Q99861: PRB: Using SetCommState() to Toggle DTR/RTS for Flow Control

Article: Q99861
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
-------------------------------------------------------------------------------

SYMPTOMS
========

When SetCommState() is used to raise the DTR and RTS lines for hardware flow
control, the first several incoming characters may be lost.

CAUSE
=====

SetCommState() calls the communications driver's setcom function, which calls
$SETCOM. $SETCOM disables interrupts from the specified port by clearing the
UART's interrupt enable register (IER). After changing the state of the UART and
the DTR and RTS lines, $SETCOM delays while interrupts from the UART are still
disabled. In Windows 3.1, this delay is approximately 200 milliseconds; in
Windows 3.0, the delay is approximately 55 milliseconds. If the DTR and RTS
lines are raised from low to high, any characters that arrive before interrupts
from the UART are enabled will be lost.

RESOLUTION
==========

SetCommState() should not be used to toggle the states of the DTR and RTS lines
for hardware flow control.

Use EscapeCommFunction() to toggle the the states of the DTR and RTS lines
because it does not delay while interrupts are disabled.

Additional query words: 3.00 3.10 comm

======================================================================
Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.