Sorry I didn't quite understand what you were trying to explain
? you have a RS232 cable from the 9 pin Serial port on your computer to the 25 Pin port on your Doosan ?
Can you upload and download programs via this cable ?
What RS232 software are you using this needs to have a drip feeding function
The easiest way on a 21i is to buy a SD Card and adapter and Drip feed from that