Communication Port Data Loss in Enhanced-Mode Windows 3.0

• Microsoft Windows operating system versions 3.0, 3.0a

SUMMARY

This article contains information about why MS-DOS applications lose communication-port data when running from enhanced mode in Microsoft Windows version 3.0, and the steps to eliminate this problem.

MORE INFORMATION

Data Loss at 2400 Bits Per Second (BPS) or Slower

Communication-port data loss should not occur when using MS-DOS applications operating at bit rates of 2400 BPS or less. However, data loss may occur at higher bit rates. If you are experiencing problems at 2400 BPS or slower, check for a hardware communication-port I/O base or interrupt assignment conflicts.

Data Loss at Rates Higher than 9600 BPS

MS-DOS applications cannot use bit rates greater than 9600 BPS when running under enhanced-mode Windows 3.00. To use higher rates, such as 19,200 BPS, use a Windows-based communications application, such as FutureSoft's DynaComm Asynchronous, or DCA's CrossTalk for Windows.

19.2 kilobits per second (Kbps) is the maximum possible data rate for a Windows-based application.

Data Loss at 9600 BPS

On many hardware platforms it is not possible to use 9600 BPS in an MS-DOS application under enhanced mode. This depends on the speed of the processor and the type of communication-port hardware being used. If you are experiencing problems at 9600 BPS, take the appropriate steps as explained below. The steps are different for text only or text/binary data.

1. Text data only

   If you are transferring text data ONLY, and your MS-DOS application supports XON/XOFF software flow control, you can configure enhanced-mode Windows to process the XON/XOFF signals internally. This may assist in preventing data loss at 9600 BPS. To enable this feature, insert the following line in the [386Enh] section of the SYSTEM.INI file:

         COMxProtocol=XOFF
   

   where x is the communication port in question. For example, if you are using COM1, use COM1Protocol=XOFF. It is important to note that this switch does NOT work if you are performing ANY binary data transfers, and actually prevents such data from being transferred correctly. In addition to adding this switch, you should increase the enhanced-mode virtual communication-port buffer size, as detailed below (in Step 2).

2. Binary data, or text and binary data combination

   If you are transferring any type of binary data, such as downloading from a computer bulletin board by modem, you may want to increase the enhanced-mode virtual communication-port buffer size. This may be done by adding a switch to the [386Enh] section of the SYSTEM.INI file, as follows

         COMxBuffer=yyy
   

   where x is the communication port in question, and yyy is the buffer size in characters (default 128). For example, if you are using COM1, and want to increase the buffer size to 256, use COM1Buffer=256. Since the buffer default is 128, start with 256 and work upward in 128-character increments until the data loss is eliminated. Note that data loss cannot be eliminated in all cases; in such situations, a Windows communications application should be used.