Here is what I found in the manual concerning RS232 settings. I think all non bold settings are correct at your side, but can you check the bold ones?
1.2 RS-232C Settings
*Full duplex, start-stop synchronization communication
Baud rate 9600bps
Data bits 8
Stop bit 1bit
*The RX-V1900 RTS port outputs at low level when the AC plug is disconnected.
If RTS output stays low even when the AC plug is connected, a problem may occur.
Are you sure RTS/CTS is selected?