the COM port for the application. To allow 16550-
      aware MS-DOS communications applications to enable the FIFO, disable
      Windows' buffering of a COM port by adding "COMxBuffer=0" in the
      [386Enh] section of SYSTEM.INI file, where x= the ID of the COM port
      (for example, "COM1Buffer=0"). This may improve serial communications
      performance under certain conditions.

      Some older 16550 UART versions do not properly support the buffer and
      may cause problems. Windows can detect many of these and will not
      enable the buffer. However, if problems are encountered, the 16550
      support may need to be manually disabled by adding "COMxFIFO=0" to
      the [386Enh] section of SYSTEM.INI file. For more information on
      COMxFIFO and other SYSTEM.INI switches, see the SYSINI.WRI file in
      the WINDOWS directory.

4. Q. A Windows communications application replaced the Windows 3.0
      driver. Should this driver continue to be used with Windows 3.1?

   A. The Windows 3.1 communications driver will probably provide
      better support than some replacement communications drivers developed
      for Windows 3.0. However, if the replacement communications driver
      provides special device support or features specific to the
      application, continue to use it with Windows 3.1. If not using a 3.1
      version communications driver, then add the following line to the
      [386Enh] section of SYSTEM.INI file: "COMMdrv30=TRUE". For more
      information on COMMdrv30 and other SYSTEM.INI switches, see the file
      SYSINI.WRI in the WINDOWS directory.

5. Q. I use Pacific Commware's TurboCom with Windows 3.0. Should I use
      TurboCom with Windows 3.1?

   A. Windows 3.1 corrects most of the problems that TurboCom was
      developed to alleviate in Windows 3.0 and provides additional
      communications features. Windows 3.1 also provides Windows
      application support for the 16550 buffer, which TurboCom provides in
      Windows 3.0.

      Unlike TurboCom, Windows 3.1 does not enable the 16550 buffer for
      MS-DOS communications applications. However, if you follow the above
      guidelines for using fast bit rates, these applications may not
      require TurboCom in Windows 3.1. Also, if you use TurboCom to run
      Windows-based applications at bit rates higher than they provide
      options to select, using TurboCom until the application is updated to
      support higher bit rates with Windows 3.1 may be desirable.

      In general, if the Windows 3.1 communications driver meets your
      needs, use it, rather than a third-party driver. This helps achieve
      full Windows 3.1 compatibility.