Search code examples
cwindowsserial-portsignals

Toggling the CD (RLSD) signal line on a serial port in Windows C

I am trying to set the Carrier Detect (Receive Line Signal Detect) pin on a serial port being controlled by my Windows application. I am already able to set the RTS line high using this function:

EscapeCommFunction(handle, SETRTS);

and then subsequently clear it by using:

EscapeCommFunction(handle, CLRRTS);

I want to be able to do this exact same thing with the CD line, but no such SET/CLR flags exist for the EscapeCommFunction.

Solution

  • The PC serial port was designed from the viewpoint of a terminal, not a modem. The CD signal is an output from a modem and an input to a terminal. Even if the port hardware allows you to change the direction (of which I'm skeptical) the standard interface would not be programmed that way.