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

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows version 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.