RTS and DTR are output pins - which you can set. DCD and CTS are input pins
and can only be read. The device is probably set for hardware handshaking by
default. You can change this using tcsetattr (see CRTSCTS). Then you can use
stty -F /dev/ttySX crtscts (modify the /dev/ttySX to match your device). This
enables RTS/CTS hardware flow control in the Linux device driver. Then when a
DTE (such as a PC) wants to stop the flow into it, it negates RTS.
I doubt CTS is possible to control (at least on a Intel 8250, NS16550 UART, which
is what you'd find in a PC or similar hardware designs), but RTS and DTR can be
controlled by software. Have a look at this page, the post by ...