Du kan bruge dette til at vente på CTS:
#include <stdio.h>
#include <windows.h>
int main(void)
{
   DCB dcb;
   HANDLE ComPort = CreateFile("COM1", GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);
   GetCommState(ComPort, &dcb);
   dcb.BaudRate = 9600;
   dcb.ByteSize = 8;
   dcb.fRtsControl = RTS_CONTROL_ENABLE;
   dcb.fErrorChar = TRUE;
   dcb.fParity = TRUE;
   dcb.Parity = ODDPARITY;
   dcb.fAbortOnError = TRUE;
   dcb.fOutxCtsFlow = 1;
   SetCommState(ComPort, &dcb);
   SetCommMask(ComPort, EV_CTS);
   DWORD Event;
   while(1)
   {
      WaitCommEvent(ComPort, &Event, 0);
      printf("%08X\n", Event);
   }
   CloseHandle(ComPort);
   return 0;
}
Du kan bruge GetCommState til at læse om porten er høj eller lav.