As Peridot points out you have a VERY odd Display if it can Input a Video signal on its 9-Pin D-Type connector - the RS232 port is used for controlling the Display from an external device its not a Video Input.
You may also have a 15-Pin D-Type 'VGA' connector on the Display and if your Display can accept Video as well as PC signals on the HD15 socket you may be able to Input a signal from a Digibox or similar.