PRB: Using SetCommState() to Toggle DTR/RTS for Flow Control (99861)
The information in this article applies to:
- Microsoft Windows Software Development Kit (SDK) 3.1
This article was previously published under Q99861 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.
| Modification Type: | Major | Last Reviewed: | 11/6/1999 |
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| Keywords: | kb16bitonly KB99861 |
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