Problem is almost solved but not quite. I have a USB-to-TTl converter and it gives me +5 volt at pin 2 and 3 with respect to pin 5. I have added a MAX3232 rs232-TTL converter and now I am having -9 v at pin 2 with respect to pin 5 and 0 at pin 3 but I need revers. I mean I need pin 3 to be at -9 and pin 2 to be at 0 with respect to pin 5. This will be recognized by my old device. I have connected TXD,RXD,GND and VCC on usb-to-TLL to the same signal pins on MAX3232 rs232-TLL converter. How can I make pin 3 to have -9 and pin 2 to have 0? This is exactly what I have on COM1 of my desktop PC. Have I done something wrong? People have claimed MAX3232 can give true RS232 but in my case it is almost true but voltages are reversed at TXD and RXD ?